Methimazole to Treat Polymyositis and Dermatomyositis

This study will test the safety and effectiveness of the drug methimazole in treating polymyositis and dermatomyositis-inflammatory muscle diseases causing weakness and muscle wasting. Although it is not known what causes of these diseases, abnormal immune function is thought to be involved. Recent studies indicate that methimazole, which has been used for many years to treat thyroid disease, may alter immune activity by affecting the interaction between white blood cells called lymphocytes and certain molecules on cell surfaces. This study will examine the effects of methimazole on immune activity and muscle strength in patients with inflammatory muscle diseases and evaluate the drug side effects. Patients with polymyositis and dermatomyositis who have normal thyroid function may be eligible for this study \[age requirement?\]. Candidates will undergo a history and physical examination; blood and urine tests; chest X-ray; muscle strength testing, daily living skills questionnaire, and speech and swallowing evaluation; magnetic resonance imaging of muscles; and muscle biopsy (removal of a small piece of muscle tissue under local anesthetic). When indicated, some candidates may also have cancer screening tests (for example, mammogram, Pap smear), a lung function test to measure breathing capacity, or an electromyogram, in which small needles are inserted into a muscle to measure the electrical activity . 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 wil