A 13-year-old male neutered domestic shorthair cat was referred to Cornell University Hospital for Animals (CUHA) Small Animal Internal Medicine Service for radioactive iodine treatment of his hyperthyroidism. Three months prior to presentation to CUHA, the patient had been diagnosed with hyperthyroidism by his primary care veterinarian based on an elevated total T4 (thyroxine) concentration and clinical signs. Oral methimazole therapy was initiated at the time of diagnosis. However, the owner was unable to medicate the patient due to patient non-compliance, and the total T4 concentration continued to increase on subsequent measurements. The patient was therefore referred to CUHA for definitive treatment of his hyperthyroidism with radioactive iodine therapy (RAIT). Diagnostics performed at the time of presentation to CUHA showed a significantly increased total T4 concentration, bilateral hyperplasia of the thyroid gland, and hypertrophic cardiomyopathy. The patient was found to be a good candidate for RAIT, and underwent treatment. One month later, the patient’s total T4 level was decreased significantly and clinical signs had resolved. These results illustrate the efficacy of radioactive iodine for treatment of feline hyperthyroidism.