Patients with hypothyroidism frequently present with musculoskeletal complaints and elevated serum levels of muscle enzymes, like creatine kinase (CK). With adequate hormonal treatment, the maintenance of high values of CK and muscle symptoms are uncommon. This work reports a 47-year-old woman with hypothyroidism, proximal muscle weakness and increased CK, who didn't improved with levothyroxine treatment. The diagnosis of polymyositis was based on electromyography and muscle biopsy. After treatment with prednisone plus methotrexate and increase of the levothyroxine dose, there were a clinical improvement and a normalization of CK values.435330333Duyff RF, Van den Bosch J, Laman DM, van Loon BJ, Linseen WH. Neuromuscular findings in thyroid dysfunction: a prospective clinical and electrodiagnostic study. J Neurol Neurosurg Psychiatry 000;68:750-5Mastaglia, F.L., Ojeda, V.J., Sarnat, H.B., Kakulas, B.A., Myopathies associated with hypothyroidism: A review based upon 13 cases (1988) Aust NZJ Med, 18, pp. 799-806Finsterer, J., Stollberger, C., Grossenger, C., Kroiss, A., Hypothyroid myopathy with unusually high serum creatine kinase values (1999) Horm Res, 52, pp. 205-208Fujitake, J., Ishikawa, Y., Ishimaru, S., Koh, T., Tatsuoka, Y., Localized polymyositis associated with chronic thyroiditis (1994) J Rheumatol, 21, pp. 1147-1149Taniguchi, A., Niwa, A., Nakayama, S., Hamaguchi, Y., Kuzuhara, S., Polymyositis with rimmed vacuoles in a case of chronic thyroiditis [abstract] (1995) Rinsho Shinkeigaku, 35, pp. 403-407Gamsky, T.E., Chan, M.K., Coexistent dennatomyositis and autoimmune thyroiditis (1988) West J Med, 148, pp. 213-214Hochberg, M.C., Koppes, G.M., Edwards, C.Q., Barnes, H.V., Arnett, F.C., Hypothyroidism presenting as a polymyositis-like syndrome: Report of two cases (1976) Arthritis Rheum, 19, pp. 1363-1366Ciompi, M.L., Zuccotti, M., Bazzichi, L., Pucetti, L., Polymyositis-like syndrome in hypothryroidism: Report of two cases (1994) Thyroidology, 6, pp. 33-36Rodolico, C., Toscano, A., Benvenga, S., Myopathy as the persistently isolated symptomatology of primary autoimmune hypothryroidism (1998) Thryroid, 8, pp. 1033-1038Lin, R.T., Liu, C.K., Tai, C.T., Lai, C.L., Hypothyroid myopathypathological and ultrastructural study (2000) Kaohsiung J Med Sci, 16, pp. 68-75Modi, G., Cores in hypothyroid myopathy: A clinical, histological and immunofluorescence study (2000) J Neurol Sci, 175, pp. 28-32Monforte, R., Femandez-Sola, J., Casademont, J., Vemet, M., Grau, J.M., Urbano-Marquez, A., Hypothyroid myopathy. A clinical and histological prospective study of 19 patients (1990) Med Clin (Barc), 95, pp. 126-129Singer, P.A., Cooper, D.A., Levy, E.G., Treatment guidelines for patients with hyperthyroidism and hypothyroidism (1995) JAMA, 273, pp. 808-812