radioactive iodine I-131 therapy
radioactive iodine I-131 therapy. The use of radioactive iodine I-131 to treat an overactive thyroid, a condition called hyperthyroidism. When a small amount of I-131 is swallowed, it is absorbed into the bloodstream in the gastrointestinal (GI) tract and concentrated from the blood by the thyroid gland, where it begins destroying the gland’s cells.