A 59-year-old man was admitted because of progressive proximal muscle weakness over 4 months. Serum creatine kinase was markedly increased and electromyography revealed myopathic pattern. Muscle biopsy showed nonspecific myopathic changes. No inflammatory change was shown. Thyroid function tests revealed very low thyroxine and low triiodo-L-thyronine, whereas thyroid-stimulating hormone was greatly increased as well as serum anti-thyroglobulin and microsome antibodies. The patient had hypothyroid myopathy due to Hashimoto's thyroiditis, which improved with L-thyroxine replacement.