The thyroid gland absorbs nearly all of the iodine in your blood. When a large dose of radioactive iodine (RAI), also 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, with little effect on the rest of your body. The radioactive iodine is usually given as a capsule or liquid.
The indication for radioactive iodine is 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. Radioactive iodine therapy cannot typically 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), which stimulates thyroid tissue (and cancer cells) to take up radioactive iodine.
The short-term side effects of RAI treatment include nausea, salivary gland swelling, dry mouth, and, rarely, pain. Radioactive iodine may remain in the tissues for a period after treatment, so it is recommended that women avoid becoming pregnant for 6 months to a year after treatment. No ill effects have been noted in the children born of parents who received radioactive iodine in the past.