Radioactive iodine provides a simple, effective, and safe treatment for cats with hyperthyroidism and is regarded by most authorities to be the treatment of choice. It is a particularly useful treatment for cats with bilateral thyroid involvement (found in approximately 70% of cats), cats with ectopic (intrathoracic) thyroid tissue, and the rare patient with thyroid carcinoma. Treatment with radioiodine avoids the inconvenience of daily, oral administration of an antithyroid drug, as well as the side effects commonly associated with these drugs. Use of radioiodine also avoids the risks and perioperative complications associated with anesthesia and surgical thyroidectomy. Radioiodine treatment involves a single, nonstressful procedure that is without associated morbidity or mortality. A single treatment restores euthyroidism in most cats with hyperthyroidism. Whereas the therapy is simple and relatively stress-free for cats, it does require special licensing and hospitalization facilities, nuclear medicine equipment, and extensive compliance with local and state radiation safety laws.