We report a case of subacute thyroiditis complicated by thyrotoxic myopathy. Previously thyrotoxic myopathy has been described as being associated with Graves' disease. The patient in this study was a 35-year-old man who developed proximal dominant muscular weakness and atrophy during the course of subacute thyroiditis. His myopathic symptoms regressed as his serum thyroxine and triiodothyronine levels returned to normal, however it took a relatively long period of time for them to do so. This suggests that marked myopathy may develop even in cases of subacute thyroiditis if the thyrotoxic state persists for a long period of time.