Iodine (I-131) therapy, also known as radioactive iodine therapy, uses a form of iodine that sends out radiation to treat thyroid cancer. Radioactive iodine is also called I-131.

Your thyroid gland absorbs nearly all of the iodine in your blood. With iodine therapy, I-131 is taken into your body’s thyroid gland and destroys the thyroid gland and other thyroid cells—including cancer cells—with minimal effect on the rest of your body. It can be used to destroy any thyroid tissue not removed by surgery or to treat thyroid cancer that has spread to other parts of your body.

Radioactive iodine is taken as a capsule or liquid to treat slow-growing, differentiated thyroid cancers, also called papillary and follicular thyroid cancer. It is not used to treat medullary or anaplastic thyroid cancer, as these cancer types do not take up iodine. You may have thyroid scans after this type of radiation to see if there are still cancerous cells present.

Radioactive iodine therapy is most effective if you have high blood levels of thyroid stimulating hormone (TSH), which causes thyroid cells, including thyroid cancer cells, to take up the iodine.

Ask your physician for more information on the precautions or side effects of iodine (I-131) therapy.