Your thyroid gland absorbs nearly all of the iodine in your blood. When a form of radioactive iodine (RAI) known as I-131 is taken into the body, it can destroy the thyroid gland and any other thyroid cells (including cancer cells) that take up iodine, without affecting the rest of your body. (The radiation dose used here is much stronger than the one used in radioiodine scans, which were described in "How is thyroid cancer diagnosed?") The radioactive iodine is usually given as a capsule or liquid.

This treatment can be used to destroy (ablate) any thyroid tissue not removed by surgery or to treat thyroid cancer that has spread to lymph nodes and other parts of the body.

Radioactive iodine therapy has been shown to improve the survival rate of patients with papillary or follicular thyroid cancer (differentiated thyroid cancer) that has spread to the neck or other body parts, and this treatment is now standard practice in such cases. But the benefits of RAI therapy are less clear for patients with small cancers of the thyroid gland that have not spread. Discuss the risks and benefits of RAI therapy in your particular case with your doctor. Radioactive iodine therapy is not used to treat anaplastic (undifferentiated) and medullary thyroid carcinomas because these types of cancer do not take up iodine.

For RAI therapy to be most effective, patients must have high levels of thyroid-stimulating hormone (TSH, or thyrotropin) in the blood. This substance stimulates thyroid tissue (and cancer cells) to take up radioactive iodine. After surgery, TSH levels can be raised by stopping thyroid hormone pills for several weeks. This causes very low thyroid hormone levels (a condition known as hypothyroidism), which in turn causes the pituitary gland to release more TSH. Although this intentional hypothyroidism is temporary, it can cause symptoms like tiredness, depression, weight gain, sleepiness, constipation, muscle aches, and reduced concentration. An injectable form of thyrotropin is now available that can increase a patient's TSH. It is sometimes used before thyroid scans, but it's not clear if it's as effective for RAI therapy as stopping thyroid hormones.

Risks and side effects

Depending on the dose of radioiodine used and where you are being treated, you may need to be in the hospital for up to a few days after treatment, staying in a special isolation room to prevent others from being exposed to radiation. Some people may not need to be hospitalized. You may be allowed to go home after treatment. If this is the case, you will be given instructions on how to protect others from radiation exposure.

Short-term side effects of RAI treatment may include:

• neck tenderness
• nausea and upset stomach
• swelling and tenderness of the salivary glands
• dry mouth
• taste changes
• pain (this is rare)

Chewing gum or sucking on hard candy may help with salivary gland problems. Radioiodine treatment also reduces tear formation in some people, leading to dry eyes. If you wear contact lenses ask your doctor how long you should keep them out.

Men who receive large total doses because of many treatments with RAI may have lower sperm counts or, rarely, become infertile. Radioactive iodine may also affect a woman's ovaries, and some women may have irregular periods for up to a year after treatment. Many doctors recommend that women avoid becoming pregnant for 6 months to a year after treatment. No ill effects have been noted in children whose parents received radioactive iodine in the past.

Both men and women who have had RAI therapy may have a slightly increased risk of developing leukemia in the future. Doctors disagree on exactly how much this risk is increased, but most of the largest studies have found that this is an extremely rare complication. Some research even suggests the risk of leukemia may not be significantly increased. Again, discuss your individual risks and benefits with your doctor.

Last Medical Review: 04/28/2009
Last Revised: 05/14/2009