Testosterone

Testosterone is the major male sex hormone and is produced by the male testes in men and to a lesser extent by the adrenal glands in both men and women. In men, testosterone is thought to regulate sex drive (libido), bone mass, fat distribution, promotes gains in muscle mass and strength when combined with resistance training, the production of red blood cells and controlling fertility and the development of sperm (spermatogenesis) ¹. Testosterone also plays an important part in the development of ‘male sex characteristics’ such as a deeper voice and certain patterns of muscle development and hair growth. Testosterone helps bring on the physical changes that turn a boy into a man. This time of life is called puberty. Adolescent boys with too little testosterone may not experience normal masculinization. For example, the genitals may not enlarge, facial and body hair may be scant and the voice may not deepen normally. In women or anyone with ovaries, testosterone impacts overall growth as well as development of muscle and reproductive tissue. Testosterone is produced in the Leydig cells in the testis in men and to a lesser extent by the adrenal glands in both men and women ². In males, the normal range for early morning testosterone is between 300 ng/dL to 1000 ng/dL but may vary from laboratory to laboratory ³, ⁴, ⁵. Low testosterone levels in men is defined as total testosterone < 350 ng/dL and free testosterone < 225 pmol/L are associated with sexual dysfunction such as low sexual desire (low libido), erectile dysfunction (fewer or diminished spontaneous erections, decreased nocturnal penile tumescence), reduced skeletal muscle mass and strength, decreased bone mineral density, sparse beard growth, shrinking testes, increased cardiovascular risk and alterations of the glycometabolic profile ⁶, ³, ⁷, ⁸, ⁹. Testosterone is used only for men with low testosterone levels caused by certain medical conditions, including disorders of the testicles, pituitary gland (a small gland in the brain), or hypothalamus (a part of the brain) that cause male hypogonadism (a condition in which the body does not produce enough natural testosterone) ¹⁰, ¹¹, ¹², ¹³, ¹⁴, ¹⁵, ¹⁶, ¹⁷, ¹⁸. Your doctor will order certain lab tests to check your testosterone levels to see if they are low before you begin to take testosterone.

Testosterone should not be used treat the symptoms of low testosterone in men who have low testosterone due to aging also called ‘age-related hypogonadism‘ or ‘late-onset hypogonadism‘ ¹⁹, ²⁰, ²¹. Testosterone production and testicular function tend to decrease in aging men by 1% to 2% per year after the fifth decade ²², ²³. Similarly, testicular volume decreases about 15% from age 25 to 90 years, impacting on quantity and quality of sperm ²⁴, ²⁵, ²⁶. Hypogonadism occurs in 19% of men in their 60s, 28% of men in their 70s, and 49% of men in their 80s ²⁷. Testosterone deficiency is very common in elderly men and is linked to different signs and symptoms such as reduced libido, reduced sexual functioning, decreases in mobility and energy that could heavily affect the aging process and the quality of life ²⁸, ²⁹, ³⁰.

Unmodified testosterone is not orally available, so it must be given intramuscularly (e.g., testosterone cypionate [Depo-Testosterone], testosterone enanthate (Xyosted, available generically), testosterone undecanoate (Aveed), and testosterone pellet (Testopel) are forms of testosterone injection used to treat symptoms of low testosterone in men who have hypogonadism), sublingually (e.g., testosterone buccal), intranasally (e.g., testosterone nasal gel) or by transcutaneous patch (e.g., testosterone transdermal patch) ³¹.

A small amount of circulating testosterone is converted to estradiol (E2), a form of estrogen. As men age, they often make less testosterone, and so they produce less estradiol as well. Thus, changes often attributed to testosterone deficiency might be partly or entirely due to the accompanying decline in estradiol.

Testosterone is now widely prescribed to men whose bodies naturally produce low levels. But the levels at which testosterone deficiency become medically relevant still aren’t well understood. Normal testosterone production varies widely in men, so it’s difficult to know what levels have medical significance. Testosterone hormone’s mechanisms of action are also unclear.

The amount of testosterone synthesized is regulated by the hypothalamic–pituitary–testicular axis (Figure 1) ³². The brain and pituitary gland, a small gland at the base of the brain, control production of testosterone by the testes. In males, testosterone is synthesized primarily in the testes (testicles) by the Leydig cells. The number of Leydig cells in turn is regulated by luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In addition, the amount of testosterone produced by existing Leydig cells is under the control of LH, which regulates the expression of 17β-hydroxysteroid dehydrogenase ³³. When testosterone levels are low, gonadotropin-releasing hormone (GnRH) is released by the hypothalamus, which in turn stimulates the pituitary gland to release FSH (follicle-stimulating hormone) and LH (luteinizing hormone). These latter two hormones stimulate the testis to synthesize testosterone. Finally, increasing levels of testosterone through a negative feedback loop act on the hypothalamus and pituitary to inhibit the release of GnRH and FSH/LH, respectively (Figure 1).

Prior to puberty, there is no sex difference in circulating testosterone concentrations or athletic performance, but from puberty onward a clear sex difference in athletic performance emerges as circulating testosterone concentrations rise in men because testes (testicles) produce 30 times more testosterone than before puberty with circulating testosterone exceeding 15-fold that of women at any age ³⁴. There is a wide sex difference in circulating testosterone concentrations and a reproducible dose-response relationship between circulating testosterone and muscle mass and strength as well as circulating hemoglobin in both men and women. These dichotomies largely account for the sex differences in muscle mass and strength and circulating hemoglobin levels that result in at least an 8% to 12% performance-enhancing advantage in men.

Testosterone is the major sex hormone in males and testosterone makes a man look and feel like a man. In a man, testosterone hormone helps ³⁵:

• The development of the penis and testes (testicles)
• The deepening of the voice during puberty
• The appearance of facial and pubic hair starting at puberty; later in life, it may play a role in balding (androgenic alopecia)
• Promote gains in muscle mass and strength when combined with resistance training ³⁶.
• Bone growth and strength
• Growth spurts in puberty
• Sex drive (libido) and erections
• Sperm production (spermatogenesis)
• Fat distribution
• Make red blood cells
• Maintain normal mood and boost energy

There may be other important functions of testosterone hormone that have not yet been discovered.

Testosterone in the blood can be either bound or free:

• Bound testosterone is attached to proteins such as sex-hormone-binding-globulin (SHBG) and albumin. Most testosterone (approximately 60–70%) is transported in the blood to target tissues bound to sex hormone-binding globulin (SHBG), which in men is also called testosterone-binding globulin. About 30–40% of testosterone is loosely bound to albumin. This majority supply of protein-bound testosterone acts as a surplus of testosterone hormone for the body ³⁷.
• Free testosterone, the active form, is all the remaining testosterone that is not bound to other substances. The small amounts of free testosterone (approximately 0.5–2%) in the blood act at the level of the tissues, primarily the seminal vesicles, bone, muscle, and prostate gland ³⁷.

Historically, only free testosterone was thought to be the biologically active component. However, testosterone is weakly bound to serum albumin and dissociates freely in the capillary bed, thereby becoming readily available for tissue uptake. All non-SHBG-bound testosterone is therefore considered bioavailable.

Decreased testosterone levels indicate partial or complete hypogonadism. Hypogonadism is a clinical syndrome (group of symptoms) that is characterized by low serum testosterone levels and the presence of symptoms including low libido, fatigue, fewer or diminished spontaneous erections, decreased nighttime penile tumescence, sparse beard growth, and shrinking testes ³, ³⁸, ³⁹, ⁴⁰. The incidence of hypogonadism increases with age and affects approximately 4-5 million men in the United States ⁴¹.

Some men with low testosterone do not have any symptoms. Early symptoms (changes you feel) and signs (abnormalities that your doctor finds) of low testosterone in men include ⁴²:

• A drop in sex drive. It’s normal to feel less interested in sex as you get older. But, it is not usually normal to have no interest in sex.
• Problems having an erection
• Low sperm count
• Enlarged or tender breasts

Later, low testosterone can lead to decreased muscle size and strength, bone loss, less energy, sleep problems such as insomnia, lower fertility, increase in body fat, depression and trouble concentrating. Some things can temporarily lower testosterone, for instance, too much exercise, poor nutrition, or serious illness. Living a healthy lifestyle with regular exercise and a good diet helps maintain normal testosterone levels.

Factors that may decrease production of testosterone can occur with:

• Aging: Testosterone levels gradually reduce as men age, by 1%–2% per year after the age of 30 -40 years ²⁷, ⁴³. This effect is sometimes referred to as andropause or late-onset hypogonadism (LOH) ⁴⁴. However, on average serum levels of testosterone still remain within the normal range of young men ⁴⁵. Furthermore, decreases in testosterone concentrations with age are gradual and vary between individuals, with higher rates of decline in men with adiposity and comorbid diseases ⁴⁶. The age-dependent reference range for testosterone has not been defined and the criteria on cutoff levels for hypogonadism remain somewhat controversial ⁴³.
• Exercise: Resistance training increases testosterone levels ⁴⁷, however, in older men, that increase can be avoided by protein ingestion ⁴⁸. Chronic stress from too much exercise (overtraining syndrome) and endurance training in men may lead to lower testosterone levels ⁴⁹.
• Nutrients:
  • Vitamin A deficiency may lead to sub-optimal plasma testosterone levels ⁵⁰.
  • Vitamin D supplementation might increase testosterone levels ⁵¹. However, results from preliminary randomized controlled trials are conflicting. Vitamin D treatment had no effect on testosterone levels in middle-aged healthy men with normal baseline testosterone ⁵². In male patients with advanced heart failure and low vitamin D, a daily vitamin D3 supplement of 4000 IU for 3 years did not prevent the decline in testosterone indices ⁵³.
  • Zinc deficiency lowers testosterone levels ⁵⁴, but over-supplementation has no effect on serum testosterone ⁵⁵.
• Diets: There is limited evidence that low-fat diets may reduce total and free testosterone levels in men ⁵⁶ and moderate evidence that very high protein diets (≥35% protein) decrease total testosterone levels in men (∼5.23 nmol/L) ⁵⁷.
• Weight loss: Reduction in weight may result in an increase in testosterone levels, as well as sex-hormone binding globulin (SHBG) ⁵⁸, ⁵⁹, ⁶⁰. However, it’s unclear whether the improvement in semen quality is a result of the reduction in body weight per se or improved lifestyles remains unknown ⁵⁸. Fat cells synthesize the enzyme aromatase, which converts testosterone, the male sex hormone, into estradiol, the female sex hormone. A 2021 meta-analysis indicated that overweight and/or obesity were associated with lower sperm quality (i.e., semen volume, sperm count and concentration, sperm vitality, total motility and normal morphology), and underweight was likewise associated with low sperm normal morphology ⁶¹. In conclusion, these results suggest that maintaining a healthy body weight is important for increasing sperm quality parameters and potentially male fertility.
• Too much body fat (obesity). Ask your doctor whether you need a test called free testosterone.
• Sleep: REM (rapid-eye movement) sleep increases nocturnal testosterone levels ⁶². Peak testosterone levels coincide with rapid-eye movement (REM) sleep onset. The decreasing sleep efficiency and numbers of REM sleep episodes with altered REM sleep latency observed in older men are associated with lower concentrations of circulating testosterone ⁶².
• Behavior: Dominance challenges can, in some cases, stimulate increased testosterone release in men ⁶³.
• Antiandrogens: Natural or man-made antiandrogens including spearmint tea reduce testosterone levels ⁶⁴. Licorice can decrease the production of testosterone and this effect is greater in females ⁶⁵.
• Medicine side effects, such as from chemotherapy.
• Diseases of the testicles (trauma, cancer, infection, immune, iron overload and undescended testicle [cryptorchidism])
• Problems with glands in the brain (hypothalamus and pituitary) that control hormone production. The cause is either primary or secondary/tertiary (pituitary/hypothalamic) testicular failure.
  
  • Primary testicular failure is associated with increased luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels, and decreased total, bioavailable, and free testosterone levels. Causes include:
    • Genetic causes (eg, Klinefelter syndrome, XXY males)
    • Developmental causes (eg, testicular maldescent)
    • Testicular trauma or ischemia (eg, testicular torsion, surgical mishap during hernia operations)
    • Infections (eg, mumps)
    • Autoimmune diseases (eg, autoimmune polyglandular endocrine failure)
    • Metabolic disorders (eg, hemochromatosis, liver failure)
    • Orchidectomy
  • Secondary/tertiary hypogonadism, also known as hypogonadotrophic hypogonadism, shows low testosterone and low, or inappropriately “normal,” LH/FSH levels; causes include:
    • Inherited or developmental disorders of hypothalamus and pituitary (eg, Kallmann syndrome, congenital hypopituitarism)
    • Pituitary or hypothalamic tumors
    • Hyperprolactinemia of any cause
    • Malnutrition or excessive exercise
    • Cranial irradiation
    • Head trauma
    • Medical or recreational drugs (eg, estrogens, gonadotropin releasing hormone [GnRH] analogs, cannabis)
• Low thyroid function.
• Genetic disorders such as Klinefelter syndrome and Kallmann syndrome. Klinefelter syndrome is the most common congenital abnormality that results in primary hypogonadism. In Klinefelter syndrome, there is dysgenesis of seminiferous tubules and loss of Sertoli cells which leads to a decrease in inhibin levels and a resultant increase in follicle-stimulating hormone (FSH). FSH upregulates aromatase leading to increased conversion of androgens to estrogens. In Klinefelter, there is also Leydig cell dysfunction which leads to decreased testosterone levels and an increase in luteinizing hormone (LH) due to loss of negative feedback. In Kallmann syndrome, failed migration of gonadotropin-releasing hormone (GnRH)-producing neurons leads to lack of GnRH (gonadotropin-releasing hormone). No GnRH results in a decrease in luteinizing hormone (LH), follicle-stimulating hormone (FSH), testosterone, and sperm count. Specific to Kallmann syndrome, in comparison to other causes of hypogonadotropic hypogonadism, is defects in the sensation of smell (hyposmia or anosmia) ⁹.
• Other disorders, chronic diseases, treatments, or infection.

Increased total testosterone levels may be due to:

• Resistance to the action of male hormones (androgen resistance)
• Tumor of the ovaries
• Adrenal tumors
• Cancer of the testes
• Congenital adrenal hyperplasia (CAH). In classic CAH (95% of cases), due to 21 hydroxylase deficiency, newborns usually present with ambiguous genitalia and later develop salt wasting, vomiting, hypotension, and acidosis. A marked increase in 17-hydroxyprogesterone is diverted towards adrenal androgen synthesis and leads to hyperandrogenism. Hyperandrogenism impairs hypothalamic sensitivity to progesterone leading to a rapid rise in GnRH synthesis and thus increased LH and FSH. Elevations in LH and FSH lead to increased gonadal steroid production (17-hydroxyprogesterone, dehydroepiandrosterone [DHEA], testosterone, LH, and FSH). Diagnosis is with adrenocorticotropic hormone stimulation test showing exaggerated 17 hydroxyprogesterone response ⁶⁶.
• Taking medicines or drugs that increase testosterone level (including some supplements)

During childhood, excessive production of testosterone induces premature puberty (puberty too early before age 9) in boys and masculinization in girls ⁴². Some rare conditions, such as certain types of tumors, cause boys to make testosterone earlier than normal. Young boys also can have too much testosterone if they touch testosterone gel that an adult man is using for treatment.

In females, the ovaries produce most of the testosterone. The adrenal glands can also produce too much of other androgens that are converted to testosterone. In adult women, excess testosterone production results in varying degrees of virilization, including hirsutism, acne, oligo-amenorrhea, or infertility. Levels are most often checked to evaluate signs of higher testosterone levels, such as:

• Acne, oily skin
• Change in voice
• Decreased breast size
• Excess hair growth (dark, coarse hairs in the area of the moustache, beard, sideburns, chest, buttocks, inner thighs)
• Increased size of the clitoris
• Irregular or absent menstrual periods
• Male-pattern baldness or hair thinning.

Mild-to-moderate testosterone elevations are usually asymptomatic in males but can cause distressing symptoms in females. The exact causes for mild-to-moderate elevations in testosterone often remain obscure. Common causes of pronounced elevations of testosterone include genetic conditions (eg, congenital adrenal hyperplasia); adrenal, testicular, and ovarian tumors; polycystic ovarian syndrome (PCOS) and abuse of testosterone or gonadotrophins by athletes. Testosterone-producing ovarian or adrenal neoplasms often produce total testosterone values greater than 200 ng/dL. High estrogen values also may be observed, and LH and FSH are low or “normal.” In polycystic ovarian syndrome (PCOS), hirsutism, acne, menstrual disturbances, insulin resistance and, frequently, obesity, form part of this syndrome. Total testosterone levels may be normal or mildly elevated and, uncommonly, greater than 200 ng/dL.

Figure 1. Hypothalamic–pituitary–gonadal axis

Footnotes: In puberty, the hypothalamic-pituitary-gonadal axis plays a major role in regulating testosterone levels and gonadal function. Gonadotropin-releasing hormone (GnRH) is secreted from the hypothalamus by GnRH-expressing neurons. The GnRH travels down the hypothalamohypophyseal portal system to the anterior pituitary, which secretes luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH are two gonadotropic hormones that travel through the blood and act on receptors in the gonads. The largest amounts of testosterone (>95%) are produced by the testes in men, while the adrenal glands account for most of the remainder. In the testes, testosterone is produced by the Leydig cells ⁶⁷. The male testes also contain Sertoli cells, which require testosterone for spermatogenesis (sperm cell development). Luteinizing hormone (LH), in particular, acts on the Leydig cells to increase testosterone production. Testosterone limits its own secretion via negative feedback. High levels of testosterone in the blood feedback to the hypothalamus to suppress the secretion of GnRH and also feedback to the anterior pituitary, making it less responsive to GnRH stimuli ⁶⁸. Throughout the reproductive life of males, the hypothalamus releases GnRH in pulses every 1 to 3 hours. Despite this pulsatile release, however, average plasma levels of FSH and LH remain fairly constant from the start of puberty, where levels spike, to the third decade of life, where levels peak and slowly begin to decline. Prior to puberty, testosterone levels are low, reflecting the low secretion of GnRH and gonadotropins. Changes in neuronal input to the hypothalamus and brain activity during puberty cause a dramatic rise in GnRH secretion.

Testosterone production

Testosterone is synthesized from cholesterol in the Leydig cells in the testis with further metabolism to dihydrotestosterone (DHT). The alternative or “backdoor” pathway shows dihydrotestosterone (DHT) production without going through testosterone. 5-alpha reductase is an enzyme that converts testosterone to dihydrotestosterone (DHT). Testosterone production in the fetal human testis starts during the 6th week of pregnancy. Leydig cell differentiation and the initial early testosterone biosynthesis in the fetal testis are independent of luteinizing hormone (LH) ⁶⁹, ⁷⁰, ⁷¹. During testis development production of testosterone comes under the control of luteinizing hormone (LH) which is produced by the pituitary gland. Synthesis and release of luteinizing hormone (LH) is under control of the hypothalamus through gonadotropin-releasing hormone (GnRH) and inhibited by testosterone via a negative feedback mechanism (Figure 1) ⁷². The biosynthetic conversion of cholesterol to testosterone involves several discrete steps, of which the first one includes the transfer of cholesterol from the outer to the inner mitochondrial membrane by the steroidogenic acute regulatory protein (Star) and the subsequent side chain cleavage of cholesterol by the enzyme P450scc ⁷³. This conversion, resulting in the synthesis of pregnenolone, is the rate-limiting step in testosterone biosynthesis. Subsequent steps require several enzymes including, 3β-hydroxysteroid dehydrogenase, 17α-hydroxylase/C17-20-lyase and 17β-hydroxysteroid dehydrogenase type 3 (Figure 2) ⁷⁴.

Metabolism of testosterone to dihydrotestosterone (DHT) occurs through the classical pathway (Figures 2 and 3) and is essential for initiation of the differentiation and development of the urogenital sinus into the prostate ². Differentiation of male external genitalia (penis, scrotum and urethra) also strongly depends on the conversion of testosterone to dihydrotestosterone (DHT) in the urogenital tubercle, labioscrotal swellings, and urogenital folds ⁷⁵. In recent research there has been considerable interest in the alternative or ‘backdoor’ pathway of DHT production. This pathway has been found to have a significant role in the normal masculinization of the male fetus and abnormal virilization of the female fetus in cases of congenital adrenal hyperplasia resulting from mutations in the enzyme P450 oxidoreductase ⁷⁶.

The irreversible conversion of testosterone to dihydrotestosterone (DHT) is catalyzed by the microsomal enzyme 5-alpha reductase and is NADPH dependent ⁷⁷. The cDNA of the gene for 5-alpha reductase codes for a protein of 254 amino acid residues with a predicted molecular mass of 28,398 Dalton ⁷⁸, ⁷⁹. Male patients with 5-alpha reductase deficiency present with normal female or male genitalia or ambiguous genitalia at birth due to lack of dihydrotestosterone (DHT). These patients have a male internal urogenital tract (anti-Mullerian hormone is still present). At puberty, adolescents with 5-alpha reductase deficiency, who may have been raised as girls due to lack of secondary male characteristics, begin to develop male secondary sex characteristics and have primary amenorrhea. These patients will have normal testosterone and LH, low dihydrotestosterone (DHT), and an increased testosterone-to-DHT ratio. In contrast to 5-alpha reductase deficiency, androgen insensitivity is a condition in which patients lack functional androgen receptors resulting in under-virilization. These patients, like those with 5-alpha reductase deficiency, have a 46 XY karyotype. In contrast, however, these patients have normal female external genitalia and usually undescended testes. In adolescence, they experience primary amenorrhea and breast development but have no pubic or axillary hair and lack the deepening voice changes that occur with puberty. They will have a blind vaginal pouch and abnormal internal reproductive organs (fallopian tubes, uterus, and the upper portion of the vagina) due to the production of the Mullerian inhibiting factor. These patients will have high levels of testosterone and LH ⁸⁰.

Impaired testosterone metabolism can occur in certain cases of congenital adrenal hyperplasia (CAH). In classic CAH (95% of cases), due to 21 hydroxylase deficiency, newborns usually present with ambiguous genitalia and later develop salt wasting, vomiting, hypotension, and acidosis. A marked increase in 17-hydroxyprogesterone is diverted towards adrenal androgen synthesis and leads to hyperandrogenism. Hyperandrogenism impairs hypothalamic sensitivity to progesterone leading to a rapid rise in GnRH synthesis and thus increased LH and FSH. Elevations in LH and FSH lead to increased gonadal steroid production (17-hydroxyprogesterone, dehydroepiandrosterone [DHEA], testosterone, LH, and FSH). Diagnosis is with adrenocorticotropic hormone stimulation test showing exaggerated 17 hydroxyprogesterone response ⁶⁶.

Figure 2. Testosterone biosynthetic pathways

Footnotes: Biosynthetic pathways for testosterone and dihydrotestosterone (DHT) synthesis. The classic pathway show testosterone synthesized from cholesterol with further metabolism to dihydrotestosterone (DHT). The alternative or “backdoor” pathway shows DHT production without going through testosterone. Note only some of the enzymes are shown for clarity.

[Source ² ]

Figure 3. Enzymes involved in the biosynthesis of the adrenal steroid hormones and the gonadal steroid hormones

Footnotes: Figure 3 illustrates all of the enzymes involved in the biosynthesis of the adrenal steroid hormones, corticosterone, cortisol, and aldosterone; and the gonadal steroid hormones, progesterone, estradiol, and testosterone. Leydig cells in the testes function to turn cholesterol into testosterone ⁸¹. Luteinizing hormone (LH) regulates the initial step in this process. Two important intermediates in this process are dehydroepiandrosterone (DHEA) and androstenedione. Androstenedione is converted to testosterone by the enzyme 17-beta-hydroxysteroid dehydrogenase. At the cellular level, testosterone gets converted to dihydrotestosterone (DHT) by the enzyme 5-alpha-reductase. Dihydrotestosterone (DHT) is an androgen, which means it is a hormone that triggers the development of male characteristics ⁸². About 10% of the testosterone in the bodies of both men and women is converted into dihydrotestosterone (DHT) in adults, with a much higher amount in puberty. This may be why it is so closely related to the triggering of puberty. The dihydrotestosterone hormone (DHT) is much more powerful than testosterone. Dihydrotestosterone initiates the start of puberty in boys. It causes the genitals to develop and can cause the growth of pubic and body hair. It also causes the prostate to grow during puberty and may work together with testosterone to begin the expression of sexual desires and behavior. Women also have dihydrotestosterone (DHT), but its role in their bodies is not as well known. Some research has shown that it can lead to pubic hair growth after puberty in girls. It may also play a role in determining when puberty will start for a girl. Both men and women also produce weak acting androgens in the zona reticularis of the adrenal cortex. These weak-acting androgens are known as dehydroepiandrosterone (DHEA and androstenedione. They bind to testosterone receptors with weaker affinity but can also be converted to testosterone in the peripheral tissues if produced at high amounts ⁸².

Testosterone function

Testosterone is the main male hormone that is responsible for regulating sex differentiation producing male sex characteristics with male primary sexual development, which includes testicular descent, spermatogenesis, enlargement of the penis and testes, and increasing libido ⁸¹. Testosterone’s effects are first seen in the fetus. During the first 6 weeks of development, the reproductive tissues of males and females are identical. At around week 7 in utero, the sex-related gene on the Y chromosome (SRY gene) initiates the development of the testicles. Sertoli cells from the testis cords (fetal testicles) eventually develop into seminiferous tubules. Sertoli cells produce a Mullerian-inhibiting substance (MIS), which leads to the regression of the Fallopian tubes, uterus, and upper segment of the vagina (Mullerian structures normally present in females). Fetal Leydig cells and endothelial cells migrate into the gonad and produce testosterone, which supports the differentiation of the Wolffian duct (mesonephric duct) structures that go on to become the male urogenital tract ⁸¹. Testosterone also gets converted to dihydrotestosterone (DHT) in the periphery and induces the formation of the prostate and male external genitalia. Testosterone is also responsible for testicular descent through the inguinal canal, which occurs in the last 2 months of fetal development. When an embryo lacks a Y chromosome and thus the SRY gene, ovaries develop. Fetal ovaries do not produce adequate amounts of testosterone, thus the Wolffian ducts do not develop. There is also an absence of Mullerian-inhibiting substance (MIS) in these individuals, leading to the development of the Mullerian ducts and female reproductive structures ⁸³.

The testes usually begin the descent into the scrotum around 7 months of gestation, where the testes begin secreting reasonable quantities of testosterone. If a male child is born with undescended but normal testes that do not descend by 4 to 6 months of age, administration of testosterone can help the testes descend through the inguinal canals ⁸⁴.

Testosterone is also involved in regulating secondary male characteristics, which are those responsible for masculinity. These secondary sex characteristics include male hair patterns, vocal changes, and voice deepening, anabolic effects, which include growth spurts in puberty (testosterone increases tissue growth at the epiphyseal plate early on and eventual closure of plate later in puberty) and skeletal muscle growth (testosterone stimulates protein synthesis). Testosterone also stimulates erythropoiesis, which results in a higher hematocrit in males versus females. Testosterone levels tend to drop with increasing age; because of this, men tend to experience a decrease in testicular size, a drop in libido, lower bone density, muscle mass decline, increased fat production, and decreased red blood cells production (erythropoiesis), which leads to possible anemia.

Normal testosterone levels

A total testosterone test measures both bound and free testosterone in a sample of blood. Total testosterone is the most commonly used test for diagnostic and monitoring purposes and levels are commonly reported in nanograms per deciliter of blood (ng/dL). Less often, a testosterone test may be performed for free testosterone, which is reported in picograms per deciliter of blood (pg/dL). Another less common test is for bioavailable testosterone, which is testosterone that can be used more readily by the body. Bioavailable testosterone includes all testosterone that is not bound to sex-hormone-binding-globulin (SHBG) , including free testosterone and albumin-bound testosterone. Bioavailable testosterone is also commonly reported in nanograms per deciliter of blood (ng/dL).

To measure your testosterone level, your doctor can order a blood test. The blood test should be done in the morning between 7:00 AM and 10:00 AM.

Normal testosterone levels

• Normal total testosterone levels in males: 300 to 1,000 nanograms per deciliter (ng/dL) or 10 to 35 nanomoles per liter (nmol/L). The harmonized normal serum total testosterone range in a healthy nonobese population of European and American men, 19 to 39 years, is 264 to 916 ng/dL ⁸⁵. Furthermore, in randomized testosterone trials, the men with testosterone levels below the harmonized threshold would be more likely to respond to testosterone therapy than those with testosterone levels above the harmonized threshold.
  • 0-5 months: 75-400 ng/dL
  • 6 months-9 years: <7-20 ng/dL
  • 10-11 years: <7-130 ng/dL
  • 12-13 years: <7-800 ng/dL
  • 14 years: <7-1,200 ng/dL
  • 15-16 years: 100-1,200 ng/dL
  • 17-18 years: 300-1,200 ng/dL
  • Over 19 years: 240-950 ng/dL
• Normal total testosterone levels in females: 15 to 70 ng/dL or 0.5 to 2.4 nmol/L
  • 0-5 months: 20-80 ng/dL
  • 6 months to 9 years: <7-20 ng/dL
  • 10-11 years: <7-44 ng/dL
  • 12-16 years: <7-75 ng/dL
  • 17-18 years: 20-75 ng/dL
  • Over 19 years: 8-60 ng/dL
• Normal free testosterone levels in males:
  • 20-25 years: 5.25-20.7 ng/dL
  • 25-30 years: 5.05-19.8 ng/dL
  • 30-35 years: 4.85-19.0 ng/dL
  • 35-40 years: 4.65-18.1 ng/dL
  • 40-45 years: 4.46-17.1 ng/dL
  • 45-50 years: 4.26-16.4 ng/dL
  • 50-55 years: 4.06-15.6 ng/dL
  • 55-60 years: 3.87-14.7 ng/dL
  • 60-65 years: 3.67-13.9 ng/dL
  • 65-70 years: 3.47-13.0 ng/dL
  • 70-75 years: 3.28-12.2 ng/dL
  • 75-80 years: 3.08-11.3 ng/dL
  • 80-85 years: 2.88-10.5 ng/dL
  • 85-90 years: 2.69-9.61 ng/dL
  • 90-95 years: 2.49-8.76 ng/dL
  • 95-100+ years: 2.29-7.91 ng/dL
• Normal free testosterone levels in females:
  • 20-25 years: 0.06-1.08 ng/dL
  • 25-30 years: 0.06-1.06 ng/dL
  • 30-35 years: 0.06-1.03 ng/dL
  • 35-40 years: 0.06-1.00 ng/dL
  • 40-45 years: 0.06-0.98 ng/dL
  • 45-50 years: 0.06-0.95 ng/dL
  • 50-55 years: 0.06-0.92 ng/dL
  • 55-60 years: 0.06-0.90 ng/dL
  • 60-65 years: 0.06-0.87 ng/dL
  • 65-70 years: 0.06-0.84 ng/dL
  • 70-75 years: 0.06-0.82 ng/dL
  • 75-80 years: 0.06-0.79 ng/dL
  • 80-85 years: 0.06-0.76 ng/dL
  • 85-90 years: 0.06-0.73 ng/dL
  • 90-95 years: 0.06-0.71 ng/dL
  • 95-100+ years: 0.06-0.68 ng/dL

Talk to your doctor about the meaning of your specific test results. Testosterone levels change from hour to hour. Testosterone levels tend to be highest in the morning and lowest at night. Testosterone levels are highest by age 20 to 30 and slowly go down after age 30 to 35. In addition, normal testosterone level ranges may vary slightly among different laboratories. Some labs use different measurements or test different specimens.

If the result is not normal, you should repeat the test to make sure of the result. In healthy men, testosterone levels can change a lot from day to day, so a second test could be normal.

Low testosterone / testosterone deficiency

Testosterone deficiency also known as male hypogonadism, is defined as having one or more symptoms (e.g. decreased libido, decreased erections, decreased energy, decreased physical stamina, decreased lean muscle mass) attributable to low circulating levels of testosterone (morning serum total testosterone less than 300 ng/dL on two separate occasions) ¹², ¹⁰, ⁸⁶. An estimated 25% of men have low levels of testosterone ⁸⁷. Besides being fundamental for the development and maintenance of male characteristics and male sexual organs, testosterone also has effects on most major organs, such as the brain, muscle, kidney, bone, liver and skin ⁸⁸. Recent studies have also suggested that testosterone deficiency is independently and robustly associated with various obesity-related chronic diseases in men, including type 2 diabetes, hypertension, cardiovascular disease, hyperlipidemia, asthma, chronic obstructive pulmonary disease (COPD) ⁸⁹, ⁹⁰. However, there is uncertainty as to what constitutes optimal physiological levels of total testosterone among men across different age categories, and to the effects that varying total testosterone levels have on disease risk ⁹¹. This debate is partially fueled by inconsistent findings in the literature and differences across expert clinical recommendations, as well as an incomplete understanding of the underlying mechanisms and temporal sequence of events leading to and stemming from testosterone deficiency. For example, the authors of a well-known systematic review ⁹² assert that although testosterone deficiency is robustly associated with obesity, insulin resistance, low high-density lipoprotein (HDL) cholesterol and elevated triglycerides, low-density lipoprotein (LDL) cholesterol, and plasminogen activator type 1, there is insufficient evidence to demonstrate a causal role of endogenous testosterone deficiency on coronary artery disease-the leading cause of preventable death among men in the U.S ⁹³.

Incidence of hypogonadism (symptomatic low testosterone) increases with advancing male age such that levels of circulating testosterone decline annually by approximately 1% from the age of 40 years according to the European Male Ageing Study ⁴⁶. When hypogonadism occurs in an older man, the condition is often called andropause or androgen deficiency of the aging male or late onset hypogonadism (LOH) ⁹⁴. The most easily recognized clinical signs of late onset hypogonadism (andropause) in older men are a decrease in muscle mass and strength, a decrease in bone mass and osteoporosis, decreased libido, erectile dysfunction, anemia, increase in central body fat, and impaired memory, mood, concentration, and sleep ⁹⁵, as well as increased mortality ⁹⁶. Sexual symptoms and fatigue are the earliest and most common presentations ⁹⁷. Other symptoms include depression, sleep alterations, poor concentration, and metabolic disorders are seen at borderline testosterone levels ⁹⁸, ⁹⁹. However, symptoms such as a decrease in libido and sexual desire, forgetfulness, loss of memory, anemia, difficulty in concentration, insomnia, and a decreased sense of well-being are more difficult to measure and differentiate from hormone-independent aging ¹⁰⁰. Androgen deficiency of the aging male may result in significant detriment to quality of life and adversely affect the function of multiple organ systems ¹⁰¹.

The pathophysiology of male hypogonadism can be characterized into two types ¹⁰²:

1. Primary hypogonadism also known as primary testicular failure or hypergonadotropic hypogonadism. Primary hypogonadism is a testosterone deficiency due to a testicular abnormality impairing testosterone production; for example, Klinefelter syndrome, testicular tumors, infection, trauma, impaired Leydig cell function, or those caused by certain medications ¹⁰³.
2. Secondary hypogonadism also known as hypogonadotropic hypogonadism. Secondary hypogonadism is a testosterone deficiency due to a problem in the hypothalamus or the pituitary gland, parts of the brain that signal the testicles to produce testosterone (i.e., luteinizing hormone (LH) and follicle-stimulating hormone (FSH)); for example, hypothalamic or pituitary lesions, GnRH deficiency, or hyperprolactinemia ¹⁰³. Another important cause of hypogonadism is late-onset hypogonadism, an age-related (i.e., >40 years) decline in testosterone accompanied by clinical symptoms that is often associated with obesity, diabetes, and other chronic health conditions ¹⁰⁴.

Risk factors for hypogonadism include:

• HIV/AIDS
• Previous chemotherapy or radiation therapy
• Aging
• Obesity
• Malnutrition

Late-onset hypogonadism (andropause) has the combined features of both primary and secondary hypogonadism ¹⁰⁵. The diagnosis of andropause (late-onset hypogonadism) requires the presence of characteristic signs and symptoms, in combination with decreased serum total testosterone. Based on the recent guidelines by the International Society for the Study of Aging Male (ISSAM), the European Association of Urology (EAU), the European Society of Endocrinology (ESE), the European Academy of Andrology (EAA), and the American Association of Urology (AUA), a total testosterone of 250–350 ng/dL is the proper threshold value to define low testosterone. The optimal indication for TRT in late-onset hypogonadism (andropause) is the presence of signs and symptoms of hypogonadism, and low testosterone without contraindications for TRT ¹⁰⁶.

Testosterone deficiency signs and symptoms

Testosterone deficiency signs and symptoms may include ¹⁰⁶:

• Physical
  • Anemia
  • Reduced energy
    Reduced endurance
  • Diminished work performance
  • Diminished physical performance
  • Fatigue
  • Reduced lean muscle mass
  • Obesity
  • Loss of body hair
  • Reduced beard growth
  • Development of breast tissue (gynecomastia)
  • Loss of bone mass (osteoporosis)
  • Hot flashes
• Cognitive
  • Depressive symptoms
  • Cognitive dysfunction
  • Reduced motivation
  • Poor concentration
  • Poor memory
  • Irritability

• Sexual
  • Reduced sex drive
  • Reduced erectile function

Symptoms of hypogonadism may be categorized as sexual or non-sexual. Sexual symptoms include erectile dysfunction, diminished frequency of morning erections, and decrease in sexual thoughts (low libido), as well as difficulty in achieving orgasm and reduced intensity of orgasm ¹⁰⁷. Non-sexual symptoms include fatigue, impotence, impaired concentration, depression, and decreased sense of vitality and/or well-being. Signs of hypogonadism also include anemia, osteopenia and osteoporosis, abdominal obesity, and metabolic syndrome ¹⁰⁸.

Figure 4. Testosterone deficiency signs and symptoms

[Source ⁸⁸ ]

Testosterone deficiency causes

Testosterone deficiency also known as male hypogonadism, is defined as having one or more symptoms (e.g. decreased libido, decreased erections, decreased energy, decreased physical stamina, decreased lean muscle mass) attributable to low circulating levels of testosterone (morning serum total testosterone less than 300 ng/dL on two separate occasions) ¹², ¹⁰, ⁸⁶.

There are two basic types of hypogonadism:

1. Primary. This type of hypogonadism — also known as primary testicular failure — originates from a problem in the testicles.
2. Secondary. This type of hypogonadism indicates a problem in the hypothalamus or the pituitary gland — parts of the brain that signal the testicles to produce testosterone. The hypothalamus produces gonadotropin-releasing hormone, which signals the pituitary gland to make follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Luteinizing hormone then signals the testes to produce testosterone.

Either type of hypogonadism can be caused by an inherited (congenital) trait or something that happens later in life (acquired), such as an injury or an infection. At times, primary and secondary hypogonadism occur together.

Primary hypogonadism

Common causes of primary hypogonadism include:

• Klinefelter syndrome also known as 47,XXY or XXY male. Klinefelter syndrome results from a congenital abnormality of the sex chromosomes, X and Y. A male normally has one X and one Y chromosome. In Klinefelter syndrome, two or more X chromosomes are present in addition to one Y chromosome. The Y chromosome contains the genetic material that determines the sex of a child and related development. The extra X chromosome typically affects physical, neurodevelopmental, behavioral, and neurocognitive functioning. Common physical features may include tall stature, reduced muscle tone, abnormal development of the testicles (hypogonadism), which in turn results in underproduction of testosterone with delayed pubertal development and lack of secondary male sex characteristics such as decreased facial and body hair. Increased breast growth (gynecomastia) may occur later in puberty without appropriate biological care.
• Undescended testicles (cryptorchidism). Before birth, the testicles develop inside the abdomen and normally move down into their permanent place in the scrotum. Sometimes one or both of the testicles aren’t descended at birth. This condition often corrects itself within the first few years of life without treatment. If not corrected in early childhood, it can lead to malfunction of the testicles and reduced production of testosterone.
• Mumps orchitis. A mumps infection involving the testicles that occurs during adolescence or adulthood can damage the testicles, affecting the function of the testicles and testosterone production.
• Hemochromatosis. Too much iron in the blood can cause testicular failure or pituitary gland dysfunction, affecting testosterone production.
• Injury to the testicles. Because they’re outside the abdomen, the testicles are prone to injury. Damage to both testicles can cause hypogonadism. Damage to one testicle might not impair total testosterone production.
• Cancer treatment. Chemotherapy or radiation therapy for the treatment of cancer can interfere with testosterone and sperm production. The effects of both treatments often are temporary, but permanent infertility may occur. Although many men regain their fertility within a few months after treatment, preserving sperm before starting cancer therapy is an option for men.

Secondary hypogonadism

In secondary hypogonadism, the testicles are normal but don’t function properly due to a problem with the pituitary or hypothalamus. A number of conditions can cause secondary hypogonadism, including:

• Kallmann’s syndrome. This is an abnormal development of the area of the brain that controls the secretion of pituitary hormones (hypothalamus). This abnormality can also affect the ability to smell (anosmia) and cause red-green color blindness.
• Pituitary disorders. An abnormality in the pituitary gland can impair the release of hormones from the pituitary gland to the testicles, affecting normal testosterone production. A pituitary tumor or other type of brain tumor located near the pituitary gland may cause testosterone or other hormone deficiencies. Also, treatment for a brain tumor, such as surgery or radiation therapy, can affect the pituitary gland and cause hypogonadism.
• Inflammatory disease. Certain inflammatory diseases, such as sarcoidosis, histiocytosis and tuberculosis, involve the hypothalamus and pituitary gland and can affect testosterone production.
• HIV/AIDS. HIV/AIDS can cause low levels of testosterone by affecting the hypothalamus, the pituitary and the testes.
• Medications. The use of certain drugs, such as opiate pain medications and some hormones, can affect testosterone production.
• Obesity. Being significantly overweight at any age might be linked to hypogonadism.
• Aging. As men age, there’s a slow, progressive decrease in testosterone production. The rate varies greatly.

Low testosterone diagnosis

Your doctor will conduct a physical examination and note whether your sexual development, the amount and distribution of body hair (including beard growth and pubic hair), muscle mass and size and consistency of your testes, is consistent with your age. The prostate should be examined in older patients for size, consistency, symmetry, and presence of nodules or induration; it should be noted that the prostate may be enlarged in older men, despite a low testosterone level ¹⁰⁹. Weight, height, body mass index (BMI), and waist circumference should also be measured, since signs and symptoms potentially indicative of testosterone deficiency in men include height loss, reduced muscle bulk and strength, and increased body fat, particularly increased BMI and abdominal fat accumulation ¹⁴.

Your doctor will test your blood level of testosterone if you have signs or symptoms of hypogonadism. Because testosterone levels vary and are generally highest in the morning, blood testing is usually done early in the day, before 10 a.m. (8 AM to 10 AM), possibly on more than one day. If both measurements are low, then certain studies should be ordered to rule out secondary hypogonadism. Further testing includes FSH (follicle-stimulating hormone), LH (luteinizing hormone), prolactin, TSH (thyroid-stimulating hormone), complete blood count (CBC), and comprehensive metabolic panel. In cases of low normal testosterone with clinical symptoms, further testing to assess free or bioavailable testosterone should be done. These tests include sex hormone binding globulin (SHBG) and albumin to calculate the bioavailable testosterone which can be affected by obesity, type 2 diabetes, hypothyroidism, and liver disease. Furthermore, semen analysis, pituitary MRI, testicular ultrasound and biopsy, and genetic studies can be ordered, if there is clinical suspicion of a secondary cause.

Testosterone in blood is mainly bound to serum proteins with only 2% of the hormone circulating as free testosterone. Sex hormone binding globulin (SHBG) accounts for 60-70% of testosterone binding, and 30-40% of the total testosterone is bound by albumin or other proteins. Bioavailable testosterone, referring to free and albumin-bound testosterone together, is thought to reflect an individual’s biologically active, circulating testosterone ¹¹⁰. Biochemical parameters commonly used to assess androgen deficiency include total testosterone (TT), free testosterone, calculated free testosterone, bioavailable testosterone (bT), and free androgen index ¹¹¹. Theoretically, bioavailable testosterone (bT) is the most accurate; however, a standard method to measure bioavailable testosterone (bT) has not been established ¹¹⁰. Practically, morning total testosterone is the most widely accepted parameter, but results have been found to be misleading when sex-hormone-binding-globulin (SHBG) is elevated. The diagnosis of low testosterone should be made only after two total testosterone measurements have been taken on separate occasions between 8 and 11 AM due to the circadian rhythm of testosterone production by the testicles ¹¹⁰. Free testosterone measurement is recommended when total testosterone measurement is not diagnostic. In individuals with clinically suspected testosterone deficiency, sex-hormone-binding-globulin (SHBG) levels should be assessed if total testosterone is low to normal or borderline, especially in obese or older men ¹¹². Calculation of free testosterone based on serum levels of testosterone and SHBG may be used to avoid inaccuracies in assessing the individual degree of androgenicity in a cost-effective manner ¹¹³. Since normal total testosterone ranges vary significantly between laboratories depending on the methods used and/or the assay kits employed, it is better to measure testosterone in the same laboratory ¹¹⁴.

Recently, other hormones are also being measured as per recommendations. In patients with low testosterone, serum luteinizing hormone (LH) should be measured to establish the cause of testosterone deficiency and may be an important factor in determining if adjunctive tests should be ordered ¹⁰². Serum prolactin levels should be measured in patients with low testosterone levels combined with low or low/normal LH levels to screen for hyperprolactinemia. Persistently elevated prolactin levels can indicate the presence of pituitary tumors, such as prolactinomas ¹¹⁵. Men with total testosterone levels <150 ng/dL in combination with a low or low/normal LH should undergo a pituitary MRI regardless of prolactin levels, as this may indicate non-secreting adenomas ¹¹⁶.

Table 1. Cutoff values of testosterone for low testosterone (testosterone deficiency) diagnosis

	Cutoff Values	Year of Release and Update
Expert opinion	Total testosterone or free testosterone below the lower limits of normal	Before Official Guideline
International Society for the Study of the Aging Male (ISSAM)	Total testosterone < 231 ng/dL (8 nmol/L) Total testosterone: 231–346 ng/dL (8–12 nmol/L) or free testosterone < 52 pg/mL	2005
	Total testosterone < 230 ng/dL (8 nmol/L) total testosterone: 230–350 ng/dL (8–12 nmol/L) or free testosterone < 52 pg/mL, sex-hormone-binding-globulin (SHBG)	2008
	Total testosterone < 350 ng/dL (12 nmol/L) or free testosterone < 65 pg/mL	2015
Endocrine Society	Total testosterone < 300 ng/dL or free testosterone < 5 ng/dL	2006
	Total testosterone < 280–300 ng/dL or free testosterone < 5–9 ng/dL	2010
	Total testosterone < 300 ng/dL or free testosterone < 5 ng/dL	2018
International Society for Sexual Medicine (ISSM)	Total testosterone < 350 ng/dL (12 nmol/L)	International Consultation for Sexual Medicine (ICSM) 2015
American Urological Association (AUA)	Total testosterone < 300 ng/dL	2018

[Source ¹¹⁰ ]

Low testosterone treatment

If you have low testosterone, man-made testosterone may help. This treatment is called testosterone replacement therapy or TRT. Testosterone replacement therapy can be given as a pill, gel, patch, injection, or implant.

TRT may relieve or improve symptoms in some men. It may help keep bones and muscles strong. Testosterone replacement therapy seems to be more effective in young men with very low testosterone levels. TRT can also be helpful for older men. Testosterone replacement therapy is FDA-approved as replacement therapy only for men who have low testosterone levels due to disorders of the testicles, pituitary gland, or brain that cause a condition called hypogonadism. Examples of these disorders include failure of the testicles to produce testosterone because of genetic problems, or damage from chemotherapy or infection ¹¹⁷, ¹¹⁸.

Testosterone replacement therapy (TRT) has risks. These may include ¹¹⁹:

• Infertility
• Enlarged prostate leading to difficulty urinating
• Blood clots ¹²⁰
• Erythrocytosis or polycythemia (increased production of red blood cells) ¹²¹, ¹²²
• Heart attacks
• Worsening heart failure
• Obstructive sleep apnea
• Cholesterol problems

At this time, it is unclear whether testosterone replacement therapy increases the risk of heart attack, stroke, or prostate cancer ¹¹⁹, ¹²³.

Talk with your doctor about whether TRT is right for you. If you do not notice any change in symptoms after treatment for 3 months, it is less likely that TRT treatment will benefit you.

If you decide to start testosterone replacement therapy, be sure to see your doctor for regular checkups.

Prior to commencing TRT, all patients should undergo a baseline measurement of hemoglobin (Hb)/hematocrit (Hct). If the hematocrit (Hct) exceeds 50%, clinicians should consider withholding TRT until the cause of the high Hct is explained ¹²¹. Men with elevated Hct and on-treatment low/normal total and free testosterone levels should be referred to a hematologist for further evaluation, and possible coordination of phlebotomy. Testosterone has a stimulating effect on erythropoiesis, and elevation of Hb/Hct is the most frequent adverse event related to TRT ¹²². During TRT, levels of hemoglobin (Hb)/hematocrit (Hct) generally rise for the first six months, then tend to plateau ¹²⁴.

Prostate-specific antigen (PSA) should be measured in men over 40 years of age prior to commencement of TRT to exclude a prostate cancer diagnosis ¹²⁵. PSA secretion is an androgen-dependent phenomenon, and the rise of PSA levels in patients on TRT is primarily dependent on baseline total testosterone levels. A previous study found that men with lower baseline testosterone levels were more likely to experience PSA level increases after TRT ¹²⁶. In that study, participants (N = 451) received 5–10 g of 1% Testosterone gel daily for 12 months. Patients were divided into two groups: A (n = 197 with total testosterone <250 ng/dL) and B (n = 254 with total testosterone ≥250 ng/dL). In Group A, but not Group B, baseline PSA levels correlated significantly with total testosterone levels. At the end of follow-up, PSA increased significantly in Group A (21.9% change; 0.19 ± 0.61 ng/mL), but not in Group B (14.1% change; 0.28 ± 1.18 ng/mL), with the greatest PSA change observed after one month of treatment ¹²⁶.

The effects of TRT on cardio vascular disease (CVD) remain a point of concern. Ferrucci et al. ¹²⁷ showed that low testosterone levels had an independent influence on the development of anemia in older adults. Testosterone stimulates the production of erythropoietin-responsive cells and burst-forming units in the bone marrow, which boosts iron absorption and erythropoiesis ¹²⁸. The effects of testosterone on the bone marrow affecting the hematopoietic growth factors and iron absorption show the association between testosterone and erythropoiesis. In an earlier study ¹²⁹, data demonstrated that subjects with low total testosterone and free testosterone levels had low Hb and Hct levels. This result suggests that low total testosterone and free testosterone may play a significant role in erythropoiesis. According to a recent study ¹³⁰, TRT in older men with low testosterone levels significantly increased hemoglobin (Hb) levels of those with unexplained anemia, as well as those with anemia from known causes. Measurement of testosterone levels might be considered in men 65 years or older who have unexplained anemia and symptoms of low testosterone levels. Although men with hypogonadism were not always anemic in our previous study, the association between low testosterone and low hemoglobin (Hb) levels was statistically significant ¹²⁹. Furthermore, our previous study noted that the prevalence of anemia decreased, and patients with anemia showed increased erythropoietin after TRT ¹³¹. TRT may be effective in men with hypogonadism to reduce the incidence of anemia and the cardiovascular disease associated with anemia Saad, F., Doros, G., Haider, K. S., & Haider, A. (2018). Hypogonadal men with moderate-to-severe lower urinary tract symptoms have a more severe cardiometabolic risk profile and benefit more from testosterone therapy than men with mild lower urinary tract symptoms. Investigative and clinical urology, 59(6), 399–409. https://doi.org/10.4111/icu.2018.59.6.399.

Testosterone replacement therapy

Testosterone replacement therapy (TRT) is an important treatment option for men with low testosterone and symptomatic hypogonadism. Male hypogonadism also known as testosterone deficiency, is defined as having one or more symptoms attributable to low circulating levels of testosterone (serum total testosterone <300 ng/dL) ¹², ¹⁰. Although controversy remains regarding indications for testosterone replacement therapy (TRT) in aging men due to lack of large-scale, long-term studies assessing the benefits and risks of testosterone-replacement therapy in men, reports indicate that TRT has been shown to increase serum testosterone to physiologic levels, which may produce a wide range of benefits for men with hypogonadism that include improve libido, improve erectile dysfunction, improve overall sexual function, increase bone mineral density, decrease body fat mass, and increase lean body muscle mass, increase energy, improve mood, erythropoiesis, cognition, quality of life and cardiovascular disease ¹³². The TIMES2 (Testosterone replacement In hypogonadal men with either MEtabolic Syndrome or type 2 diabetes) study ¹³³ showed that testosterone replacement therapy (TRT) improved glycemic control, lipid levels, sexual function and libido in men with type 2 diabetes and/or metabolic syndrome, with no corresponding increase in adverse events.

The most controversial area for TRT is the issue of risk, especially possible stimulation of prostate cancer by testosterone, even though no evidence to support this risk exists. A 2021 study ¹³⁴ concluded that compared to TRT-untreated patients, TRT-treated patients may not have increased risks for disease progression in prostate cancer. However, the quality of currently available evidence is extremely poor. TRT may be harmful in men with advanced prostate cancer, in those with untreated prostate cancer undergoing active surveillance, and in those with successfully treated prostate cancer but having high-risk disease ¹³⁴. The United States Food and Drug Administration (FDA) stated, in all testosterone package inserts, that TRT is contraindicated in men with known or suspected prostate cancer, but it did not substantiate this contraindication ¹³⁵. The clinical guidelines by Endocrine Society recommend against treating hypogonadism in men with prostate cancer, citing lack of sufficient data to make a general recommendation in men previously treated for prostate cancer (‘recommendation with low quality evidence’) ¹⁴. Meanwhile, the European Association of Urology stated that there is no conclusive evidence that TRT increases the risk of prostate cancer (‘level of evidence=4’), and that men with prostate cancer can receive TRT with careful monitoring for prostate safety (‘level of evidence=3’) ¹³⁶. Similarly, the recent treatment guideline by the American Urologic Association stated that patients should be informed that there is inadequate evidence for TRT (‘expert opinion’), but TRT can be considered in men who have undergone radical prostatectomy with favorable pathology (e.g., negative margins, negative seminal vesicles, negative lymph nodes), without PSA recurrence ¹².

Currently, there are a variety of widely available testosterone formulations, including topical gels and patches, intramuscular injections, subcutaneous pellets, and oral/buccal formulations that provide clinicians and male patients the opportunity to personalize replacement therapy to reestablish testosterone levels and improve testosterone deficiency symptoms ¹³⁷:

1. Exogenous Testosterone Replacement Therapy
   • Mechanism of action: Restore testosterone levels by supplementing various preparations of natural testosterone
   • Types: Oral, buccal, intramuscular, transdermal, subdermal, and nasal preparations
     • Gel. There are several gels and solutions available, with different ways of applying them. Depending on the brand, you rub the testosterone into your skin on your upper arm or shoulder (AndroGel, Testim, Vogelxo) or apply it to the front and inner thigh (Fortesta). Your body absorbs testosterone through your skin. Don’t shower or bathe for several hours after a gel application, to be sure it gets absorbed. Side effects include skin irritation and the possibility of transferring the medication to another person. Avoid skin-to-skin contact until the gel is completely dry, or cover the area after an application.
     • Injection. Testosterone cypionate (Depo-Testosterone) and testosterone enanthate are given in a muscle or under the skin. Your symptoms might waver between doses depending on the frequency of injections. You or a family member can learn to give testosterone injections at home. If you’re uncomfortable giving yourself injections, member of your care team can give the injections. Testosterone undecanoate (Aveed) is given by deep intramuscular injection, typically every 10 weeks. It must be given at your provider’s office and can have serious side effects.
     • Patch. A patch containing testosterone (Androderm) is applied each night to your thighs or torso. A possible side effect is severe skin reaction.
     • Gum and cheek (buccal cavity). A small putty-like substance, gum-and-cheek testosterone replacement delivers testosterone through the natural depression above your top teeth where your gum meets your upper lip (buccal cavity). This product, taken three times a day, sticks to your gumline and allows testosterone to be absorbed into your bloodstream. It can cause gum irritation.
     • Nasal. This testosterone gel (Natesto) can be pumped into the nostrils. This option reduces the risk that medication will be transferred to another person through skin contact. Nasal-delivered testosterone must be applied twice in each nostril, three times daily, which might be more inconvenient than other delivery methods.
     • Implantable pellets. Testosterone-containing pellets (Testopel) are surgically implanted under the skin every three to six months. This requires an incision.
   • Advantages:
     • Multiple routes of administration depending on patient preference and needs
     • Improves low testosterone levels and maintains virilization in hypogonadal men
     • Particularly useful for treating hypergonadotropic hypogonadism in which testosterone levels are low but LH levels are high
   • Limitations:
     • Because it mimics functions of endogenous testosterone, it also exerts negative feedback on the hypothalamus and pituitary gland, reducing LH and decreasing endogenous intratesticular testosterone production
     • Decreases intratesticular testosterone, impairs spermatogenesis, and results in male infertility
2. Therapies that Increase Endogenous Testosterone
   • Mechanism of action: Multiple mechanisms of action by affecting various points along the hypothalamic-pituitary-gonadal axis to increase
   • Types: Selective estrogen receptor modulators (SERMs), Gonadotropins, Aromatase inhibitors, Selective androgen receptor modulators (SARMs), Leydig stem cell transplantation
   • Advantages:
     • Able to increase serum and intratesticular testosterone levels
     • Greater likelihood of preserving hypothalamic-pituitary-gonadal axis axis function and, subsequently, spermatogenesis and fertility
   • Limitations:
     • Potential unintended decline in quality of semen and complications related to changes in estrogen levels
     • None have been approved by the US FDA specifically for men with hypogonadism or infertility.

Table 2. Testosterone replacement therapy formulations

Names	Strengths	Limitations	Comparative Efficacy	References
Testosterone undecanoate	Bypasses first-pass liver metabolism due to aliphatic side chain and is absorbed directly by the lymphatic system and then by blood	Requires large and frequent dosing to mimic physiologic testosterone levels Serum concentration difficult to predict due to high intra- and inter-individual variability Risk of increased blood pressure and cardiac events	A small but significant increase in systolic blood pressure versus topical testosterone (gel or patch), A greater number of gastrointestinal-associated side effects compared to topical testosterone 138	Nieschlag 2015 139, Swerdloff et al. 2020 140
Striant (testosterone buccal)	Requires only 2x/day dosing to result in stable serum testosterone concentrations and mimics physiologic circadian rhythm release Medication effects can easily be reversed due to easy access of tablets in the oral cavity	Side effects of local irritation, inflammation, or gingivitis in some patients	Superior to testosterone patch (Androderm) 141 Equal to testosterone gel (AndroGel) 142	Tsametis & Isidori 2018 143, Dinsmore & Wyllie 2012 144
Long acting testosterone: Testosterone enanthate, Testosterone cypionate Extra-long acting testosterone: Testosterone undecanoate	More lipophilic than other formulations allowing for extended storage, gradual release, and prolonged presence of testosterone in the blood Requires fewer administrations 2-4x/month (long acting) or 1-2x/2 months (extra-long acting) Easy access Cost effective	Pain associated with deep intramuscular administration Fluctuations in energy, mood, and libido in many patients Risk of pulmonary oil microembolism and anaphylaxis with extra-long acting formulation	Equal to testosterone gel (AndroGel) 145	Nieschlag 2015 139, Tsametis & Isidori 2018 143, Middleton et al. 2015 146
Testosterone Gel: AndroGel®, Testim®, and Fortesta® Testosterone Solution: Axiron® Testosterone Patch: Androderm®	Mimics physiologic diurnal release Short duration of action, allowing for quick discontinuation User friendly	Requires daily administration High cost Side effects of skin irritation and potential contact transfer to another person Less effective in obese individuals	AndroGel equal to Androderm 147 AndroGel equal to Depo Testosterone 145 AndroGel equal to Delatestryl (Testosterone Enanthate) 145 AndroGel Equal to Striant (testosterone buccal) 142 Testim superior to Androderm 148 Testim superior to AndroGel 149	Tsametis & Isidori 2018 143
Testosterone implant: Testopel®	Only requires implantation of pellets every 4-6 months No risk of contact transference	Necessity for surgical procedure to implant sub dermally Risk of pellet extrusion, infection, and fibrosis		Tsametis & Isidori 2018 143
Testosterone nasal: Natesto®	Decreased risk of transference due to inconspicuous site of application Non-invasive and user-friendly Increases testosterone, while maintaining FSH, LH, and semen parameters	Side effects of runny nose (rhinorrhea), nose bleed (epistaxis), nasopharyngitis, sinusitis, and nasal scabbing	Maintenance of spermatogenesis in >95% of men, in contrast to injections or gels. Short duration of action, frequency of administration and clinical pharmacokinetic profile compared to long-acting testosterone injections, gels and pellets 150.	Tsametis & Isidori 2018 143, Gronski et al. 2019 151, Rogol et al. 2018 152

Oral testosterone

Currently, the most commonly available oral preparation is testosterone undecanoate ¹³⁷. This preparation is able to avoid first passing through the liver due to its aliphatic side chain which allows it to be absorbed by the lymph system and directly enter the circulatory system ¹⁵³. This formulation requires more frequent and larger doses in order to effectively replete testosterone therapy ¹⁵⁴. Resent study reported that although oral testosterone undecanoate is a safe and effective means to treat hypogonadal men, it was related to a small but significant increased systolic blood pressure ¹⁴⁰. While it has shown efficacy, pharmacokinetic analysis has proven that it is difficult to predict absorption due to high intra- and inter-individual variability in serum concentrations. Oral Testosterone undecanoate also includes a black box warning regarding blood pressure increases and risk of cardiac events, which makes its use a major concern for older men. An additional group of oral agents exist that have been available for decades; these are 17-alpha alkylated androgens (methyltestosterone and fluoxymesterone). However, many reports have implicated these oral preparations in cholestatic jaundice, a hepatic cystic disease called peliosis hepatis, and hepatoma ¹³⁸. Therefore, oral testosterone undecanoate are no longer suggested for use due to the discovery of extreme liver toxicity and minimal effectiveness in resolving hypogonadal symptoms.

Buccal testosterone

Currently, the most commonly available buccal preparation is a buccal tablet (Striant®) which exists as a potential therapeutic option for patients who can tolerate it ¹³⁷. The tablets are mucoadhesive and are applied to a depression in the gum above the upper incisors, slowly releasing testosterone into the circulatory system. Twice daily applications of 30 mg tablets result in stable serum levels of testosterone. The benefits of this method are that the effects can quickly be reversed and the release of medication mimics circadian rhythms ¹⁵⁵. However, it has also been shown that 3-15% of patients may experience irritation, inflammation, or gingivitis while also having difficulty from complaints due to the unique method of delivery ¹⁴³. These potential side effects should be taken into consideration when opting for this method of TRT.

Testosterone intramuscular injections

Intramuscular injection of testosterone has proven to be effective in initiating and maintaining virilization in all hypogonadal men ¹⁵⁵. Current options include long-acting (testosterone enanthate and testosterone cypionate) and extra-long-acting (testosterone undecanoate) variants. The long-acting options are esters of testosterone which make the molecule more lipophilic than the natural variant. This results in their storage and gradual release, thereby prolonging the presence of testosterone in the blood ¹⁵⁶. Administration of long-acting intramuscular testosterone usually occurs as 50–100 mg every week, 100–200 mg every 2 weeks, or 250 mg every 3 weeks and has shown consistent results ¹⁵⁵. The advantages of long-acting options are that they are extremely effective, easily accessible, and cost effective, and they free patients from daily administration. Conversely, the disadvantages are the need for patients to be comfortable with deep intramuscular administration of the testosterone solution and fluctuations in energy, mood, and libido in many patients.

An extra-long-acting option exists with testosterone undecanoate in oil, which is also one of the common oral options. When injected alongside castor oil, a half-life of 34 days is observed ¹⁵³. Each dose is 750 mg in 3mL of oil. The recommendations for administration in the United States are to administer a dose into the buttocks, followed by a second dose 4 weeks later, and then subsequent doses every 10 weeks. Access to this option is regulated by a restricted program called the AVEED Risk Evaluation and Mitigation Strategy (REMS) Program and is not recommended consistently. Although the injectable preparation is generally considered safe, patients must be monitored for pulmonary oil micro-embolism (POME) ¹⁵⁷. Symptoms of POME (pulmonary oil micro-embolism), such as sudden onset of non-productive cough associated with faintness following injections, have been observed in 1.5% of patients ¹⁵⁸. Additionally, anaphylactic responses have been observed in some patients, requiring 30-min post-dose monitoring to rule out this side effect ¹⁵⁵. The advantages of less frequent administration come at the cost of various systemic side effects that require regular monitoring.

Transdermal testosterone

Transdermal testosterone preparations are known to provide very stable and effective improvements in serum testosterone levels due to their pharmacokinetic profile which mimics physiological diurnal variations. Transdermal testosterone preparations include preparations that incorporate solutions, gels, and patches. Three testosterone gels (AndroGel®, Testim®, and Fortesta®), one solution (Axiron®), and one patch (Androderm®) are available in the United States ¹⁵⁵. Transdermal testosterone substitution has the following advantages: a) since they have a short duration of action, the preparations can be quickly discontinued if any adverse effect occurs and the testosterone levels will then quickly decline, and b) they are easy to use and easily accessible to many. Conversely, the disadvantages include the need for daily administration, relatively high cost, skin irritation, and the risk of contact transfer to another person ¹⁴⁴. The high risk of unintentional transfer led the FDA to require testosterone gel products to include a Blackbox warning. They have also been shown to be less effective in obese individuals and require higher doses ¹⁵⁵. However, due to the ease of use, accessibility, and reversibility, these are often a first-line choice for patients who require testosterone substitution.

Subdermal testosterone

Subdermal pellet implants (Testopel®) are a viable option for testosterone replacement and result in stable testosterone levels. The pellets are implanted into the subdermal fat of the buttocks, lower abdominal wall, or thigh with a tunneling technique using a local anesthetic ¹⁵⁹. It is recommended that three to six 75 mg pellets should be implanted every 4-6 months (150 mg-450 mg) to achieve stable testosterone levels in the normal range ¹⁵⁵. The benefits of this method include guaranteed compliance and lack of transference to persons who may come in contact with it ¹⁵⁵. Adverse effects include pellet extrusion, infection, and fibrosis ¹⁶⁰. Due to the need for a surgical procedure for implantation and the potential side effects, this form of TRT is usually not recommended as first-line treatment ¹⁴⁶.

Nasal testosterone

A nasal testosterone gel, known as Natesto®, is now available in the United States for treatment of male hypogonadism. Studies have shown successful treatment with this new method and have exhibited consistent improvements in testosterone deficiency ¹⁶¹. This gel is administered in the nostrils via a metered dose pump which delivers 5.5mg per actuation. The recommended dosage is to administer one actuation in each nostril three times daily, resulting in a daily dosage of 33 mg/day. This method has shown a reduction in gel transfer to a partner or child due to the inconspicuous application site, and it provides ease of use as a non-invasive option. Most importantly, preliminary studies have demonstrated that Natesto® increases serum testosterone while also maintaining FSH, LH, and semen parameters ¹⁶². Natesto nasal testosterone gel can maintain spermatogenesis in >95% of men, in contrast to injections or gels ¹⁵⁰. Also, significant differences have been observed between duration of action, frequency of administration and clinical pharmacokinetic profile of Natesto compared to long-acting testosterone injections, gels and pellets ¹⁵². On the other hand, more than 3% of men have reported experiencing rhinorrhea, epistaxis, nasopharyngitis, sinusitis, and nasal scab as a result of this medication ¹⁵⁵. Unique among the formulations of exogenous TRT, nasal formulations of testosterone are showing promise in alleviating symptoms of low testosterone without adversely affecting the hypothalamic-pituitary-gonadal (HPG) axis.

Table 3. Therapies that increase endogenous testosterone

Class	Mechanism of Action	Strengths	Limitations	References
Selective Estrogen Receptor Modulators (SERMs)	Mixed estrogen agonist/antagonist properties in different tissues In the pituitary gland, acts as an antagonist, thus stimulating the release of LH and FSH, which drive both testosterone production and spermatogenesis	Successful in treatment of azoospermia, oligospermia, male hypogonadism, and unexplained male infertility	Increased risk of thromboembolic events and carcinogenesis	El Meliegy et al. 2018 163, Chehab et al. 2015 164
Gonadotropins	Mimics effects of endogenous gonadotropins and is particularly useful in the treatment of hypogonadotropic hypogonadism Like endogenous gonadotropins, stimulates testosterone production and spermatogenesis	Successful in treatment of hypogonadal men, while maintaining semen parameters, intratesticular testosterone, and fertility Useful for preserving spermatogenesis in men undergoing exogenous TRT by maintaining intratesticular testosterone levels	Side effects of gynecomastia, headache, fatigue, and mood changes	Lee & Ramasamy 2018 165, El Meliegy et al. 2018 163, Dohle et al. 2016 166
Aromatase Inhibitors	Blocks aromatase conversion of testosterone to estradiol, thereby increasing testosterone levels and normalizing the testosterone/estradiol ratio	Useful in treating infertility in obese men with hypogonadotropic hypogonadism	Potential increase in PSA level in elderly men with low or borderline hypogonadism Limited long-term data to assess efficacy and adverse effects	Crosnoe et al. 2013 167, El Meliegy et al. 2018 163, Ribeiro et al. 2016 168
Selective Androgen Receptor Modulators (SARMs)	Selective agonist of tissue-specific androgen receptors, mimicking the effect of testosterone in certain tissues	Potential use in the treatment of male hypogonadism and infertility	Currently in investigational phase; further studies are required to assess safety and efficacy	Narayanan et al. 2018 169, Solomon et al. 2018 170
Leydig Stem Cell Transplantation	Leydig stem cells can differentiate into adult Leydig cells, which produce intratesticular testosterone under stimulation of LH	Successful studies showing increased testosterone production in various animal models	Further studies are required to prove effectiveness and safety in humans	Zang et al. 2017 171, Beattie et al. 2015 172, Arora et al. 2019 173

Selective estrogen receptor modulators

Selective estrogen receptor modulators (SERMs) are a group of pharmaceuticals that act as competitive inhibitors of estrogen receptors in the hypothalamus and pituitary with the increasing release of GnRH and gonadotrophins, subsequently increasing production of intra-testicular testosterone and improving spermatogenesis ¹⁷⁴. Clomiphene citrate is a SERM that has been used in the treatment of men with azoospermia, oligozoospermia, and unexplained infertility, in addition to treating male hypogonadism. It has been demonstrated that Clomiphene citrate leads to a moderate increase in FSH, LH, total testosterone, and the concentration of sperm in these men ¹⁷⁵. Also, in men with late-onset hypogonadism, long-term Clomiphene citrate therapy can be prescribed as a testosterone replacement ¹⁷⁴. Although thromboembolic events and carcinogenesis have been reported as adverse effects of SERMs ¹⁷⁶, current ongoing research shows that this group of pharmaceuticals has great potential for future applications ¹⁷⁷.

Gonadotropins

Gonadotropin therapy is used to manage infertile patients with low testosterone concentrations, with the aim of recovering normal sperm production. In such patients, stimulating spermatogenesis requires human chorionic gonadotropin (hCG) therapy alone or in combination with human menopausal gonadotrophins (hMGs), urinary FSH, or recombinant FSH ¹⁷⁴. It has been recommended that TRT with concomitant use of hCG can be prescribed in hypogonadal men who seek fertility treatment ¹⁷⁸. Treatment with hCG can preserve normal sperm production in men undergoing TRT by maintaining intratesticular testosterone concentrations ¹⁷⁹. Potential side effects of gonadotropin therapy include gynecomastia, headache, fatigue, and mood changes ¹⁸⁰. A retrospective study found that treatment of hypogonadal men with TRT and co-administration of low dose hCG maintains semen parameters in these men ¹⁸¹. Coviello et al. ¹⁸² evaluated intratesticular testosterone concentrations from men with normal reproductive physiology who received testosterone enanthate in combination with hCG (125, 250, or 500 IU). A linear increase in intratesticular testosterone concentrations with increased hCG confirms that moderately low dose hCG preserves testicular function in endogenous gonadotropin-suppressed healthy men ¹⁸². A systematic review showed that hypogonadal men desiring fertility preservation while benefiting from TRT can be prescribed SERMs or testosterone plus a low dose of hCG. Prescription of hCG alone or in combination with FSH preparations can be recommended to hypogonadotropic hypogonadal patients who need to be treated for infertility ¹⁷⁴. In younger men with low serum testosterone levels, treatment with hCG or with Clomiphene citrate will increase serum and intra-testicular testosterone levels without resulting in declining gonadotropin levels via feedback inhibition ¹⁶⁷. Overall, human chorionic gonadotropin (hCG) is an efficient and safe alternative or adjunct to TRT in men desiring both fertility and treatment for symptoms of hypogonadism. However, a drawback of hCG is that this is an injectable agent.

Aromatase inhibitors

Aromatase inhibitors are a class of drugs that act by blocking aromatase P450 enzymes, consequently normalizing the testosterone/E2 ratio and improving spermatogenesis ¹⁶³. It has been shown that an aromatase inhibitor (Testolactone) is an effective drug for alleviating infertility in hypogonadotropic hypogonadal obese men ¹⁶⁴. However, Testolactone is no longer commercially available. Some men with severe spermatogenic failure who have too much aromatase activity, decreased serum levels of testosterone, increased E2 levels, or impaired testosterone/E2 ratio might benefit from the use of aromatase inhibitors in treating infertility ¹⁷⁴. Although no important side effects have been observed with consumption of an aromatase inhibitor for elevating serum testosterone concentrations (46), one study has reported a slight insignificant increase in the level of serum prostate-specific antigen (PSA) in elderly men with low or borderline hypogonadism ¹⁸³. However, the use of aromatase inhibitors is not suggested due to the absence of long-term data and a limited amount of published literature ¹⁸⁴.

Selective androgen receptor modulators

The pathological effects arising from testosterone has resulted in the search for tissue-selective agonists of the androgen receptor which could potentially trigger the androgen receptor in a tissue-selective manner ¹⁶⁹. Selective androgen receptor modulators (SARMs) are a new class of androgen receptor ligands that bind to androgen receptors, depending on the chemical structure of each SARM. Consequently, SARMs lead to anabolic activity while avoiding most of the side effects caused by anabolic steroids ¹⁷⁰. The tissue specificity of SARMs has led to the notion that they may be a promising treatment for a wide variety of diseases ¹⁷⁰. SARMs have been studied in the treatment of male hypogonadism and their use is being investigated in clinical trials (6, 60). Miner et al. (2007), in a rat model of sexual behavior, showed that SARMs can lead to increased sexual function. In this model treatment of male rats with 100 mg/kg of a synthetic SARM, LGD226, increased number of mounts, intromissions, and ejaculation ¹⁶⁶. Jones et al. ¹⁸⁵ demonstrated that administration of S-23 with estradiol benzoate in male rats led to inhibition in spermatogenesis and the suppression of FSH and LH levels. However, these effects are completely reversible. Chen et al. ¹⁸⁶ reported that C-6, a novel SARM, can mimic in vivo endocrine effects of testosterone. Administration of C-6 significantly inhibited spermatogenesis and decreased testosterone concentration, epididymal and testicular size (63). Another study showed that SARM-2f, a novel SARM compound, can restore the sexual behavior in castrated male rats ¹⁸⁷. Although SARMs represent an attractive therapeutic approach, further studies are required for the safe use of these medications ¹⁷⁰.

Leydig stem cell transplantation

Adult Leydig cells found in the interstitial space of the testis, are the primary source of testosterone production in males ¹⁸⁸. Adult Leydig cells undergo a four-stage process of differentiation, beginning with undifferentiated mesenchymal cells (also known as Leydig stem cells [LSCs]) and progressing to progenitor Leydig cells, immature Leydig cells, and finally to adult Leydig cells ¹⁸⁹. Leydig stem cells do not have steroidogenic capabilities; however, they are unique in that they are capable of replicating indefinitely and differentiating to functioning adult Leydig cells ¹⁸⁹. This suggests the possibility of having an endless pool of progenitor cells capable of replacing dysfunctional adult Leydig cells and restoring testosterone production in men with low testosterone ¹⁹⁰. Arora et al. ¹⁷³ showed that Leydig stem cells in combination with Sertoli and myoid cells underwent self-renewal as well as differentiation into mature Leydig cells. Adult Leydig cells act in the hypothalamic-pituitary-gonadal axis: an increase in LH stimulates the first step of testosterone synthesis in adult Leydig cells; conversely, an increase in testosterone production by adult Leydig cells decreases LH production by the pituitary ¹⁹¹. Dysfunction at any point along the hypothalamic-pituitary-gonadal axis axis can lead to testosterone deficiency ¹⁷¹.

Beattie et al. ¹⁷² demonstrated increased testosterone production in rat models by directly stimulating aged adult Leydig cells with translocator protein, a ligand involved in transporting cholesterol from the cytosol to mitochondria. This process was independent of LH regulation and treated aged adult Leydig cells had equivalent testosterone production to young adult Leydig cells. Other studies have shown great promise in increasing testosterone production at more upstream regulation points. Jiang et al. ¹⁹² showed that transplantation of nestin-positive Leydig stem cells into the testis of mice with impaired adult Leydig cells caused differentiation into functional adult Leydig cells and an increase in testosterone production. Zhang et al. ¹⁴¹ identified that transplanting human p75+ Leydig stem cells into EDS-treated rats (inducing ALC dysfunction) successfully replaced the impaired adult Leydig cells and partially restored testosterone production. Arora et al. ¹⁷³ demonstrated a novel form of Leydig stem cell transplantation by autografting Leydig stem cells in combination with Sertoli cells and peritubular myoid cells into the subcutaneous, rather than the testicular, tissue of hypogonadal mouse models. This also resulted in differentiation of Leydig stem cells into adult Leydig cells and increased testosterone production ¹⁷³.

Testosterone replacement therapy contraindications

Contraindications to testosterone replacement therapy include ¹⁹³:

• History of breast cancer
• Prostate cancer
• Uncontrolled heart failure
• Myocardial infarction or cerebrovascular accident within the past 6 months
• Untreated obstructive sleep apnea
• Hematocrit over 48%
• Men planning fertility
• A palpable undiagnosed prostate nodule
• An elevated PSA above 4 ng/mL
• An elevated PSA level above 3vng/mL in high-risk patients including African Americans and men with a first-degree relative with prostate cancer.

Testosterone replacement therapy dosage

Adult Male Dose for Hypogonadism

Uses:

• Primary hypogonadism (congenital or acquired): Testicular failure due to conditions such as cryptorchidism, bilateral torsion, orchitis, vanishing testis syndrome, orchiectomy, Klinefelter Syndrome, chemotherapy, or toxic damage from alcohol or heavy metals. These men usually have low serum testosterone concentrations and gonadotropins (FSH, LH) above the normal range.
• Hypogonadotropic hypogonadism (congenital or acquired): Gonadotropin or luteinizing hormone-releasing hormone (LHRH) deficiency or pituitary-hypothalamic injury from tumors, trauma, or radiation. These men have low testosterone serum concentrations but have gonadotropins in the normal or low range.

IM INJECTION:

• Testosterone Undecanoate: 750 mg (3 mL) IM injection followed by 750 mg (3 mL) injected after 4 weeks, then 750 mg (3 mL) every 10 weeks thereafter
• Testosterone Enanthate and Cypionate: 50 to 400 mg IM injection every 2 to 4 weeks

IMPLANT:

• 2 to 6 pellets (75 mg each) implanted subcutaneously every 3 to 6 months.
• The number of pellets to be implanted depends upon the minimal daily requirements of testosterone propionate administered parenterally. Thus, implant two 75 mg pellets for each 25 mg testosterone propionate required weekly.

Comments:

• The chronological and skeletal ages must be taken into consideration, both in determining the initial dose and in adjusting the dose.
• This drug should be used only if the benefits outweigh the serious risks of pulmonary oil microembolism and anaphylaxis.
• Injections more frequently than every two weeks are not recommended.
• Adequate effect of the implants (pellets) continues for three to four months, sometimes as long as six months.

BUCCAL:

• Mucoadhesive Oral Patch: Apply a 30 mg patch to the gum region twice a day; morning and evening (about 12 hours apart).

TOPICAL:

• Transdermal Film: 2 to 6 mg applied to the back, abdomen, upper arm, or upper thigh once a day, preferably at night.
• Gel (in tubes, packets or spray): 5 g applied once a day, preferably in the morning. Consult the manufacturer product information for specific dosage and additional instructions of use.
• Transdermal Solution: Initial dose is 60 mg of testosterone (1 pump actuation of 30 mg of testosterone to each axilla), applied once a day, at the same time each morning. Consult the manufacturer product information for specific dosage and additional instructions of use.

Comments: Prior to initiating therapy with this drug, confirm the diagnosis of hypogonadism by ensuring that serum testosterone concentrations have been measured in the morning on at least two separate days, and that these serum testosterone concentrations are below the normal range.

Pediatric Male Dose for Delayed Puberty

Use: To stimulate puberty in selected males with clearly delayed puberty

IM INJECTION:

• Testosterone Enanthate: 50 to 200 mg every 2 to 4 weeks for 4 to 6 months

IMPLANT:

• 2 pellets (each pellet contain 75 mg of testosterone) implanted subcutaneously every 3 to 6 months
• Duration of therapy: 4 to 6 months

Comments:

• The chronological and skeletal ages should be taken into consideration when determining the initial dose and when adjusting the dose.
• An X-ray of the hand and wrist to determine bone age should be obtained every six months to assess the effect of treatment on the epiphyseal centers.
• Report frequent or persistent erections.
• Androgen therapy should be used very cautiously in children and only by specialists who are aware of the adverse effects on bone maturation.

Renal Dose Adjustments

• Testosterone Cypionate IM injection: Contraindicated in renal disease

Liver Dose Adjustments

• Testosterone Cypionate IM injection: Contraindicated in liver disease

Dose Adjustments

• The doses of this drug should be adjusted according to the patient’s response and the appearance of adverse reactions.

Testosterone replacement therapy monitoring

Lab tests needed before starting androgen replacement include hemoglobin, hematocrit (red blood cell count), liver function tests, lipid panel, digital rectal exam, PSA level, 2-morning testosterone levels, and consider a DEXA scan.

Monitoring should be done as follows ¹⁹³:

• One month after treatment: morning testosterone level
• Three to 6 months after treatment during 1 year: morning testosterone level, liver function tests, lipid profile, PSA, digital rectal exam, estradiol, hemoglobin, and hematocrit, blood pressure
• Annually after 1 year: morning testosterone level, liver function tests, lipid profile, digital rectal exam, PSA, estradiol, Hgb, and hematocrit (red blood cell count), blood pressure

Referral to urology is recommended if there is an increase in PSA level greater than 1.4 ng/mL within any 12-month period. If hematocrit rises above 54%, then stop therapy as soon as possible ¹⁹³. It is important to look out for signs of sleep apnea on annual follow-up visits. DEXA scans need to be repeated 1 to 2 years after initiating therapy in hypogonadal men with osteoporosis ¹⁹⁴. Hyperestrogenism can be a side effect of replacement therapy because testosterone can be aromatized to estrogen ¹⁹⁵. Aromatase inhibitors may need to be prescribed. Therefore, estradiol levels in men need to be assessed to rule out hyperestrogenism. Physicians need to regularly monitor patients receiving testosterone therapy and should discontinue therapy in those who fail to follow up.

Testosterone replacement therapy risks

Testosterone replacement therapy has various risks. For example, testosterone therapy may:

• Elevate red blood cell count (erythrocytosis), thereby increasing the risk of blood clots or venous thromboembolism ¹⁹⁶
• Cause acne or other skin reactions
• Contribute to sleep apnea — a potentially serious sleep disorder in which breathing repeatedly stops and starts
• Stimulate noncancerous growth of the prostate (benign prostatic hyperplasia) and growth of existing prostate cancer
• Enlarge breasts
• Limit sperm production or cause testicle shrinkage
• Increase the risk of a blood clot forming in a deep vein (deep vein thrombosis), which could break loose, travel through your bloodstream and lodge in your lungs, blocking blood flow (pulmonary embolism)

In addition, testosterone therapy may impact your risk of heart disease. Research has had conflicting results, so the exact risk isn’t clear yet. Most notable are  the TOM (Testosterone in Older Men) trial and the TEAAM (Testosterone’s Effects on Atherosclerosis Progression in Aging Men) trials. Low testosterone levels have been associated with increased risk of coronary artery disease ¹⁹⁷. Published in JAMA 2017, a study ¹⁹⁸ found that testosterone replacement was associated with a lower risk of cardiovascular outcomes. In 2015, the FDA concluded a possible increased cardiovascular risk associated with testosterone use requiring labeling change to inform the public. The FDA requires that you are made aware that the possibility of cardiovascular events may exist during treatment.

The American Association of Clinical Endocrinologists ¹⁹⁹ issued a guideline in response to the 2015 FDA labeling requirement on cardiovascular risk and stated that there is no compelling evidence that testosterone therapy increases cardiovascular risk. On the other hand, testosterone deficiency has been associated with falls, sarcopenia, frailty, osteopenia, and osteoporosis ²⁰⁰.

Serum PSA levels can increase in response to testosterone treatment, so it is important to rule out prostate cancer before starting therapy as it can worsen the disease process. Patients on replacement therapy need to be reevaluated for prostate cancer at 3 months and 1 year after beginning treatment ²⁰¹.

There is no firm scientific evidence that long-term testosterone replacement is associated with either prostate cancer or cardiovascular events. Prostate cells are stimulated by testosterone, so be extra vigilant about cancer screenings. African American men over age 45 — especially those with family history of cancer — are already at risk for prostate cancer.

Testosterone booster

Testosterone boosters also known as “T-Boosters”, “natural steroids”, “pro-hormones” or “designer steroids”, are supplementary substances that are often taken with the hopes of increasing endogenous testosterone production and doing this in a more “natural” manner, presumably by activation of the hypothalamo-pituitary-gonadal (HPG) axis ²⁰². Testosterone boosters are used by many athletes worldwide to achieve a significant muscle mass increase within a short period of time ²⁰³. There are a number of naturally occurring substances that can boost testosterone levels in the body. Foods containing such substances are known as testosterone-foods; and they tend to be rich in vitamins, antioxidants, and minerals like zinc, which plays a key role in testosterone production ²⁰⁴, ²⁰⁵. Testosterone boosters are not regulated by the US Food and Drug Administration (FDA) and lack a standardized composition, being composed of a myriad of ingredients ²⁰⁶. Many of these compounds have been used in ancient Indian Ayurvedic and Unani medicine systems for their aphrodisiac or anti-aging properties ²⁰⁷. In addition, many “testosterone boosters” contain a proprietary blend of various ingredients that allegedly increase endogenous testosterone levels. In most cases there is little data, either on the complete product itself or the various active ingredients typically contained within a proprietary blend, to substantiate the manufacturers’ claims ²⁰⁴. Furthermore many testosterone boosters had supra-therapeutic doses of vitamins and minerals, occasionally over the tolerable upper intake level (the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects to almost all individuals in the general population) ²⁰⁸. Over the years, some consumers of testosterone boosters have complained of kidney and liver abnormalities that could be linked to their use of boosters ²⁰⁹. A 2017 case report ²¹⁰ described new-onset, bilateral pulmonary embolisms secondary to over-the-counter fenugreek-extract-containing testosterone booster. Another 2018 case report ²⁰² described abdominal pain caused by liver injury following the consumption of two courses of a commercial testosterone booster. According to researchers based in the US, about 13% of the annual cases of acute liver failure are attributable to idiosyncratic drug- and/or supplement-induced liver injury ²¹¹. In 2014, the FDA issued a general warning for the risk of venous blood clots associated with testosterone booster use ¹¹⁸.

In many cases, the studies that are available which are used to substantiate “testosterone boosters” product claims involve animal or cell culture models; therefore, the results may not be relevant to humans ²⁰⁴. Furthermore, these products usually have no data available, yet are still being marketed on the premise that increases in endogenous testosterone will result in increases in muscle mass, especially when ingested in conjunction with a resistance training program. Despite this variability, prominent ingredients in many testosterone boosters include horny goat weed, saw palmetto and fenugreek ²¹². An investigation has revealed that limited human studies have evaluated testosterone boosters, resulting in no definitive findings of efficacy ²¹². In the absence of additional human studies, patients should be cautioned before considering testosterone boosters, given the availability of highly effective Food and Drug Administration approved therapies (e.g., testosterone undecanoate or testosterone enanthate and cypionate).

Recommended Dietary Allowance (RDA or the average daily level of intake sufficient to meet the nutrient requirements of 97%–98% healthy individuals; often used to plan nutritionally adequate diets for individuals) information was not available for many of the individual components for the “testosterone boosters” ²⁰⁸. The FDA does not issue RDA (Recommended Dietary Allowance) and upper tolerable limit (UL) data for herbal supplements. Supplements contained a median 1,291% of the RDA for vitamin B12, 807.6% of vitamin B6, 272% of zinc, 200% of vitamin B5, and 187.5% of vitamin B3 (Table 6). It is also worrisome that two testosterone boosters had greater than the upper tolerable limit (UL) of zinc. Relatively high levels of zinc supplementation have been shown to lower copper status, altered iron function and to adversely affect high-density lipoprotein cholesterol concentrations ²¹³. Slightly higher zinc doses have been shown to result in anemia and neutropenia, as well as reduced immune function ²¹⁴.

In humans, testosterone is synthesized and released through an elaborate hormonally-orchestrated signaling cascade referred to as the hypothalamic-pituitary-gonadal axis (see Figure 1 above). In order for this axis to be up-regulated, D-aspartic acid (D-Asp), an endogenous amino acid present in nervous tissues and endocrine glands of humans ²¹⁵, plays an important neuromodulating role. In males, D-aspartic acid (D-Asp) endogenously synthesizes testosterone by first converting to N-methyl-D-aspartate (NMD-Asp) by D-aspartate methyltransferase (NMDA synthetase) ²¹⁶. In the hypothalamus, N-methyl-D-aspartate (NMD-Asp) binds to its receptor, a subtype of the L-glutamate receptor, and potentiates transmission via glutaminergic neurons ²¹⁷, which results in the release of gonadotropin-releasing hormone (GnRH) ²¹⁵. The release of GnRH from the hypothalamus then triggers the release of prolactin, follicle stimulating hormone (FSH), and luteinizing hormone (LH) from the anterior pituitary gland. The effect of the latter two hormones on the testes is that FSH stimulates spermatogenesis and LH stimulates testosterone synthesis ²¹⁵. In addition, D-aspartic acid (D-Asp)-induced neuromodulation induces aromatase activity, the enzyme responsible for the conversion of testosterone to 17-beta-estradiol ²¹⁸. In a 2017 systematic review showed that exogenous D-aspartic acid (D-Asp) enhances testosterone levels in male animal studies, whereas studies in human yielded inconsistent results ²¹⁹. Based on limited and contrary data in humans, the paucity and often low quality of primary studies, more high-quality randomized controlled trials are needed to be conducted on D-aspartic acid (D-Asp) supplementation regarding their ability to increase endogenous levels of testosterone ²¹⁹.

Table 4. Individual components for testosterone booster supplements and the percentage of surveyed supplements containing those components

Supplement	Supplements containing the component (n=45)
Zinc	29 (64.4%)
Fenugreek extract	22 (48.9%)
Vitamin B6	20 (44.4%)
Tribulus	19 (42.2%)
Magnesium	17 (37.8%)
Boron	12 (26.7%)
Diindolemethane	12 (26.7%)
Eurycoma longifolia extract	12 (26.7%)
Maca extract	10 (22.2%)
D-Aspartic acid	9 (20.0%)
Ashwagandha extract	8 (17.8%)
Vitamin D	8 (17.8%)
Black pepper fruit extract	7 (15.6%)
Mucuna purines	7 (15.6%)
Tongkat ali extract	7 (15.6%)
Vitamin B12	7 (15.6%)
L-Arginine	6 (13.3%)
Vitamin B3	6 (13.3%)
Vitamin B9	6 (13.3%)
Calcium	5 (11.1%)
Ginseng	5 (11.1%)
Nettle root extract	5 (11.1%)
Saw palmetto	5 (11.1%)
Selenium	5 (11.1%)
Shilajit	5 (11.1%)
Avena sativa extract	4 (8.9%)
Bioperine	4 (8.9%)
Damiana leaf	4 (8.9%)
Epimedium	4 (8.9%)
Ginkgo biloba	4 (8.9%)
Rhodiola rosea extract	4 (8.9%)
Grape extract	3 (6.7%)
Horney goat weed extract	3 (6.7%)
Safed musli extract	3 (6.7%)
Agaricus bisporus fruit extract	2 (4.4%)
Akarkara extract	2 (4.4%)
Alpha-Glycerolphosphorylcholine	2 (4.4%)
Astragalus root powder	2 (4.4%)
Broccoli	2 (4.4%)
Bulbine natalensis	2 (4.4%)
Caffeine	2 (4.4%)
Cnidium monnieri	2 (4.4%)
Coleus extract	2 (4.4%)
Dehydroepiandrosterone	2 (4.4%)
Green tea leaf extract	2 (4.4%)
L-Carnitine	2 (4.4%)
L-Citrulline	2 (4.4%)
Laxosterone	2 (4.4%)
Melatonin	2 (4.4%)
Resveratrol	2 (4.4%)
Rosemary extract	2 (4.4%)
Vitamin E	2 (4.4%)
Yohimbe extract	2 (4.4%)
Agmatine	1 (2.2%)
Aminoethanesulfonic acid	1 (2.2%)
Anacyclus pyrethrum	1 (2.2%)
Androsta	1 (2.2%)
Apigenin	1 (2.2%)
Apple extract	1 (2.2%)
Calcium fructoborate	1 (2.2%)
Calcium glucarate	1 (2.2%)
Chrysin	1 (2.2%)
Eleutherococcus senticosus	1 (2.2%)
Epicatechin	1 (2.2%)
Fadogia agrestis	1 (2.2%)
Fumarate	1 (2.2%)
GABA	1 (2.2%)
Garlic extract	1 (2.2%)
Glycyrrhiza glabra (licorice)	1 (2.2%)
Hesperidin	1 (2.2%)
HTP (5-HTP)	1 (2.2%)
Huperzia serrata extract	1 (2.2%)
Indole-3-carbinol	1 (2.2%)
Kava	1 (2.2%)
L-Histidine	1 (2.2%)
L-Phenylalanine	1 (2.2%)
L-Theanine	1 (2.2%)
L-Tyrosine	1 (2.2%)
Longjack extract	1 (2.2%)
Medium chain triglyceride oil	1 (2.2%)
Methylliberine	1 (2.2%)
N-MDA	1 (2.2%)
Orchic substance	1 (2.2%)
Oyster extract	1 (2.2%)
Panax notoginseng	1 (2.2%)
Pantothenic acid	1 (2.2%)
Phosphorus	1 (2.2%)
Phytosterol	1 (2.2%)
Piper nigrum	1 (2.2%)
Pyrroloquinoline quinone	1 (2.2%)
Prolinsis	1 (2.2%)
Prunella vulgaris	1 (2.2%)
Quercetin	1 (2.2%)
Red clover	1 (2.2%)
S-2-Amino-5-guanidinopentanoic acid	1 (2.2%)
Sarsaparilla extract	1 (2.2%)
Schizonepeta	1 (2.2%)
Sodium alpha lipoic acid	1 (2.2%)
Succinate	1 (2.2%)
Theobroma cacao	1 (2.2%)
Trigonella foenum	1 (2.2%)
Turmeric	1 (2.2%)
Valerian root	1 (2.2%)
Vitamin B2	1 (2.2%)
Vitamin C	1 (2.2%)
Vitamin K	1 (2.2%)
White tea extract	1 (2.2%)
Wild yam extract	1 (2.2%)
Withania somnifera	1 (2.2%)

Abbreviations: GABA = gamma-aminobutyric acid; HTP = hydroxytryptophan; N-MDA = N-Methyl-D-aspartate.

[Source ²⁰⁸ ]

Table 5. Published evidence showing an increase, decrease or no change in testosterone with supplementation

Supplement	Increase testosterone	Decrease testosterone	No change in testosterone	Conflicting data
Anacyclus pyrthrum	X	–	–	–
Apigenin	–	X	–	–
Ashwagandha extract	X	–	X	X
Astragalus	–	X	–	–
Bioperine	–	X	–	–
Boron	X	–	X	X
Broccoli	–	X	–	–
Bulbine natalensis	X	–	–	–
Caffeine	X	–	X	X
Calcium	X	–	X	X
Chrysin	–	–	X	–
D-Aspartic acid	X	X	X	X
Dehydroepiandrosterone	X	–	X	X
Epicatechin	X	–	–	–
Epimedium	–	–	X	–
Eurycoma longifolia extract	X	–	–	–
Fenugreek extract	X	–	–	–
Garlic	X	X	–	X
Ginkgo biloba	–	–	X	–
Ginseng	X	–	X	X
Glycyrrhiza glabra	–	X	X	X
Green tea leaf extract	–	X	X	X
Horney goat weed extract	X	–	–	–
L-Arginine	X	–	–	–
L-Carnitine	X	–	–	–
Maca extract	X	–	X	X
Magnesium	X	–	–	–
Melatonin	X	X	–	X
Mucuna purine	X	–	–	–
Pantothenic acid	X	–	–	–
Quercetin	–	–	X	–
Red clover	–	–	X	–
Resveratrol	X	–	X	X
Rhodiola rosea extract	–	–	X	–
Rosemary extract	–	X	–	–
Saw palmetto	–	–	X	–
Selenium	X	–	–	–
Shilajit	X	–	–	–
Tongkat ali extract	X	–	–	–
Vitamin B6	–	X	–	–
Vitamin D	X	–	X	X
Zinc	X	–	X	X

Footnotes: For 24.8%, or 27 out of 109 individual supplements, there was data showing an increase in testosterone with supplementation. However, for many of these (15 of the 27, or 55.5%) there was also conflicting data showing no change or a decrease in testosterone with supplementation. Of concern, 10.1% of components had published data that found a decrease in testosterone with supplementation.

[Source ²⁰⁸ ]

Table 6. Recommended Daily Allowance (RDA) and upper tolerable limit (UL) for each of the individual supplement components in testosterone boosters

Supplement (vitamins & minerals)	Containing (%)	Recommended Daily Allowance (RDA)	Upper tolerable limit (UL)	Range	Median dose	Compared to median (%)
Calcium	11.1	1,000 mg	2,500 mg	11–175 mg	31.5 mg	3.1
Folate	13.3	400 μg	1,000 μg	100–800 μg	400 μg	100
Magnesium	37.7	320 mg	350 mg	9–450 mg	450 mg	140.6
Phosphorus	2.2	700 mg	4,000 mg	135 mg	135 mg	19.2
Selenium	11.1	55 μg	400 μg	50–70 μg	50 μg	90.9
Vitamin B2	2.2	1.3 mg	NA	2 mg	2 mg	153.8
Vitamin B3	11.1	16 mg	35 mg	15–41 mg	30 mg	187.5
Vitamin B5	2.2	5 mg	NA	10 mg	10 mg	200
Vitamin B6	44.4	1.3 mg	100 mg	2–50 mg	10.5 mg	807.6
Vitamin B12	15.5	2.4 μg	NA	6–500 μg	31 μg	1291
Vitamin C	2.2	90 mg	2,000 mg	90 mg	90 mg	100
Vitamin D	17.7	600 IU	4,000 IU	400–3,000 IU	1,000 IU	166.6
Vitamin E	4.4	33 IU (synthetic)	1,100 IU (synthetic)	30–50 IU	40 IU	121.2
Vitamin K	2.2	120 μg	NA	50 μg	50 μg	41.6
Zinc	64.4	11 mg	40 mg	1.05–50 mg	30 mg	272

Footnote: NA = “not available” as there is no RDA or upper tolerable limit for these compounds.

[Source ²⁰⁸ ]

Table 7. Review of 10 most common testosterone booster ingredients

Ingredients	# of supplements included	Inclusion in PubMed searchable studies					Impact on Testosterone Levels in Human Studies
		Human Studies	Animal Studies	In vitro studies	Other	Total	Positive Effect	Negative Effect	No effect	Indeterminate
Tongkat Ali Extract	5	3	15	5	4	27	2	0	1	0
Horny Goat Weed	5	0	8	0	0	8	0	0	0	0
Saw Palmetto Extract	3	10	23	15	6	54	2	0	6	2
Boron	3	9	11	1	7	28	2	0	6	1
Nettle Extract	3	1	4	1	1	7	0	0	1	0
Maca Root Powder	2	4	20	2	2	28	0	1	3	0
Ashwagandha Root Extract	2	1	9	1	2	13	1	0	0	0
Fenugreek	2	7	3	0	1	11	4	0	0	3
Bioperine	2	1	5	2	0	8	0	0	0	1
Diindolymethane (DIIM)	2	1	5	1	0	7	0	0	0	1
	Total	37	103	28	23	191	11	1	17	8

[Source ²¹² ]

Table 8. Top 5 testosterone booster supplements based on cost and ingredient profile

Testosterone booster information

Ingredients

Name

Manufacturer

Number of ingredients

Cost ($) per pill

Ashwagand ha root extract

Bioperine

Boron

Boron Citrate

Calcium

Diindolymethane (DIIM)

Fenugree k extract

Horney goat weed

Maca root powder

Nettle extract

Orchic substance

Sarsaparil la

Saw palemett o extract

Tongkat Ali extract

Tribulus Terrestris extract

Vitamin B3

Vitamin D

Wild Yam extract

Zinc

Prime Labs

Prime labs

7

$0.33

x

x

x

x

x

x

x

Alpha boost

Invictus labs

9

$0.32

x

x

x

x

x

x

x

x

x

Extra strength

Dr. Martin’s nutrition

9

$0.20

x

x

x

x

x

x

x

x

x

Iron brothers

Iron Brothers supplements

12

$0.27

x

x

x

x

x

x

x

x

x

x

x

x

Pro-T

Prometheus Wellness LLC

12

$0.28

x

x

x

x

x

x

x

x

x

x

x

x

Footnotes: Three of the 5 testosterone boosters did not report quantities of individual ingredients, but rather only provided a list of all ingredients. A total of 19 unique ingredients were identified across the group of supplements; each supplement consisted of an average of 9.8 ± 2.2 ingredients (Range 7-12). Boron, horny goat weed, and tongkat ali extract were present in all 5 supplements. Despite being marketed under different names, several testosterone boosterrs, namely Alpha Boost and Extra Strength as well as Pro-T and Iron Brothers all had the same ingredients, including quantities of each ingredient. The average cost per bottle was $20.16 ± $5.34 (Range $11.99-$25.23). The average suggested serving size was 2.4 pills (Range: 2-3 pills). The average cost per pill was $0.28 ± $0.05 (Range $0.19-$0.33).

[Source ²¹² ]

1. Nassar GN, Leslie SW. Physiology, Testosterone. [Updated 2023 Jan 2]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526128[↩]
2. McEwan IJ, Brinkmann AO. Androgen Physiology: Receptor and Metabolic Disorders. [Updated 2021 Jul 2]. In: Feingold KR, Anawalt B, Blackman MR, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279028[↩][↩][↩]
3. Sizar O, Leslie SW, Pico J. Androgen Replacement. [Updated 2023 Nov 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK534853[↩][↩][↩]
4. Vermeulen A, Kaufman JM. Diagnosis of hypogonadism in the aging male. Aging Male. 2002 Sep;5(3):170-6. Erratum in: Aging Male. 2002 Dec;5(4):iv.[↩]
5. Vermeulen A. Hormonal cut-offs of partial androgen deficiency: a survey of androgen assays. J Endocrinol Invest. 2005;28(3 Suppl):28-31.[↩]
6. Abbasi A.A., Drinka P.J., Mattson D.E., Rudman D. Low circulating levels of insulin-like growth factors and testosterone in chronically institutionalized elderly men. J. Am. Geriatr. Soc. 1993;41:975–982. doi: 10.1111/j.1532-5415.1993.tb06764.x[↩]
7. Spaziani M, Mileno B, Rossi F, Granato S, Tahani N, Anzuini A, Lenzi A, Radicioni AF. Endocrine and metabolic evaluation of classic Klinefelter syndrome and high-grade aneuploidies of sexual chromosomes with male phenotype: are they different clinical conditions? Eur J Endocrinol. 2018 Apr;178(4):343-352. doi: 10.1530/EJE-17-0902[↩]
8. Bozzola M, Bozzola E, Montalbano C, Stamati FA, Ferrara P, Villani A. Delayed puberty versus hypogonadism: a challenge for the pediatrician. Ann Pediatr Endocrinol Metab. 2018 Jun;23(2):57-61. doi: 10.6065/apem.2018.23.2.57[↩]
9. Hauser LJ, Jensen EL, Mirsky DM, Chan KH. Pediatric anosmia: A case series. Int J Pediatr Otorhinolaryngol. 2018 Jul;110:135-139. doi: 10.1016/j.ijporl.2018.05.011[↩][↩]
10. Carrasquillo R, Chu K, Ramasamy R. Novel Therapy for Male Hypogonadism. Curr Urol Rep. 2018 Jun 9;19(8):63. doi: 10.1007/s11934-018-0816-x[↩][↩][↩][↩]
11. Basaria S. Male hypogonadism. Lancet. 2014 Apr 5;383(9924):1250-63. doi: 10.1016/S0140-6736(13)61126-5[↩]
12. Mulhall JP, Trost LW, Brannigan RE, Kurtz EG, Redmon JB, Chiles KA, Lightner DJ, Miner MM, Murad MH, Nelson CJ, Platz EA, Ramanathan LV, Lewis RW. Evaluation and Management of Testosterone Deficiency: AUA Guideline. J Urol. 2018 Aug;200(2):423-432. doi: 10.1016/j.juro.2018.03.115[↩][↩][↩][↩][↩]
13. Morgentaler A, Traish AM, Khera M. A Critique of the AUA Guidelines on Testosterone Deficiency. J Sex Med. 2020 Apr;17(4):561-564. doi: 10.1016/j.jsxm.2019.10.019[↩]
14. Bhasin S, Brito JP, Cunningham GR, Hayes FJ, Hodis HN, Matsumoto AM, Snyder PJ, Swerdloff RS, Wu FC, Yialamas MA. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018 May 1;103(5):1715-1744. doi: 10.1210/jc.2018-00229[↩][↩][↩]
15. Mulhall JP, Trost LW, Brannigan RE, Kurtz EG, Redmon JB, Chiles KA, et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. Journal of Urology [Internet]. 2018 Aug 1 [cited 2024 Aug 6];200(2):423–32. Available from: https://doi.org/10.1016/j.juro.2018.03.115[↩]
16. Elmlinger MW, Kühnel W, Döller PC. Evaluation of direct and indirect markers to assess the androgen status in healthy males during aging. Clin Lab. 2006;52(9-10):491-6.[↩]
17. Gheorghiu I, Moshyk A, Lepage R, Ahnadi CE, Grant AM. When is bioavailable testosterone a redundant test in the diagnosis of hypogonadism in men? Clin Biochem. 2005 Sep;38(9):813-8. doi: 10.1016/j.clinbiochem.2005.05.005[↩]
18. Tenover JS. Declining testicular function in aging men. Int J Impot Res. 2003 Aug;15 Suppl 4:S3-8. doi: 10.1038/sj.ijir.3901029[↩]
19. Ross A, Bhasin S. Hypogonadism: Its Prevalence and Diagnosis. Urol Clin North Am. 2016 May;43(2):163-76. doi: 10.1016/j.ucl.2016.01.002[↩]
20. Salonia A, Bettocchi C, Boeri L, Capogrosso P, Carvalho J, Cilesiz NC, Cocci A, Corona G, Dimitropoulos K, Gül M, Hatzichristodoulou G, Jones TH, Kadioglu A, Martínez Salamanca JI, Milenkovic U, Modgil V, Russo GI, Serefoglu EC, Tharakan T, Verze P, Minhas S; EAU Working Group on Male Sexual and Reproductive Health. European Association of Urology Guidelines on Sexual and Reproductive Health-2021 Update: Male Sexual Dysfunction. Eur Urol. 2021 Sep;80(3):333-357. doi: 10.1016/j.eururo.2021.06.007[↩]
21. Isidori AM, Aversa A, Calogero A, Ferlin A, Francavilla S, Lanfranco F, Pivonello R, Rochira V, Corona G, Maggi M. Adult- and late-onset male hypogonadism: the clinical practice guidelines of the Italian Society of Andrology and Sexual Medicine (SIAMS) and the Italian Society of Endocrinology (SIE). J Endocrinol Invest. 2022 Dec;45(12):2385-2403. doi: 10.1007/s40618-022-01859-7[↩]
22. Wu F.C.W., Tajar A., Pye S., Silman A.J., Finn J.D., O’Neill T., Bartfai G., Casanueva F., Forti G., Giwercman A., et al. Hypothalamic-pituitary-testicular axis disruptions in older men are differentially linked to age and modifiable risk factors: The european male aging study. J. Clin. Endocrinol. Metab. 2008;93:2737–2745. doi: 10.1210/jc.2007-1972[↩]
23. Feldman H.A., Longcope C., Derby C.A., Johannes C.B., Araujo A.B., Coviello A.D., Bremner W.J., McKinlay J.B. Age trends in the level of serum testosterone and other hormones in middle-aged men: Longitudinal results from the massachusetts male aging study. J. Clin. Endocrinol. Metab. 2002;87:589–598. doi: 10.1210/jcem.87.2.8201[↩]
24. Perheentupa A., Huhtaniemi I. Aging of the human ovary and testis. Mol. Cell. Endocrinol. 2009;299:2–13. doi: 10.1016/j.mce.2008.11.004[↩]
25. Well D., Yang H., Houseni M., Iruvuri S., Alzeair S., Sansovini M., Wintering N., Alavi A., Torigian D.A. Age-Related Structural and Metabolic Changes in the Pelvic Reproductive End Organs. Semin. Nucl. Med. 2007;37:173–184. doi: 10.1053/j.semnuclmed.2007.01.004[↩]
26. Kidd S.A., Eskenazi B., Wyrobek A. Effects of male age on semen quality and fertility: A review of the literature. Fertil. Steril. 2001;75:237–248. doi: 10.1016/S0015-0282(00)01679-4[↩]
27. Harman SM, Metter EJ, Tobin JD, Pearson J, Blackman MR; Baltimore Longitudinal Study of Aging. Longitudinal effects of aging on serum total and free testosterone levels in healthy men. Baltimore Longitudinal Study of Aging. J Clin Endocrinol Metab. 2001 Feb;86(2):724-31. doi: 10.1210/jcem.86.2.7219[↩][↩]
28. Baum NH, Crespi CA. Testosterone replacement in elderly men. Geriatrics. 2007 Sep;62(9):15-8.[↩]
29. Moncada I. Testosterone and men’s quality of life. Aging Male. 2006 Dec;9(4):189-93. doi: 10.1080/13685530601003180[↩]
30. Kaufman JM, Vermeulen A. The decline of androgen levels in elderly men and its clinical and therapeutic implications. Endocr Rev. 2005 Oct;26(6):833-76. doi: 10.1210/er.2004-0013[↩]
31. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet]. Bethesda (MD): National Institute of Diabetes and Digestive and Kidney Diseases; 2012-. Androgenic Steroids. [Updated 2020 May 30]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK548931[↩]
32. Swerdloff RS, Wang C, Bhasin S. Developments in the control of testicular function. Baillieres Clin Endocrinol Metab. 1992 Apr;6(2):451-83. https://doi.org/10.1016/S0950-351X(05)80158-2[↩]
33. Payne AH, O’Shaughnessy P (1996). “Structure, function, and regulation of steroidogenic enzymes in the Leydig cell”. In Payne AH, Hardy MP, Russell LD (eds.). Leydig Cell. Vienna [Il]: Cache River Press. pp. 260–85. ISBN 978-0-9627422-7-9[↩]
34. Handelsman, D. J., Hirschberg, A. L., & Bermon, S. (2018). Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance. Endocrine reviews, 39(5), 803–829. https://doi.org/10.1210/er.2018-00020[↩]
35. Cunningham, G. R., Stephens-Shields, A. J., Rosen, R. C., Wang, C., Bhasin, S., Matsumoto, A. M., Parsons, J. K., Gill, T. M., Molitch, M. E., Farrar, J. T., Cella, D., Barrett-Connor, E., Cauley, J. A., Cifelli, D., Crandall, J. P., Ensrud, K. E., Gallagher, L., Zeldow, B., Lewis, C. E., Pahor, M., … Snyder, P. J. (2016). Testosterone Treatment and Sexual Function in Older Men With Low Testosterone Levels. The Journal of clinical endocrinology and metabolism, 101(8), 3096–3104. https://doi.org/10.1210/jc.2016-1645[↩]
36. Brown GA, Vukovich MD, Sharp RL, Reifenrath TA, Parsons KA, King DS. Effect of oral DHEA on serum testosterone and adaptations to resistance training in young men. J Appl Physiol (1985). 1999 Dec;87(6):2274-83. doi: 10.1152/jappl.1999.87.6.2274[↩]
37. Winter, A. G., Zhao, F., & Lee, R. K. (2014). Androgen deficiency and metabolic syndrome in men. Translational andrology and urology, 3(1), 50–58. https://doi.org/10.3978/j.issn.2223-4683.2014.01.04[↩][↩]
38. Wu FCW, Tajar A, Beynon JM, Pye SR, Silman AJ, Finn JD, et al. Identification of Late-Onset Hypogonadism in Middle-Aged and Elderly Men. N Engl J Med 2010;363:123–35. doi:10.1056/NEJMoa0911101[↩]
39. Wang C, Nieschlag E, Swerdloff R, Behre HM, Hellstrom WJ, Gooren LJ, et al. Investigation, treatment and monitoring of late-onset hypogonadism in males. Int J Androl 2009;32:1–10. doi:10.1111/j.1365-2605.2008.00924.x[↩]
40. Araujo AB, Esche GR, Kupelian V, O’Donnell AB, Travison TG, Williams RE, Clark RV, McKinlay JB. Prevalence of symptomatic androgen deficiency in men. J Clin Endocrinol Metab. 2007 Nov;92(11):4241-7. doi: 10.1210/jc.2007-1245[↩]
41. Seftel AD. Male hypogonadism. Part I: Epidemiology of hypogonadism. Int J Impot Res 2006;18:115–20. doi:10.1038/sj.ijir.3901397[↩]
42. Testosterone. https://medlineplus.gov/ency/article/003707.htm[↩][↩]
43. Huhtaniemi I. (2014). Late-onset hypogonadism: current concepts and controversies of pathogenesis, diagnosis and treatment. Asian journal of andrology, 16(2), 192–202. https://doi.org/10.4103/1008-682X.122336[↩][↩]
44. Huhtaniemi IT. Andropause–lessons from the European Male Ageing Study. Ann Endocrinol (Paris). 2014 May;75(2):128-31. doi: 10.1016/j.ando.2014.03.005[↩]
45. Wu FC, Tajar A, Pye SR, Silman AJ, Finn JD, O’Neill TW, Bartfai G, Casanueva F, Forti G, Giwercman A, Huhtaniemi IT, Kula K, Punab M, Boonen S, Vanderschueren D; European Male Aging Study Group. Hypothalamic-pituitary-testicular axis disruptions in older men are differentially linked to age and modifiable risk factors: the European Male Aging Study. J Clin Endocrinol Metab. 2008 Jul;93(7):2737-45. doi: 10.1210/jc.2007-1972[↩]
46. Wu FC, Tajar A, Beynon JM, Pye SR, Silman AJ, Finn JD, O’Neill TW, Bartfai G, Casanueva FF, Forti G, Giwercman A, Han TS, Kula K, Lean ME, Pendleton N, Punab M, Boonen S, Vanderschueren D, Labrie F, Huhtaniemi IT; EMAS Group. Identification of late-onset hypogonadism in middle-aged and elderly men. N Engl J Med. 2010 Jul 8;363(2):123-35. doi: 10.1056/NEJMoa0911101[↩][↩]
47. Vingren JL, Kraemer WJ, Ratamess NA, Anderson JM, Volek JS, Maresh CM. Testosterone physiology in resistance exercise and training: the up-stream regulatory elements. Sports Med. 2010 Dec 1;40(12):1037-53. doi: 10.2165/11536910-000000000-00000[↩]
48. Hulmi JJ, Ahtiainen JP, Selänne H, Volek JS, Häkkinen K, Kovanen V, Mero AA. Androgen receptors and testosterone in men–effects of protein ingestion, resistance exercise and fiber type. J Steroid Biochem Mol Biol. 2008 May;110(1-2):130-7. doi: 10.1016/j.jsbmb.2008.03.030[↩]
49. Hackney AC, Moore AW, Brownlee KK. Testosterone and endurance exercise: development of the “exercise-hypogonadal male condition”. Acta Physiol Hung. 2005;92(2):121-37. doi: 10.1556/APhysiol.92.2005.2.3[↩]
50. Livera G, Rouiller-Fabre V, Pairault C, Levacher C, Habert R. Regulation and perturbation of testicular functions by vitamin A. Reproduction. 2002 Aug;124(2):173-80.[↩]
51. Pilz S, Frisch S, Koertke H, Kuhn J, Dreier J, Obermayer-Pietsch B, Wehr E, Zittermann A. Effect of vitamin D supplementation on testosterone levels in men. Horm Metab Res. 2011 Mar;43(3):223-5. doi: 10.1055/s-0030-1269854[↩]
52. Elisabeth Lerchbaum, Stefan Pilz, Christian Trummer, Verena Schwetz, Oliver Pachernegg, Annemieke C Heijboer, Barbara Obermayer-Pietsch, Vitamin D and Testosterone in Healthy Men: A Randomized Controlled Trial, The Journal of Clinical Endocrinology & Metabolism, Volume 102, Issue 11, 1 November 2017, Pages 4292–4302, https://doi.org/10.1210/jc.2017-01428[↩]
53. Zittermann A, Ernst JB, Prokop S, Fuchs U, Dreier J, Kuhn J, Knabbe C, Berthold HK, Gouni-Berthold I, Gummert JF, Börgermann J, Pilz S. Vitamin D supplementation does not prevent the testosterone decline in males with advanced heart failure: the EVITA trial. Eur J Nutr. 2019 Mar;58(2):673-680. doi: 10.1007/s00394-018-1666-5[↩]
54. Prasad AS, Mantzoros CS, Beck FW, Hess JW, Brewer GJ. Zinc status and serum testosterone levels of healthy adults. Nutrition. 1996 May;12(5):344-8. doi: 10.1016/s0899-9007(96)80058-x[↩]
55. Koehler K, Parr MK, Geyer H, Mester J, Schänzer W. Serum testosterone and urinary excretion of steroid hormone metabolites after administration of a high-dose zinc supplement. Eur J Clin Nutr. 2009 Jan;63(1):65-70. doi: 10.1038/sj.ejcn.1602899[↩]
56. Whittaker J, Wu K. Low-fat diets and testosterone in men: Systematic review and meta-analysis of intervention studies. J Steroid Biochem Mol Biol. 2021 Jun;210:105878. doi: 10.1016/j.jsbmb.2021.105878[↩]
57. Whittaker J, Harris M. Low-carbohydrate diets and men’s cortisol and testosterone: Systematic review and meta-analysis. Nutr Health. 2022 Mar 7:2601060221083079. doi: 10.1177/02601060221083079[↩]
58. Håkonsen, L. B., Thulstrup, A. M., Aggerholm, A. S., Olsen, J., Bonde, J. P., Andersen, C. Y., Bungum, M., Ernst, E. H., Hansen, M. L., Ernst, E. H., & Ramlau-Hansen, C. H. (2011). Does weight loss improve semen quality and reproductive hormones? Results from a cohort of severely obese men. Reproductive health, 8, 24. https://doi.org/10.1186/1742-4755-8-24[↩][↩]
59. A.A. MacDonald, G.P. Herbison, M. Showell, C.M. Farquhar, The impact of body mass index on semen parameters and reproductive hormones in human males: a systematic review with meta-analysis, Human Reproduction Update, Volume 16, Issue 3, May-June 2010, Pages 293–311, https://doi.org/10.1093/humupd/dmp047[↩]
60. Pasquali R. Obesity and androgens: facts and perspectives. Fertil Steril. 2006 May;85(5):1319-40. doi: 10.1016/j.fertnstert.2005.10.054[↩]
61. Salas-Huetos A, Maghsoumi-Norouzabad L, James ER, Carrell DT, Aston KI, Jenkins TG, Becerra-Tomás N, Javid AZ, Abed R, Torres PJ, Luque EM, Ramírez ND, Martini AC, Salas-Salvadó J. Male adiposity, sperm parameters and reproductive hormones: An updated systematic review and collaborative meta-analysis. Obes Rev. 2021 Jan;22(1):e13082. doi: 10.1111/obr.13082[↩]
62. Andersen ML, Tufik S. The effects of testosterone on sleep and sleep-disordered breathing in men: its bidirectional interaction with erectile function. Sleep Med Rev. 2008 Oct;12(5):365-79. doi: 10.1016/j.smrv.2007.12.003[↩][↩]
63. Schultheiss OC, Campbell KL, McClelland DC. Implicit power motivation moderates men’s testosterone responses to imagined and real dominance success. Horm Behav. 1999 Dec;36(3):234-41. doi: 10.1006/hbeh.1999.1542[↩]
64. Grant P. Spearmint herbal tea has significant anti-androgen effects in polycystic ovarian syndrome. A randomized controlled trial. Phytother Res. 2010 Feb;24(2):186-8. doi: 10.1002/ptr.2900[↩]
65. Armanini D, Fiore C, Mattarello MJ, Bielenberg J, Palermo M. History of the endocrine effects of licorice. Exp Clin Endocrinol Diabetes. 2002 Sep;110(6):257-61. doi: 10.1055/s-2002-34587[↩]
66. Burdea L, Mendez MD. 21 Hydroxylase Deficiency. [Updated 2021 Aug 9]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK493164[↩][↩]
67. Saez, J. M., Forest, M. G., Morera, A. M., & Bertrand, J. (1972). Metabolic clearance rate and blood production rate of testosterone and dihydrotestosterone in normal subjects, during pregnancy, and in hyperthyroidism. The Journal of clinical investigation, 51(5), 1226–1234. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC292254/pdf/jcinvest00201-0192.pdf[↩]
68. Plant TM, Marshall GR. The functional significance of FSH in spermatogenesis and the control of its secretion in male primates. Endocr Rev. 2001 Dec;22(6):764-86. doi: 10.1210/edrv.22.6.0446[↩]
69. El-Gehani F, Zhang FP, Pakarinen P, Rannikko A, Huhtaniemi I. Gonadotropin-independent regulation of steroidogenesis in the fetal rat testis. Biol Reprod. 1998 Jan;58(1):116-23. doi: 10.1095/biolreprod58.1.116[↩]
70. Brinkmann AO, van Straalen RJ. Development of the LH-response in fetal guinea pig testes. Biol Reprod. 1979 Nov;21(4):991-7. doi: 10.1095/biolreprod21.4.991[↩]
71. O’Shaughnessy PJ, Baker P, Sohnius U, Haavisto AM, Charlton HM, Huhtaniemi I. Fetal development of Leydig cell activity in the mouse is independent of pituitary gonadotroph function. Endocrinology. 1998 Mar;139(3):1141-6. doi: 10.1210/endo.139.3.5788[↩]
72. Curtin D, Jenkins S, Farmer N, Anderson AC, Haisenleder DJ, Rissman E, Wilson EM, Shupnik MA. Androgen suppression of GnRH-stimulated rat LHbeta gene transcription occurs through Sp1 sites in the distal GnRH-responsive promoter region. Mol Endocrinol. 2001 Nov;15(11):1906-17. doi: 10.1210/mend.15.11.0723[↩]
73. Stocco DM, Clark BJ. Regulation of the acute production of steroids in steroidogenic cells. Endocr Rev. 1996 Jun;17(3):221-44. doi: 10.1210/edrv-17-3-221[↩]
74. Miller WL. Molecular biology of steroid hormone synthesis. Endocr Rev. 1988 Aug;9(3):295-318. doi: 10.1210/edrv-9-3-295[↩]
75. George FW, Wilson JD 1994 Sex determination and differentiation. In: Knobil E, Neill JD, eds. The Physiology of Reproduction. New York.: Raven Press Ltd.; Chapter 1.[↩]
76. Reisch N, Taylor AE, Nogueira EF, Asby DJ, Dhir V, Berry A, Krone N, Auchus RJ, Shackleton CHL, Hanley NA, Arlt W. Alternative pathway androgen biosynthesis and human fetal female virilization. Proc Natl Acad Sci U S A. 2019 Oct 29;116(44):22294-22299. doi: 10.1073/pnas.1906623116[↩]
77. Russell DW, Wilson JD. Steroid 5 alpha-reductase: two genes/two enzymes. Annu Rev Biochem. 1994;63:25-61. doi: 10.1146/annurev.bi.63.070194.000325[↩]
78. Andersson S, Berman DM, Jenkins EP, Russell DW. Deletion of steroid 5 alpha-reductase 2 gene in male pseudohermaphroditism. Nature. 1991 Nov 14;354(6349):159-61. doi: 10.1038/354159a0[↩]
79. Andersson S, Bishop RW, Russell DW. Expression cloning and regulation of steroid 5 alpha-reductase, an enzyme essential for male sexual differentiation. J Biol Chem. 1989 Sep 25;264(27):16249-55. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4448950/[↩]
80. Zhang JR, Zhang PY, Sun LG. [Mild androgen insensitivity syndrome: a case report]. Zhonghua Nei Ke Za Zhi. 2018 Aug 1;57(8):600-602. Chinese. doi: 10.3760/cma.j.issn.0578-1426.2018.08.014[↩]
81. Nassar GN, Leslie SW. Physiology, Testosterone. [Updated 2022 Jan 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526128[↩][↩][↩]
82. Clark BJ, Prough RA, Klinge CM. Mechanisms of Action of Dehydroepiandrosterone. Vitam Horm. 2018;108:29-73. doi: 10.1016/bs.vh.2018.02.003[↩][↩]
83. Basaria S. Reproductive aging in men. Endocrinol Metab Clin North Am. 2013 Jun;42(2):255-70. doi: 10.1016/j.ecl.2013.02.012[↩]
84. Kalfa N, Gaspari L, Ollivier M, Philibert P, Bergougnoux A, Paris F, Sultan C. Molecular genetics of hypospadias and cryptorchidism recent developments. Clin Genet. 2019 Jan;95(1):122-131. doi: 10.1111/cge.13432[↩]
85. Travison, T. G., Vesper, H. W., Orwoll, E., Wu, F., Kaufman, J. M., Wang, Y., Lapauw, B., Fiers, T., Matsumoto, A. M., & Bhasin, S. (2017). Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies in the United States and Europe. The Journal of clinical endocrinology and metabolism, 102(4), 1161–1173. https://doi.org/10.1210/jc.2016-2935[↩]
86. Bhasin S, Cunningham GR, Hayes FJ, Matsumoto AM, Snyder PJ, Swerdloff RS, Montori VM; Task Force, Endocrine Society. Testosterone therapy in men with androgen deficiency syndromes: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2010 Jun;95(6):2536-59. doi: 10.1210/jc.2009-2354. Erratum in: J Clin Endocrinol Metab. 2021 Jun 16;106(7):e2848.[↩][↩]
87. Morley JE, Charlton E, Patrick P, Kaiser FE, Cadeau P, McCready D, Perry HM 3rd. Validation of a screening questionnaire for androgen deficiency in aging males. Metabolism. 2000 Sep;49(9):1239-42. doi: 10.1053/meta.2000.8625[↩]
88. Grober E. D. (2014). Testosterone deficiency and replacement: Myths and realities. Canadian Urological Association journal = Journal de l’Association des urologues du Canada, 8(7-8 Suppl 5), S145–S147. https://doi.org/10.5489/cuaj.2309[↩][↩]
89. Peterson, M. D., Belakovskiy, A., McGrath, R., & Yarrow, J. F. (2018). Testosterone Deficiency, Weakness, and Multimorbidity in Men. Scientific reports, 8(1), 5897. https://doi.org/10.1038/s41598-018-24347-6[↩]
90. Mulligan, T., Frick, M. F., Zuraw, Q. C., Stemhagen, A., & McWhirter, C. (2006). Prevalence of hypogonadism in males aged at least 45 years: the HIM study. International journal of clinical practice, 60(7), 762–769. https://doi.org/10.1111/j.1742-1241.2006.00992.x[↩]
91. Aversa A, Morgentaler A. The practical management of testosterone deficiency in men. Nat Rev Urol. 2015 Nov;12(11):641-50. doi: 10.1038/nrurol.2015.238[↩]
92. Wu FC, von Eckardstein A. Androgens and coronary artery disease. Endocr Rev. 2003 Apr;24(2):183-217. doi: 10.1210/er.2001-0025[↩]
93. Heron M. Deaths: Leading Causes for 2014. Natl Vital Stat Rep. 2016 Jun;65(5):1-96.[↩]
94. Nieschlag E, Swerdloff R, Behre HM, Gooren LJ, Kaufman JM, Legros JJ, Lunenfeld B, Morley JE, Schulman C, Wang C, Weidner W, Wu FC. Investigation, treatment and monitoring of late-onset hypogonadism in males. Aging Male. 2005 Jun;8(2):56-8. doi: 10.1080/13685530500130969[↩]
95. Traish AM, Miner MM, Morgentaler A, Zitzmann M. Testosterone deficiency. Am J Med. 2011 Jul;124(7):578-87. doi: 10.1016/j.amjmed.2010.12.027[↩]
96. Shores MM, Matsumoto AM, Sloan KL, Kivlahan DR. Low serum testosterone and mortality in male veterans. Arch Intern Med. 2006 Aug 14-28;166(15):1660-5. doi: 10.1001/archinte.166.15.1660[↩]
97. Choi, S. W., Jeon, S. H., Kwon, E. B., Zhu, G. Q., Lee, K. W., Choi, J. B., Jeong, H. C., Kim, K. S., Bae, S. R., Bae, W. J., Kim, S. J., Cho, H. J., Ha, U. S., Hong, S. H., Hwang, S. Y., & Kim, S. W. (2019). Effect of Korean Herbal Formula (Modified Ojayeonjonghwan) on Androgen Receptor Expression in an Aging Rat Model of Late Onset Hypogonadism. The world journal of men’s health, 37(1), 105–112. https://doi.org/10.5534/wjmh.180051[↩]
98. Hackett, G. Type 2 Diabetes and Testosterone Therapy. World J. Mens Health 2019, 37, 31–44.[↩]
99. Nam, Y.S.; Lee, G.; Yun, J.M.; Cho, B. Testosterone Replacement, Muscle Strength, and Physical Function. World J. Mens Health 2018, 36, 110–122.[↩]
100. Bassil, N., Alkaade, S., & Morley, J. E. (2009). The benefits and risks of testosterone replacement therapy: a review. Therapeutics and clinical risk management, 5(3), 427–448. https://doi.org/10.2147/tcrm.s3025[↩]
101. Morales A, Schulman CC, Tostain J, C W Wu F. Testosterone Deficiency Syndrome (TDS) needs to be named appropriately–the importance of accurate terminology. Eur Urol. 2006 Sep;50(3):407-9. doi: 10.1016/j.eururo.2006.07.001[↩]
102. Kumar, P., Kumar, N., Thakur, D. S., & Patidar, A. (2010). Male hypogonadism: Symptoms and treatment. Journal of advanced pharmaceutical technology & research, 1(3), 297–301. https://doi.org/10.4103/0110-5558.72420[↩][↩]
103. Seftel A. Male hypogonadism. Part II: etiology, pathophysiology, and diagnosis. Int J Impot Res. 2006 May-Jun;18(3):223-8. doi: 10.1038/sj.ijir.3901365[↩][↩]
104. Naifar M, Rekik N, Messedi M, Chaabouni K, Lahiani A, Turki M, Abid M, Ayedi F, Jamoussi K. Male hypogonadism and metabolic syndrome. Andrologia. 2015 Jun;47(5):579-86. doi: 10.1111/and.12305[↩]
105. Shin YS, Park JK. The Optimal Indication for Testosterone Replacement Therapy in Late Onset Hypogonadism. Journal of Clinical Medicine. 2019; 8(2):209. https://doi.org/10.3390/jcm8020209[↩]
106. Shin, Y. S., & Park, J. K. (2019). The Optimal Indication for Testosterone Replacement Therapy in Late Onset Hypogonadism. Journal of clinical medicine, 8(2), 209. https://doi.org/10.3390/jcm8020209[↩][↩]
107. Lunenfeld B, Arver S, Moncada I, Rees DA, Schulte HM. How to help the aging male? Current approaches to hypogonadism in primary care. Aging Male. 2012 Dec;15(4):187-97. doi: 10.3109/13685538.2012.729110[↩]
108. Male Hypogonadism. https://d56bochluxqnz.cloudfront.net/media/EAU-Guidelines-on-Male-Hypogonadism-2019v2.pdf[↩]
109. Brunton SA, Sadovsky R. Late-onset male hypogonadism and testosterone replacement therapy in primary care. J Fam Pract. 2010 Jul;59(7 Suppl):S1-8.[↩]
110. Park, H. J., Ahn, S. T., & Moon, D. G. (2019). Evolution of Guidelines for Testosterone Replacement Therapy. Journal of clinical medicine, 8(3), 410. https://doi.org/10.3390/jcm8030410[↩][↩][↩][↩]
111. Morales A. Andropause (or symptomatic late-onset hypogonadism): facts, fiction and controversies. Aging Male. 2004 Dec;7(4):297-303. doi: 10.1080/13685530400016664[↩]
112. Yialamas MA, Hayes FJ. Androgens and the ageing male and female. Best Pract Res Clin Endocrinol Metab. 2003 Jun;17(2):223-36. doi: 10.1016/s1521-690x(03)00018-6[↩]
113. Morales A. Testosterone Deficiency Syndrome: an overview with emphasis on the diagnostic conundrum. Clin Biochem. 2014 Jul;47(10-11):960-6. doi: 10.1016/j.clinbiochem.2013.11.024[↩]
114. Buvat, J.; Lemaire, A. Endocrine screening in 1022 men with erectile dysfunction: clinical significance and cost-effective strategy. J. Urol. 1997, 158, 1764–1767.[↩]
115. Mulhall, J.P.; Trost, L.W.; Brannigan, R.E.; Kurtz, E.G.; Redmon, J.B.; Chiles, K.A.; Lightner, D.J.; Miner, M.M.; Murad, M.H.; Nelson, C.J.; et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. J. Urol. 2018, 200, 423–432.[↩]
116. Morales, A.; Bebb, R.A.; Manjoo, P.; Assimakopoulos, P.; Axler, J.; Collier, C.; Elliott, S.; Goldenberg, L.; Gottesman, I.; Grober, E.D.; et al. Diagnosis and management of testosterone deficiency syndrome in men: clinical practice guideline. Can. Med. Assoc. J. 2015, 187, 1369–1377.[↩]
117. Testosterone Information. https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/testosterone-information[↩]
118. FDA Drug Safety Communication: FDA cautions about using testosterone products for low testosterone due to aging; requires labeling change to inform of possible increased risk of heart attack and stroke with use. https://www.fda.gov/drugs/drug-safety-and-availability/fda-drug-safety-communication-fda-cautions-about-using-testosterone-products-low-testosterone-due[↩][↩]
119. Grech, A., Breck, J., & Heidelbaugh, J. (2014). Adverse effects of testosterone replacement therapy: an update on the evidence and controversy. Therapeutic advances in drug safety, 5(5), 190–200. https://doi.org/10.1177/2042098614548680[↩][↩]
120. Holmegard HN, Nordestgaard BG, Schnohr P, Tybjaerg-Hansen A, Benn M. Endogenous sex hormones and risk of venous thromboembolism in women and men. J Thromb Haemost. 2014;12(3):297-305. doi: 10.1111/jth.12484[↩]
121. Coviello, A. D., Kaplan, B., Lakshman, K. M., Chen, T., Singh, A. B., & Bhasin, S. (2008). Effects of graded doses of testosterone on erythropoiesis in healthy young and older men. The Journal of clinical endocrinology and metabolism, 93(3), 914–919. https://doi.org/10.1210/jc.2007-1692[↩][↩]
122. Fernández-Balsells MM, Murad MH, Lane M, Lampropulos JF, Albuquerque F, Mullan RJ, Agrwal N, Elamin MB, Gallegos-Orozco JF, Wang AT, Erwin PJ, Bhasin S, Montori VM. Clinical review 1: Adverse effects of testosterone therapy in adult men: a systematic review and meta-analysis. J Clin Endocrinol Metab. 2010 Jun;95(6):2560-75. doi: 10.1210/jc.2009-2575[↩][↩]
123. No consistent evidence of an increased risk of heart problems with testosterone medicines. https://www.ema.europa.eu/en/news/no-consistent-evidence-increased-risk-heart-problems-testosterone-medicines[↩]
124. Swerdloff RS, Wang C. Three-year follow-up of androgen treatment in hypogonadal men: preliminary report with testosterone gel. Aging Male. 2003 Sep;6(3):207-11.[↩]
125. Carter, H. B., Albertsen, P. C., Barry, M. J., Etzioni, R., Freedland, S. J., Greene, K. L., Holmberg, L., Kantoff, P., Konety, B. R., Murad, M. H., Penson, D. F., & Zietman, A. L. (2013). Early detection of prostate cancer: AUA Guideline. The Journal of urology, 190(2), 419–426. https://doi.org/10.1016/j.juro.2013.04.119[↩]
126. Khera M, Bhattacharya RK, Blick G, Kushner H, Nguyen D, Miner MM. Changes in prostate specific antigen in hypogonadal men after 12 months of testosterone replacement therapy: support for the prostate saturation theory. J Urol. 2011 Sep;186(3):1005-11. doi: 10.1016/j.juro.2011.04.065[↩][↩]
127. Ferrucci, L., Maggio, M., Bandinelli, S., Basaria, S., Lauretani, F., Ble, A., Valenti, G., Ershler, W. B., Guralnik, J. M., & Longo, D. L. (2006). Low testosterone levels and the risk of anemia in older men and women. Archives of internal medicine, 166(13), 1380–1388. https://doi.org/10.1001/archinte.166.13.1380[↩]
128. Shahani S, Braga-Basaria M, Maggio M, Basaria S. Androgens and erythropoiesis: past and present. J Endocrinol Invest. 2009 Sep;32(8):704-16. doi: 10.1007/BF03345745[↩]
129. Shin YS, You JH, Cha JS, Park JK. The relationship between serum total testosterone and free testosterone levels with serum hemoglobin and hematocrit levels: a study in 1221 men. Aging Male. 2016 Dec;19(4):209-214. doi: 10.1080/13685538.2016.1229764[↩][↩]
130. Roy, C. N., Snyder, P. J., Stephens-Shields, A. J., Artz, A. S., Bhasin, S., Cohen, H. J., Farrar, J. T., Gill, T. M., Zeldow, B., Cella, D., Barrett-Connor, E., Cauley, J. A., Crandall, J. P., Cunningham, G. R., Ensrud, K. E., Lewis, C. E., Matsumoto, A. M., Molitch, M. E., Pahor, M., Swerdloff, R. S., … Ellenberg, S. S. (2017). Association of Testosterone Levels With Anemia in Older Men: A Controlled Clinical Trial. JAMA internal medicine, 177(4), 480–490. https://doi.org/10.1001/jamainternmed.2016.9540[↩]
131. Zhang LT, Shin YS, Kim JY, Park JK. Could Testosterone Replacement Therapy in Hypogonadal Men Ameliorate Anemia, a Cardiovascular Risk Factor? An Observational, 54-Week Cumulative Registry Study. J Urol. 2016 Apr;195(4 Pt 1):1057-64. doi: 10.1016/j.juro.2015.10.130[↩]
132. Corona, G., Vignozzi, L., Sforza, A., & Maggi, M. (2013). Risks and benefits of late onset hypogonadism treatment: an expert opinion. The world journal of men’s health, 31(2), 103–125. https://doi.org/10.5534/wjmh.2013.31.2.103[↩]
133. Jones, T. H., Arver, S., Behre, H. M., Buvat, J., Meuleman, E., Moncada, I., Morales, A. M., Volterrani, M., Yellowlees, A., Howell, J. D., Channer, K. S., & TIMES2 Investigators (2011). Testosterone replacement in hypogonadal men with type 2 diabetes and/or metabolic syndrome (the TIMES2 study). Diabetes care, 34(4), 828–837. https://doi.org/10.2337/dc10-1233[↩]
134. Kim, M., Byun, S. S., & Hong, S. K. (2021). Testosterone Replacement Therapy in Men with Untreated or Treated Prostate Cancer: Do We Have Enough Evidences?. The world journal of men’s health, 39(4), 705–723. https://doi.org/10.5534/wjmh.190158[↩][↩]
135. Morgentaler A. Testosterone therapy for men at risk for or with history of prostate cancer. Curr Treat Options Oncol. 2006 Sep;7(5):363-9. doi: 10.1007/s11864-006-0004-y[↩]
136. Wang C, Nieschlag E, Swerdloff R, Behre HM, Hellstrom WJ, Gooren LJ, Kaufman JM, Legros JJ, Lunenfeld B, Morales A, Morley JE, Schulman C, Thompson IM, Weidner W, Wu FC; International Society of Andrology; International Society for the Study of Aging Male; European Association of Urology; European Academy of Andrology; American Society of Andrology. Investigation, treatment, and monitoring of late-onset hypogonadism in males: ISA, ISSAM, EAU, EAA, and ASA recommendations. Eur Urol. 2009 Jan;55(1):121-30. doi: 10.1016/j.eururo.2008.08.033[↩]
137. Khodamoradi, K., Khosravizadeh, Z., Parmar, M., Kuchakulla, M., Ramasamy, R., & Arora, H. (2021). Exogenous testosterone replacement therapy versus raising endogenous testosterone levels: current and future prospects. F&S reviews, 2(1), 32–42. https://doi.org/10.1016/j.xfnr.2020.11.001[↩][↩][↩]
138. Westaby D, Ogle SJ, Paradinas FJ, Randell JB, Murray-Lyon IM. Liver damage from long-term methyltestosterone. Lancet. 1977 Aug 6;2(8032):262-3.[↩][↩]
139. Korbonits M, Kipnes M, Grossman AB. Striant SR: a novel, effective and convenient testosterone therapy for male hypogonadism. Int J Clin Pract. 2004 Nov;58(11):1073-80. doi: 10.1111/j.1368-5031.2004.00383.x[↩][↩]
140. Swerdloff, R. S., Wang, C., White, W. B., Kaminetsky, J., Gittelman, M. C., Longstreth, J. A., Dudley, R. E., & Danoff, T. M. (2020). A New Oral Testosterone Undecanoate Formulation Restores Testosterone to Normal Concentrations in Hypogonadal Men. The Journal of clinical endocrinology and metabolism, 105(8), 2515–2531. https://doi.org/10.1210/clinem/dgaa238[↩][↩]
141. Zhang, M., Wang, J., Deng, C., Jiang, M. H., Feng, X., Xia, K., Li, W., Lai, X., Xiao, H., Ge, R. S., Gao, Y., & Xiang, A. P. (2017). Transplanted human p75-positive stem Leydig cells replace disrupted Leydig cells for testosterone production. Cell death & disease, 8(10), e3123. https://doi.org/10.1038/cddis.2017.531[↩][↩]
142. Dobs AS, Matsumoto AM, Wang C, Kipnes MS. Short-term pharmacokinetic comparison of a novel testosterone buccal system and a testosterone gel in testosterone deficient men. Curr Med Res Opin. 2004 May;20(5):729-38. doi: 10.1185/030079904125003494[↩][↩]
143. Dinsmore WW, Wyllie MG. The long-term efficacy and safety of a testosterone mucoadhesive buccal tablet in testosterone-deficient men. BJU Int. 2012 Jul;110(2):162-9. doi: 10.1111/j.1464-410X.2011.10837.x[↩][↩][↩][↩][↩][↩]
144. de Ronde W. Hyperandrogenism after transfer of topical testosterone gel: case report and review of published and unpublished studies. Hum Reprod. 2009 Feb;24(2):425-8. doi: 10.1093/humrep/den372[↩][↩]
145. Wang C, Cunningham G, Dobs A, Iranmanesh A, Matsumoto AM, Snyder PJ, Weber T, Berman N, Hull L, Swerdloff RS. Long-term testosterone gel (AndroGel) treatment maintains beneficial effects on sexual function and mood, lean and fat mass, and bone mineral density in hypogonadal men. J Clin Endocrinol Metab. 2004 May;89(5):2085-98. doi: 10.1210/jc.2003-032006[↩][↩][↩]
146. Fennell C, Sartorius G, Ly LP, Turner L, Liu PY, Conway AJ, Handelsman DJ. Randomized cross-over clinical trial of injectable vs. implantable depot testosterone for maintenance of testosterone replacement therapy in androgen deficient men. Clin Endocrinol (Oxf). 2010 Jul;73(1):102-9. doi: 10.1111/j.1365-2265.2009.03744.x[↩][↩]
147. Mazer N, Bell D, Wu J, Fischer J, Cosgrove M, Eilers B; BS, RN,. Comparison of the steady-state pharmacokinetics, metabolism, and variability of a transdermal testosterone patch versus a transdermal testosterone gel in hypogonadal men. J Sex Med. 2005 Mar;2(2):213-26. doi: 10.1111/j.1743-6109.2005.20231.x[↩]
148. Steidle C, Schwartz S, Jacoby K, Sebree T, Smith T, Bachand R; North American AA2500 T Gel Study Group. AA2500 testosterone gel normalizes androgen levels in aging males with improvements in body composition and sexual function. J Clin Endocrinol Metab. 2003 Jun;88(6):2673-81. doi: 10.1210/jc.2002-021058[↩]
149. Grober ED, Khera M, Soni SD, Espinoza MG, Lipshultz LI. Efficacy of changing testosterone gel preparations (Androgel or Testim) among suboptimally responsive hypogonadal men. Int J Impot Res. 2008 Mar-Apr;20(2):213-7. doi: 10.1038/sj.ijir.3901618[↩]
150. Kim ED, McCullough A, Kaminetsky J. Oral enclomiphene citrate raises testosterone and preserves sperm counts in obese hypogonadal men, unlike topical testosterone: restoration instead of replacement. BJU Int. 2016 Apr;117(4):677-85. doi: 10.1111/bju.13337[↩][↩]
151. Rogol AD, Tkachenko N, Bryson N. Natesto™ , a novel testosterone nasal gel, normalizes androgen levels in hypogonadal men. Andrology. 2016 Jan;4(1):46-54. doi: 10.1111/andr.12137. Epub 2015 Dec 22. Erratum in: Andrology. 2017 Jul;5(4):844.[↩]
152. Rogol, A. D., Tkachenko, N., Badorrek, P., Hohlfeld, J. M., & Bryson, N. (2018). Phase 1 pharmacokinetics and phase 3 efficacy of testosterone nasal gel in subjects with seasonal allergies. Canadian Urological Association journal = Journal de l’Association des urologues du Canada, 12(7), E349–E356. https://doi.org/10.5489/cuaj.4898[↩][↩]
153. Nieschlag E. Current topics in testosterone replacement of hypogonadal men. Best Pract Res Clin Endocrinol Metab. 2015 Jan;29(1):77-90. doi: 10.1016/j.beem.2014.09.008[↩][↩]
154. Nieschlag E, Mauss J, Coert A, Kićović P. Plasma androgen levels in men after oral administration of testosterone or testosterone undecanoate. Acta Endocrinol (Copenh). 1975 Jun;79(2):366-74. doi: 10.1530/acta.0.0790366[↩]
155. Tsametis CP, Isidori AM. Testosterone replacement therapy: For whom, when and how? Metabolism. 2018 Sep;86:69-78. doi: 10.1016/j.metabol.2018.03.007[↩][↩][↩][↩][↩][↩][↩][↩][↩]
156. Behre HM, Abshagen K, Oettel M, Hübler D, Nieschlag E. Intramuscular injection of testosterone undecanoate for the treatment of male hypogonadism: phase I studies. Eur J Endocrinol. 1999 May;140(5):414-9. doi: 10.1530/eje.0.1400414[↩]
157. Middleton T, Turner L, Fennell C, Savkovic S, Jayadev V, Conway AJ, Handelsman DJ. Complications of injectable testosterone undecanoate in routine clinical practice. Eur J Endocrinol. 2015 May;172(5):511-7. doi: 10.1530/EJE-14-0891[↩]
158. Mackey MA, Conway AJ, Handelsman DJ. Tolerability of intramuscular injections of testosterone ester in oil vehicle. Hum Reprod. 1995 Apr;10(4):862-5. doi: 10.1093/oxfordjournals.humrep.a136051[↩]
159. Seftel A. Testosterone replacement therapy for male hypogonadism: part III. Pharmacologic and clinical profiles, monitoring, safety issues, and potential future agents. Int J Impot Res. 2007 Jan-Feb;19(1):2-24. doi: 10.1038/sj.ijir.3901366[↩]
160. Kelleher S, Turner L, Howe C, Conway AJ, Handelsman DJ. Extrusion of testosterone pellets: a randomized controlled clinical study. Clin Endocrinol (Oxf). 1999 Oct;51(4):469-71. doi: 10.1046/j.1365-2265.1999.00827.x[↩]
161. Gronski, M. A., Grober, E. D., Gottesman, I. S., Ormsby, R. W., & Bryson, N. (2019). Efficacy of Nasal Testosterone Gel (Natesto®) Stratified by Baseline Endogenous Testosterone Levels. Journal of the Endocrine Society, 3(9), 1652–1662. https://doi.org/10.1210/js.2019-00183[↩]
162. Ramasamy R, Masterson TA, Best JC, Bitran J, Ibrahim E, Molina M, Kaiser UB, Miao F, Reis IM. Effect of Natesto on Reproductive Hormones, Semen Parameters and Hypogonadal Symptoms: A Single Center, Open Label, Single Arm Trial. J Urol. 2020 Sep;204(3):557-563. doi: 10.1097/JU.0000000000001078[↩]
163. Ribeiro MA, Gameiro LF, Scarano WR, Briton-Jones C, Kapoor A, Rosa MB, El Dib R. Aromatase inhibitors in the treatment of oligozoospermic or azoospermic men: a systematic review of randomized controlled trials. JBRA Assist Reprod. 2016 May 1;20(2):82-8. doi: 10.5935/1518-0557.20160019[↩][↩][↩][↩]
164. Zumoff B, Miller LK, Strain GW. Reversal of the hypogonadotropic hypogonadism of obese men by administration of the aromatase inhibitor testolactone. Metabolism. 2003 Sep;52(9):1126-8. doi: 10.1016/s0026-0495(03)00186-0[↩][↩]
165. Majzoub, A., & Sabanegh, E., Jr (2016). Testosterone replacement in the infertile man. Translational andrology and urology, 5(6), 859–865. https://doi.org/10.21037/tau.2016.08.03[↩]
166. Miner JN, Chang W, Chapman MS, Finn PD, Hong MH, López FJ, Marschke KB, Rosen J, Schrader W, Turner R, van Oeveren A, Viveros H, Zhi L, Negro-Vilar A. An orally active selective androgen receptor modulator is efficacious on bone, muscle, and sex function with reduced impact on prostate. Endocrinology. 2007 Jan;148(1):363-73. doi: 10.1210/en.2006-0793[↩][↩]
167. Pasqualotto FF, Lucon AM, Sobreiro BP, Pasqualotto EB, Arap S. Effects of medical therapy, alcohol, smoking, and endocrine disruptors on male infertility. Rev Hosp Clin Fac Med Sao Paulo. 2004 Dec;59(6):375-82. doi: 10.1590/s0041-87812004000600011[↩][↩]
168. Bhasin, S., & Jasuja, R. (2009). Selective androgen receptor modulators as function promoting therapies. Current opinion in clinical nutrition and metabolic care, 12(3), 232–240. https://doi.org/10.1097/MCO.0b013e32832a3d79[↩]
169. Narayanan, R., Coss, C. C., & Dalton, J. T. (2018). Development of selective androgen receptor modulators (SARMs). Molecular and cellular endocrinology, 465, 134–142. https://doi.org/10.1016/j.mce.2017.06.013[↩][↩]
170. Solomon, Z. J., Mirabal, J. R., Mazur, D. J., Kohn, T. P., Lipshultz, L. I., & Pastuszak, A. W. (2019). Selective Androgen Receptor Modulators: Current Knowledge and Clinical Applications. Sexual medicine reviews, 7(1), 84–94. https://doi.org/10.1016/j.sxmr.2018.09.006[↩][↩][↩][↩]
171. Winters SJ, Moore JP Jr, Clark BJ. Leydig cell insufficiency in hypospermatogenesis: a paracrine effect of activin-inhibin signaling? Andrology. 2018 Mar;6(2):262-271. doi: 10.1111/andr.12459[↩][↩]
172. Beattie, M. C., Adekola, L., Papadopoulos, V., Chen, H., & Zirkin, B. R. (2015). Leydig cell aging and hypogonadism. Experimental gerontology, 68, 87–91. https://doi.org/10.1016/j.exger.2015.02.014[↩][↩]
173. Arora, H., Zuttion, M., Nahar, B., Lamb, D., Hare, J. M., & Ramasamy, R. (2019). Subcutaneous Leydig Stem Cell Autograft: A Promising Strategy to Increase Serum Testosterone. Stem cells translational medicine, 8(1), 58–65. https://doi.org/10.1002/sctm.18-0069[↩][↩][↩][↩]
174. El Meliegy, A., Motawi, A., & El Salam, M. (2017). Systematic review of hormone replacement therapy in the infertile man. Arab journal of urology, 16(1), 140–147. https://doi.org/10.1016/j.aju.2017.11.011[↩][↩][↩][↩][↩]
175. Chehab M, Madala A, Trussell JC. On-label and off-label drugs used in the treatment of male infertility. Fertil Steril. 2015 Mar;103(3):595-604. doi: 10.1016/j.fertnstert.2014.12.122[↩]
176. Martinkovich, S., Shah, D., Planey, S. L., & Arnott, J. A. (2014). Selective estrogen receptor modulators: tissue specificity and clinical utility. Clinical interventions in aging, 9, 1437–1452. https://doi.org/10.2147/CIA.S66690[↩]
177. Pickar JH, MacNeil T, Ohleth K. SERMs: progress and future perspectives. Maturitas. 2010 Oct;67(2):129-38. doi: 10.1016/j.maturitas.2010.05.009[↩]
178. Dohle G, Arver S, Bettocchi C, Jones T, Kliesch S, Punab M. EAU guidelines on male hypogonadism European Association of Urology, Arnhem, The Netherlands, accessed Mar 2016;21:2018.[↩]
179. Lee, J. A., & Ramasamy, R. (2018). Indications for the use of human chorionic gonadotropic hormone for the management of infertility in hypogonadal men. Translational andrology and urology, 7(Suppl 3), S348–S352. https://doi.org/10.21037/tau.2018.04.11[↩]
180. Crosnoe-Shipley, L. E., Elkelany, O. O., Rahnema, C. D., & Kim, E. D. (2015). Treatment of hypogonadotropic male hypogonadism: Case-based scenarios. World journal of nephrology, 4(2), 245–253. https://doi.org/10.5527/wjn.v4.i2.245[↩]
181. Hsieh TC, Pastuszak AW, Hwang K, Lipshultz LI. Concomitant intramuscular human chorionic gonadotropin preserves spermatogenesis in men undergoing testosterone replacement therapy. J Urol. 2013 Feb;189(2):647-50. doi: 10.1016/j.juro.2012.09.043[↩]
182. Coviello AD, Matsumoto AM, Bremner WJ, Herbst KL, Amory JK, Anawalt BD, Sutton PR, Wright WW, Brown TR, Yan X, Zirkin BR, Jarow JP. Low-dose human chorionic gonadotropin maintains intratesticular testosterone in normal men with testosterone-induced gonadotropin suppression. J Clin Endocrinol Metab. 2005 May;90(5):2595-602. doi: 10.1210/jc.2004-0802[↩][↩]
183. Leder BZ, Rohrer JL, Rubin SD, Gallo J, Longcope C. Effects of aromatase inhibition in elderly men with low or borderline-low serum testosterone levels. J Clin Endocrinol Metab. 2004 Mar;89(3):1174-80. doi: 10.1210/jc.2003-031467[↩]
184. Crosnoe, L. E., Grober, E., Ohl, D., & Kim, E. D. (2013). Exogenous testosterone: a preventable cause of male infertility. Translational andrology and urology, 2(2), 106–113. https://doi.org/10.3978/j.issn.2223-4683.2013.06.01[↩]
185. Jones, A., Chen, J., Hwang, D. J., Miller, D. D., & Dalton, J. T. (2009). Preclinical characterization of a (S)-N-(4-cyano-3-trifluoromethyl-phenyl)-3-(3-fluoro, 4-chlorophenoxy)-2-hydroxy-2-methyl-propanamide: a selective androgen receptor modulator for hormonal male contraception. Endocrinology, 150(1), 385–395. https://doi.org/10.1210/en.2008-0674[↩]
186. Chen J, Hwang DJ, Bohl CE, Miller DD, Dalton JT. A selective androgen receptor modulator for hormonal male contraception. J Pharmacol Exp Ther. 2005 Feb;312(2):546-53. doi: 10.1124/jpet.104.075424[↩]
187. Morimoto, M., Amano, Y., Oka, M., Harada, A., Fujita, H., Hikichi, Y., Tozawa, R., Yamaoka, M., & Hara, T. (2017). Amelioration of sexual behavior and motor activity deficits in a castrated rodent model with a selective androgen receptor modulator SARM-2f. PloS one, 12(12), e0189480. https://doi.org/10.1371/journal.pone.0189480[↩]
188. Zhou R, Wu J, Liu B, Jiang Y, Chen W, Li J, He Q, He Z. The roles and mechanisms of Leydig cells and myoid cells in regulating spermatogenesis. Cell Mol Life Sci. 2019 Jul;76(14):2681-2695. doi: 10.1007/s00018-019-03101-9[↩]
189. Chen, H., Ge, R. S., & Zirkin, B. R. (2009). Leydig cells: From stem cells to aging. Molecular and cellular endocrinology, 306(1-2), 9–16. https://doi.org/10.1016/j.mce.2009.01.023[↩][↩]
190. Chen, H., Stanley, E., Jin, S., & Zirkin, B. R. (2010). Stem Leydig cells: from fetal to aged animals. Birth defects research. Part C, Embryo today : reviews, 90(4), 272–283. https://doi.org/10.1002/bdrc.20192[↩]
191. Neaves WB. Leydig cells. Contraception. 1975 May;11(5):571-606. doi: 10.1016/0010-7824(75)90111-0[↩]
192. Jiang, M. H., Cai, B., Tuo, Y., Wang, J., Zang, Z. J., Tu, X., Gao, Y., Su, Z., Li, W., Li, G., Zhang, M., Jiao, J., Wan, Z., Deng, C., Lahn, B. T., & Xiang, A. P. (2014). Characterization of Nestin-positive stem Leydig cells as a potential source for the treatment of testicular Leydig cell dysfunction. Cell research, 24(12), 1466–1485. https://doi.org/10.1038/cr.2014.149[↩]
193. Bhasin S, Brito JP, Cunningham GR, Hayes FJ, Hodis HN, Matsumoto AM, Snyder PJ, Swerdloff RS, Wu FC, Yialamas MA. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J. Clin. Endocrinol. Metab. 2018 May 01;103(5):1715-1744[↩][↩][↩]
194. Golds G, Houdek D, Arnason T. Male Hypogonadism and Osteoporosis: The Effects, Clinical Consequences, and Treatment of Testosterone Deficiency in Bone Health. Int J Endocrinol. 2017;2017:4602129[↩]
195. Tan RS, Cook KR, Reilly WG. High estrogen in men after injectable testosterone therapy: the low T experience. Am J Mens Health. 2015 May;9(3):229-34[↩]
196. Ponce OJ, Spencer-Bonilla G, Alvarez-Villalobos N, Serrano V, Singh-Ospina N, Rodriguez-Gutierrez R, Salcido-Montenegro A, Benkhadra R, Prokop LJ, Bhasin S, Brito JP. The efficacy and adverse events of testosterone replacement therapy in hypogonadal men: A systematic review and meta-analysis of randomized, placebo-controlled trials. J. Clin. Endocrinol. Metab. 2018 Mar 17[↩]
197. Rosano GM, Sheiban I, Massaro R, Pagnotta P, Marazzi G, Vitale C, Mercuro G, Volterrani M, Aversa A, Fini M. Low testosterone levels are associated with coronary artery disease in male patients with angina. Int. J. Impot. Res. 2007 Mar-Apr;19(2):176-82[↩]
198. Cheetham TC, An J, Jacobsen SJ, Niu F, Sidney S, Quesenberry CP, VanDenEeden SK. Association of Testosterone Replacement With Cardiovascular Outcomes Among Men With Androgen Deficiency. JAMA Intern Med. 2017 Apr 01;177(4):491-499.[↩]
199. Goodman N, Guay A, Dandona P, Dhindsa S, Faiman C, Cunningham GR., AACE Reproductive Endocrinology Scientific Committee. AMERICAN ASSOCIATION OF CLINICAL ENDOCRINOLOGISTS AND AMERICAN COLLEGE OF ENDOCRINOLOGY POSITION STATEMENT ON THE ASSOCIATION OF TESTOSTERONE AND CARDIOVASCULAR RISK. Endocr Pract. 2015 Sep;21(9):1066-73.[↩]
200. Hsu B, Cumming RG, Handelsman DJ. Testosterone, frailty and physical function in older men. Expert Rev Endocrinol Metab. 2018 May;13(3):159-165.[↩]
201. Bhasin S, Brito JP, Cunningham GR, Hayes FJ, Hodis HN, Matsumoto AM, Snyder PJ, Swerdloff RS, Wu FC, Yialamas MA. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J. Clin. Endocrinol. Metab. 2018 May 01;103(5):1715-1744.[↩]
202. Almaiman A. A. (2018). Effect of testosterone boosters on body functions: Case report. International journal of health sciences, 12(2), 86–90. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5870326[↩][↩]
203. Rahnema CD, Crosnoe LE, Kim ED. Designer steroids – over-the-counter supplements and their androgenic component: review of an increasing problem. Andrology. 2015 Mar;3(2):150-5. https://doi.org/10.1111/andr.307[↩]
204. Willoughby, D. S., Spillane, M., & Schwarz, N. (2014). Heavy Resistance Training and Supplementation With the Alleged Testosterone Booster Nmda has No Effect on Body Composition, Muscle Performance, and Serum Hormones Associated With the Hypothalamo-Pituitary-Gonadal Axis in Resistance-Trained Males. Journal of sports science & medicine, 13(1), 192–199. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3918557[↩][↩][↩]
205. Pasiakos SM, Berryman CE, Karl JP, Lieberman HR, Orr JS, Margolis LM, Caldwell JA, Young AJ, Montano MA, Evans WJ, Vartanian O, Carmichael OT, Gadde KM, Harris M, Rood JC. Physiological and psychological effects of testosterone during severe energy deficit and recovery: A study protocol for a randomized, placebo-controlled trial for Optimizing Performance for Soldiers (OPS). Contemp Clin Trials. 2017 Jul;58:47-57. doi: 10.1016/j.cct.2017.05.001[↩]
206. Regan, K. S., Wambogo, E. A., & Haggans, C. J. (2011). NIH and USDA funding of dietary supplement research, 1999-2007. The Journal of nutrition, 141(1), 1–3. https://doi.org/10.3945/jn.110.134106[↩]
207. Aaberg RA, Sauer MV, Sikka S, Rajfer J. Effects of extracellular ionized calcium, diltiazem and cAMP on motility of human spermatozoa. J Urol. 1989 May;141(5):1221-4. doi: 10.1016/s0022-5347(17)41225-0[↩]
208. Clemesha, C. G., Thaker, H., & Samplaski, M. K. (2020). ‘Testosterone Boosting’ Supplements Composition and Claims Are not Supported by the Academic Literature. The world journal of men’s health, 38(1), 115–122. https://doi.org/10.5534/wjmh.190043[↩][↩][↩][↩][↩]
209. Calfee R, Fadale P. Popular ergogenic drugs and supplements in young athletes. Pediatrics. 2006 Mar;117(3):e577-89. doi: 10.1542/peds.2005-1429[↩]
210. Nguyen, S. M., Ko Ko, N., Sattar, A. S., Gucuk Ipek, E., & Ali, S. (2017). Pulmonary Embolism Secondary to Testosterone-Enhancing Herbal Supplement Use. Cureus, 9(8), e1545. https://doi.org/10.7759/cureus.1545[↩]
211. Chalasani, N., Fontana, R. J., Bonkovsky, H. L., Watkins, P. B., Davern, T., Serrano, J., Yang, H., Rochon, J., & Drug Induced Liver Injury Network (DILIN) (2008). Causes, clinical features, and outcomes from a prospective study of drug-induced liver injury in the United States. Gastroenterology, 135(6), 1924–1934.e19344. https://doi.org/10.1053/j.gastro.2008.09.011[↩]
212. Balasubramanian, A., Thirumavalavan, N., Srivatsav, A., Yu, J., Lipshultz, L. I., & Pastuszak, A. W. (2019). Testosterone Imposters: An Analysis of Popular Online Testosterone Boosting Supplements. The journal of sexual medicine, 16(2), 203–212. https://doi.org/10.1016/j.jsxm.2018.12.008[↩][↩][↩][↩]
213. Hooper PL, Visconti L, Garry PJ, Johnson GE. Zinc lowers high-density lipoprotein-cholesterol levels. JAMA. 1980 Oct 24-31;244(17):1960-1[↩]
214. Fosmire GJ. Zinc toxicity. Am J Clin Nutr. 1990 Feb;51(2):225-7. doi: 10.1093/ajcn/51.2.225[↩]
215. D’Aniello A. D-Aspartic acid: an endogenous amino acid with an important neuroendocrine role. Brain Res Rev. 2007 Feb;53(2):215-34. doi: 10.1016/j.brainresrev.2006.08.005[↩][↩][↩]
216. Rahnema, C.D., Crosnoe, L.E. and Kim, E.D. (2015), Designer steroids – over-the-counter supplements and their androgenic component: review of an increasing problem. Andrology, 3: 150-155. https://doi.org/10.1111/andr.307[↩]
217. Katane M, Homma H. D-Aspartate–an important bioactive substance in mammals: a review from an analytical and biological point of view. J Chromatogr B Analyt Technol Biomed Life Sci. 2011 Nov 1;879(29):3108-21. doi: 10.1016/j.jchromb.2011.03.062[↩]
218. Raucci F, D’Aniello S, Di Fiore MM. Endocrine roles of D-aspartic acid in the testis of lizard Podarcis s. sicula. J Endocrinol. 2005 Dec;187(3):347-59. doi: 10.1677/joe.1.06115[↩]
219. Roshanzamir, F., & Safavi, S. M. (2017). The putative effects of D-Aspartic acid on blood testosterone levels: A systematic review. International journal of reproductive biomedicine, 15(1), 1–10. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5340133[↩][↩]