Sodium iodide I 131 is used to treat hyperthyroidism (an overactive thyroid) and certain kinds of thyroid cancer. sodium iodide i 131 is taken up mainly by the thyroid gland. In the treatment of overactive thyroid gland, the radiation from the radioactive iodine damages the thyroid gland to bring its activity back down to normal. Larger doses of radioiodide are usually used after thyroid cancer surgery to destroy any remaining diseased thyroid tissue or to destroy thyroid cancer that has spread to other tissues.