radioactive iodine

radioactive iodine A version (isotope) of the chemical element iodine that is radioactive. Abbreviated RAI. RAI is used in diagnostic tests as well as in radiotherapy of the thyroid. It may be used in the treatment of hyperthyroidism and cancers of the thyroid. It directly radiates thyroid tissues, thereby destroying them. RAI should not be used during pregnancy and breastfeeding. See also Graves disease.