A 13 YEAR OLD, neutered male Siamese cat was referred for 1311 therapy for hyperthyroidism. Two months prior to presentation the cat presented to its local veterinarian with a history of vomiting and an elevated resting serum T, concentration (15.2 pg/dl; normal, 0.8-3.9 pg/dl). The cat was started on 5 mg methimazole PO BID and a recheck resting serum T, concentration collected 3 weeks later was normal (3.5 pg/dl). Approximately 7 weeks later, the cat presented to its local veterinarian with a history of anorexia of 2 weeks duration. The resting serum T, concentration was elevated (6.9 pg/dl), and the platelet count was de- creased (160,OOO/pL; normal 200,000-500,000/p.L). The remainder of a CBC, serum chemistry panel, and electro- lytes were within normal limits. Whole body radiographs were normal. Methimazole was discontinued and the cat was treated with IV Lactated Ringers Solution administered at 3 times maintenance dosage (25 ml/hr). The cat was referred for l3’I therapy after several days of supportive care.