Your thyroid gland absorbs nearly all of the iodine in your body. When radioactive iodine (RAI), also known as I-131, is taken into the body in liquid or capsule form, it concentrates in thyroid cells. Suchradiation can destroy the thyroid gland and any other thyroid issue (including cancer cells) that take up iodine, with little effect on the rest of your body. (The radiation dose used here is much stronger than the one used in radioiodine scans. This treatment can be used to ablate (destroy) any thyroid tissue not removed by surgery or to treat some types of thyroid cancer that have spread to lymph nodes and other parts of the body.

Radioactive iodine therapy improves 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. Tthe benefits of RAI therapy are less clear for patients with small cancers of the thyroid gland that do not seem to have spread, which can often be removed completely with surgery. Discuss your risks and benefits of RAI therapy with your doctor. Radioactive iodine therapy cannot be 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. If the thyroid has been removed, one way to raise TSH levels is to not take 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. This intentional hypothyroidism is temporary, but it often causes symptoms like tiredness, depression, weight gain, constipation, muscle aches, and reduced concentration.

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