Older cats are at increased risk for both kidney disease and hyperthyroidism. Hyperthyroidism is usually treated by one of three methods: surgical removal of the hyperactive thyroid gland, oral anti-thyroid medication, or radioactive iodine. Radioactive iodine is an effective therapy with a high success rate, few side effects, and no requirement for anesthesia or surgery. Some hyperthyroid cats appear to be at increased risk of developing kidney failure after being treated with radioactive iodine. No investigator has evaluated specific kidney function tests or other kidney values within one to two weeks after radioactive iodine injection or for periods longer than 60 days. Therefore, these investigators will conduct a controlled study of cats receiving uniform care and treatment to determine if information can be generated that will help predict which cats are at greatest risk of developing kidney disease after radioactive iodine treatment for hyperthyroidism.