What is radioactive iodine

Iodine, in the form of iodide, is made into two radioactive forms of iodine that are commonly used in patients with thyroid diseases: Radioactive iodine I-123 (Radioiodine I-123) are harmless to thyroid cells and Radioactive iodine I-131 (Radioiodine I-131) destroys thyroid cells. The radiation emitted by each of these forms of iodine can be detected from outside the patient to gain information about thyroid function and take pictures of the size and location of thyroid tissues. Radioactive iodine is safe to use in individuals who have had allergic reactions to seafood or X-ray contrast agents, since the reaction is to the compound containing iodine, not the iodine itself. Radioactive iodine is given by mouth in pill or liquid form.

The thyroid is a gland in the neck that produces two hormones that regulate all aspects of the body’s metabolism, the chemical process of converting food into energy. When a thyroid gland is overactive, it produces too much of these hormones, accelerating the metabolism.

Radioactive iodine I-131 (I-131), an isotope of iodine that emits radiation, is used for medical purposes. When a small dose of I-131 is swallowed, it is absorbed into the bloodstream in the gastrointestinal (GI) tract and concentrated from the blood by the thyroid gland, where it begins destroying the gland’s cells.

Radioactive iodine I-131 may also be used to treat thyroid cancer.

Radioactive iodine uptake test

Radioactive iodine-123 (I-123) is the usual isotope used to take pictures and determine the activity of the intact thyroid gland (Thyroid Scan and Radioactive Iodine Uptake, RAIU), since it is harmless to thyroid cells. The 2 tests are commonly performed together, but they can be done separately. No special radiation precautions are necessary after a thyroid scan or radioactive iodine uptake test using I-123. Iodine-131 (I-131) can also be used to take pictures of the thyroid gland, although it is rarely used due to the harmful effects it has on thyroid cells.

The thyroid scan and radioactive iodine uptake test (RAIU) are used to determine the size, shape and position of the thyroid gland. The thyroid uptake is performed to evaluate the function of the thyroid gland. A whole-body thyroid scan is typically performed on people who have or had thyroid cancer.

A physician may perform these radioactive iodine uptake test tests to:

• determine if the thyroid gland is working properly
• help diagnose problems with the thyroid gland, such as an overactive thyroid gland, a condition called hyperthyroidism, cancer or other growths
• assess the nature of a nodule discovered in the gland
• detect areas of abnormality, such as lumps (nodules) or inflammation
• determine whether thyroid cancer has spread beyond the thyroid gland
• evaluate changes in the gland following medication use, surgery, radiotherapy or chemotherapy.

The radioactive iodine I-123 leaves your body through your urine. You should not need to take special precautions, such as flushing twice after urinating, for 24 to 48 hours after the test. Ask your provider or the radiology/nuclear medicine team performing the scan about taking precautions.

How should I prepare?

Follow instructions about not eating before the test. You may be told not to eat after midnight the night before your test.

Your health care provider will tell you if you need to stop taking medicines before the test that may affect your test results. DO NOT stop taking any medicine without first talking to your provider.

You may be asked to wear a gown during the exam or you may be allowed to wear your own clothing.

Women should always inform their physician or technologist if there is any possibility that they are pregnant or if they are breastfeeding.

You should inform your physician and the technologist performing your exam of any medications you are taking, including vitamins and herbal supplements. You should also inform them if you have any allergies and about recent illnesses or other medical conditions.

You should tell your physician if you:

• have had any tests, such as an x-ray or CT scan, surgeries or treatments using iodinated contrast material within the last two months.
• are taking medications or ingesting other substances that contain iodine, including kelp, seaweed, cough syrups, multivitamins or heart medications.
• have any allergies to iodine, medications and anesthetics.
• are breastfeeding.

In the days prior to your examination, blood tests may be performed to measure the level of thyroid hormones in your blood. You may be told not to eat for several hours before your exam because eating can affect the accuracy of the uptake measurement.

Jewelry and other metallic accessories should be left at home if possible, or removed prior to the exam because they may interfere with the procedure.

You will receive specific instructions based on the type of scan you are undergoing.

How is the radioactive iodine uptake test performed?

Nuclear medicine imaging is usually performed on an outpatient basis, but is often performed on hospitalized patients as well.

The radioactive iodine uptake test is done in this way:

• You are given a pill that contains a tiny amount of radioactive iodine. After swallowing it, you wait as the iodine collects in the thyroid.
• The first uptake is usually done 4 to 6 hours after you take the iodine pill. Another uptake is usually done 24 hours later. During the uptake, you lie on your back on a table. A device called a gamma probe is moved back and forth over the area of your neck where the thyroid gland is located.
• The probe detects the location and intensity of the rays given off by the radioactive material. A computer displays how much of the tracer is taken up by the thyroid gland.

The radioactive iodine uptake test takes less than 30 minutes.

You can eat beginning about 1 to 2 hours after swallowing the radioactive iodine. You can go back to a normal diet after the test.

Thyroid Scan

When it is time for the imaging to begin, you will lie down on a moveable examination table with your head tipped backward and neck extended. The gamma camera will then take a series of images, capturing images of the thyroid gland from three different angles. You will need to remain still for brief periods of time while the camera is taking pictures.

When the examination is completed, you may be asked to wait until the technologist checks the images in case additional images are needed. Occasionally, more images are obtained for clarification or better visualization of certain areas or structures. The need for additional images does not necessarily mean there was a problem with the exam or that something abnormal was found, and should not be a cause of concern for you.

If you had an intravenous line inserted for the procedure, it will usually be removed unless you are scheduled for an additional procedure that same day that requires an intravenous line.

Actual scanning time for a thyroid scan is 30 minutes or less.

Thyroid Uptake

You will be given radioactive iodine (I-123) in liquid or capsule form to swallow. The thyroid uptake will begin several hours to 24 hours later. Often, two separate uptake measurements are obtained at different times. For example, you may have uptake measurements at four to six hours and 24 hours.

When it is time for the imaging to begin, you will sit in a chair facing a stationary probe positioned over the thyroid gland in the neck.

When the examination is completed, you may be asked to wait until the technologist checks the images in case additional images are needed. Occasionally, more images are obtained for clarification or better visualization of certain areas or structures. The need for additional images does not necessarily mean there was a problem with the exam or that something abnormal was found, and should not be a cause of concern for you.

Actual scanning time for each thyroid uptake is five minutes or less.

What will I experience during and after the radioactive iodine uptake procedure?

Most thyroid scan and thyroid uptake procedures are painless. However, during the thyroid scan, you may feel uncomfortable when lying completely still with your head extended backward while the gamma camera is taking images.

When swallowed, the radioactive iodine has little or no taste. When inhaled, you should feel no differently than when breathing room air or holding your breath.

It is important that you remain still while the images are being recorded. Though nuclear imaging itself causes no pain, there may be some discomfort from having to remain still or to stay in one particular position during imaging.

Unless your physician tells you otherwise, you may resume your normal activities after your nuclear medicine scan. If any special instructions are necessary, you will be informed by a technologist, nurse or physician before you leave the nuclear medicine department.

Through the natural process of radioactive decay, the small amount of radiotracer in your body will lose its radioactivity over time. It may also pass out of your body through your urine or stool during the first few hours or days following the test. You should also drink plenty of water to help flush the radioactive material out of your body as instructed by the nuclear medicine personnel.

What are the limitations of the radioactive iodine thyroid scan and uptake?

The radioactive iodine thyroid scan and thyroid uptake are not performed on patients who are pregnant because of the risk of exposing the fetus to radiation. These tests are also not recommended for breastfeeding women.

Nuclear medicine procedures can be time consuming. It can take several hours to days for the radiotracer to accumulate in the body part of interest and imaging may take up to several hours to perform, though in some cases, newer equipment is available that can substantially shorten the procedure time.

The resolution of structures of the body with nuclear medicine may not be as high as with other imaging techniques, such as CT or MRI. However, nuclear medicine scans are more sensitive than other techniques for a variety of indications, and the functional information gained from nuclear medicine exams is often unobtainable by other imaging techniques.

What are the benefits vs. risks radioactive iodine uptake test?

Benefits

• Nuclear medicine examinations provide unique information—including details on both function and anatomic structure of the body that is often unattainable using other imaging procedures.
• For many diseases, nuclear medicine scans yield the most useful information needed to make a diagnosis or to determine appropriate treatment, if any.
• A nuclear medicine scan is less expensive and may yield more precise information than exploratory surgery.

Risks

• All radiation has possible side effects. The amount of radiation in this test is very small, and there have been no documented side effects.
• Nuclear medicine diagnostic procedures have been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure.
• The risks of the treatment are always weighed against the potential benefits for nuclear medicine therapeutic procedures. You will be informed of all significant risks prior to the treatment and have an opportunity to ask questions.
• Allergic reactions to radiopharmaceuticals may occur but are extremely rare and are usually mild. Nevertheless, you should inform the nuclear medicine personnel of any allergies you may have or other problems that may have occurred during a previous nuclear medicine exam.
• Injection of the radiotracer may cause slight pain and redness which should rapidly resolve.
• Women who are pregnant or breastfeeding should not have this test. Women should always inform their physician or radiology technologist if there is any possibility that they are pregnant or if they are breastfeeding.

Talk to your doctor if you have concerns about this test.

Normal Results radioactive iodine thyroid scan or uptake results

These are normal results at 6 and 24 hours after swallowing the radioactive iodine:

• At 6 hours: 3% to 16%
• At 24 hours: 8% to 25%

Some testing centers measure only at 24 hours. Values may vary depending on the amount of iodine in your diet. Normal value ranges may vary slightly among different labs. Talk to your provider about the meaning of your specific test results.

What abnormal results mean

Higher-than-normal uptake may be due to an overactive thyroid gland. The most common cause is Graves disease.

Other conditions can cause some areas of higher-than-normal uptake in the thyroid gland. These include:

• An enlarged thyroid gland that contains nodules producing too much thyroid hormone (toxic nodular goiter)
• A single thyroid nodule that is producing too much thyroid hormone (toxic adenoma)

These conditions often result in normal uptake, but the uptake is concentrated into a few (hot) areas while the rest of the thyroid gland does not take up any iodine (cold areas). This can only be determined if the scan is done along with the uptake test.

Lower-than-normal uptake may be due to:

• Factitious hyperthyroidism (taking too much thyroid hormone medicine or supplements)
• Iodine overload
• Subacute thyroiditis (swelling or inflammation of the thyroid gland)
• Silent (or painless) thyroiditis
• Amiodarone (medicine to treat some kinds of heart disease)

Radioactive iodine treatment

Radioactive Iodine I-131 (also called Radioiodine I-131) treatment is a nuclear medicine treatment for an overactive thyroid, a condition called hyperthyroidism, and also may be used to treat thyroid cancer. When a small dose of radioactive iodine I-131 (an isotope of iodine that emits radiation) is swallowed, it is absorbed into the bloodstream and concentrated by the thyroid gland, where it begins destroying the gland’s cells.

Your doctor will instruct you on how to prepare, how to take any necessary radiation safety precautions, and when to stop taking anti-thyroid medications. Tell your doctor if there’s a possibility you are pregnant or if you are breastfeeding and discuss any recent illnesses, medical conditions, allergies and medications you’re taking. Little to no special preparation is required for this procedure. However, you should not eat or drink anything after midnight on the day of treatment.

Patients may experience some pain in the thyroid after radioactive iodine I-131 (radioiodine I-131) therapy similar to a sore throat. You should ask your physician to recommend an over-the-counter pain reliever to treat this pain, should it occur.

Thyroid tissue

Radioactive iodine-131 (I-131) is given to destroy overactive thyroid tissue (hyperthyroidism) or to shrink thyroid glands that are functioning normally but are causing problems because of their size (goiter). Hyperthyroidism can be caused by Graves’ disease, in which the entire thyroid gland is overactive, or by nodules within the gland which are locally overactive in producing too much thyroid hormone. Patients are asked to follow some radiation precautions after treatment in order to limit radiation exposure to others (see Table 1 below). I-131 may occasionally cause mild pain in the neck that can be treated with aspirin, ibuprofen or acetaminophen. The radioactive iodine-131 (I-131) treatment may take up to several months to have its effect. Frequently, the end result of radioactive iodine-131 (I-131) treatment of hyperthyroidism is hypothyroidism, which is treated by thyroid hormone replacement (see Hypothyroidism brochure).

Thyroid cancer

Large doses of radioactive iodine-131 (I-131) are used to destroy thyroid cancer cells. This is performed after the remaining thyroid cells (including any cancer cells) are stimulated by raising TSH levels by either withdrawing the thyroid hormone pills or by treating with recombinant human TSH. Patients are asked to follow some radiation precautions after treatment in order to limit radiation exposure to others. Depending on state regulations, patients may have to stay isolated in the hospital for about 24 hours to avoid exposing other people to radiation, especially if there are young children living in the same home.

Is there any special preparation needed for the radioactive iodine treatment?

You should not eat or drink after midnight on the day of the radioactive iodine treatment procedure. If you have been taking anti-thyroid medications, you must stop at least three days before the therapy is given. Frequently, the anti-thyroid medication is stopped for five to seven days before therapy.

You will be able to return home following radioactive iodine treatment, but you should avoid prolonged, close contact with other people for several days, particularly pregnant women and small children. The majority of the radioactive iodine that has not been absorbed leaves the body during the first two days following the treatment, primarily through the urine. Small amounts will also be excreted in saliva, sweat, tears, vaginal secretions, and feces.

If your work or daily activities involve prolonged contact with small children or pregnant women, you will want to wait several days after your treatment to resume these activities. Patients with infants at home should arrange for care to be provided by another person for the first several days after treatment. Your radiologist can be more specific for your given situation, but usually this time period is only two to five days.

Your treatment team will give you a list of other precautions to take following your treatment with I-131. The following guidelines comply with the Nuclear Regulatory Commission:

• Use private toilet facilities, if possible, and flush twice after each use.
• Bathe daily and wash hands frequently.
• Drink a normal amount of fluids.
• Use disposable eating utensils or wash your utensils separately from others.
• Sleep alone and avoid prolonged intimate contact for three or four days. Brief periods of close contact, such as handshaking and hugging, are permitted.
• Launder your linens, towels, and clothes daily at home, separately. No special cleaning of the washing machine is required between loads.
• Do not prepare food for others that requires prolonged handling with bare hands.
• If you are breast-feeding, you must stop several days before to ensure that milk production has also stopped.
• You should avoid becoming pregnant from six months to one year after treatment.
• You must be sure you are not pregnant before receiving I-131. Many facilities require a pregnancy test within 24 hours prior to giving I-131 in all women of child-bearing age who have not had a surgical procedure to prevent pregnancy.

Patients who need to travel immediately after radioactive iodine treatment are advised to carry a letter of explanation from their physician. Radiation detection devices used at airports and federal buildings may be sensitive to the radiation levels present in patients up to three months following treatment with I-131. Depending on the amount of radioactivity administered during your treatment, your endocrinologist or radiation safety officer may recommend continued precautions for up to several weeks after treatment.

Radioiodine therapy is not used in a patient who is pregnant. Depending on the stage of pregnancy, I-131 given to the mother may damage the baby’s thyroid gland. In such a case, discussion of the issue with the patient’s physician is advised. When given to a nursing mother, radioactive iodine can reach a baby through her breast milk. Most physicians feel that this procedure should not be used in women who are breastfeeding unless they are willing to cease breastfeeding their newborn. Also, it is recommended that pregnancy be delayed until at least six to 12 months after I-131 treatment.

Women who have not yet reached menopause should fully discuss the use of I-131 with their physician.

How is the radioactive iodine treatment procedure performed?

Treatment for hyperthyroidism is almost always done on an outpatient basis because the dose required is relatively small.

The radioiodine I-131 is swallowed in a single dose, in capsule or liquid form, and is quickly absorbed into the bloodstream in the gastrointestinal (GI) tract and concentrated from the blood by the thyroid gland, where it begins destroying the gland’s cells. Although the radioactivity from this treatment remains in the thyroid for some time, it is greatly diminished within a few days. The effect of this treatment on the thyroid gland usually takes between one and three months to develop, with maximum benefit occurring three to six months after treatment. Usually, a single dose is successful in treating hyperthyroidism. However, rarely, a second treatment is needed, and very rarely a third treatment may be needed.

Radioactive iodine treatment side effects

It is highly likely that some or most of the thyroid gland will be destroyed with radioiodine I-131 treatment procedure. Since hormones produced by the thyroid are essential for metabolism, most patients will need to take thyroid pills for the rest of their life following the procedure. Thyroid pills are inexpensive, and patients will typically be instructed to take one per day. There are essentially no other permanent side effects from the radioiodine I-131 treatment procedure.

Special concerns for women

Radioactive iodine, whether I-123 or I-131, should never be used in a patient who is pregnant or nursing. This protects the baby who would otherwise receive radioactive milk and the mother’s breasts which concentrate radioactive iodine-131 (I-131). Breastfeeding must be stopped at least 6 weeks before administration of radioactive iodine-131 (I-131) treatment and should not be restarted after administration of radioactive iodine-131 (I-131), but can be safely done after future pregnancies. Also, pregnancy should be put off until at least 6 – 12 months after radioactive iodine-131 (I-131) treatment since the ovaries are exposed to radiation after the treatment and to ensure that thyroid hormone levels are normal and stable prior to pregnancy. There is no clear evidence that radioactive iodine-131 (I-131) leads to infertility.

Special concerns for men

Men who receive radioactive iodine-131 (I-131) treatment for thyroid cancer may have decreased sperm counts and temporary infertility for periods of roughly two years. Sperm banking is an option in a patient who is expected to need several doses of radioactive iodine-131 (I-131) for thyroid cancer.

Radioactive iodine treatment side effects long term

In general, radioactive iodine-131 (I-131) is a safe and effective treatment for the thyroid disorders mentioned above. Hypothyroidism is a common side effect of radioactive iodine-131 (I-131) for hyperthyroidism and always seen after radioactive iodine-131 (I-131) for thyroid cancer. This is usually easily treated with thyroid hormone replacement. Some studies suggest a slight increase in thyroid cancers may be seen after radioactive iodine-131 (I-131) treatment for hyperthyroidism. Loss of taste and dry mouth due to salivary gland damage may be seen. The use of lemon drops, vitamin C or sour stimulation to potentially decrease the exposure of the salivary glands to radioactive iodine-131 (I-131) is controversial and should be discussed with your physician. Importantly, once you have been treated with radioactive iodine-131 (I-131), regular medical follow-up is lifelong.

Radioactive iodine risks to others

Although the treatments with radioactive iodine-131 (I-131) are generally safe, radioactive iodine-131 (I-131) produces radiation so patients must do their best to avoid radiation exposure to others, particularly to pregnant women and young children. The amount of radiation exposure markedly decreases as the distance from the patient increases. Patients who need to travel in the days after radioactive iodine-131 (I-131) treatment are advised to carry a letter of explanation from their physician. This is because radiation detection devices used at airports or in federal buildings may pick up even very small radiation levels. Details should be discussed with a physician prior to, and at the time of, the radioactive iodine-131 (I-131) treatment.

Table 1. Instructions to reduce exposure to others after radioactive iodine-131 (I-131) treatment