In July 2004, Shadow, a 14 year old, female spayed domestic shorthair, presented to her referring veterinarian with a complaint of progressive weight loss despite a normal appetite. Total T4 concentration was found to be elevated at 11.0 ug/dl (reference 0.8-4.0), diagnosing hyperthyroidism. Methimazole treatment was initiated, however, due to the difficulties in pilling her cat every day, the owner became interested in Radioactive Iodine Therapy (RAIT). On presentation at Cornell, Shadow was thin with a 4/9 body condition score (BCS). She had a palpable right thyroid nodule, a III/IV left systolic murmur, and was hypertensive. Thyroid scintigraphy confirmed unilateral hyperthyroidism with no ectopic thyroid tissue. RAIT treatment with 131-I was administered on 9/8/06. At the time of discharge, on 9/19/06 Shadow was mildly hypothyroid with total T4 level of 1.23 ug/dl(reference 1.5-4.0). Follow up communication with the owner indicate Shadow is doing well at home.