Participants will take 30 mg of methimazole by mouth twice a day for 6 months. They will have blood tests weekly for the first 2 weeks and then every other week for the rest of the study to measure blood counts and liver and thyroid function. Blood will also be drawn for white blood cell studies during the screening evaluation, at the beginning of therapy, 6 to 12 weeks after therapy starts, at the end of the 6-month treatment period, and 1 and 3 months after therapy ends. Muscle enzyme and urine tests will be done once a month.. During drug treatment, patients will have periodic physical examinations and blood and muscle function tests to evaluate the response to therapy.