Pseudothyrotoxic myopathy: a complication of thionamide therapy in hyperthyroidism

A hyperthyroid patient treated with methimazole for three weeks developed proximal muscle weakness, myalgia, arthralgia, and fever, and thyrotoxic myopathy was diagnosed. The signs and symptoms spontaneously abated when antithyroid medication was discontinued. This case identified a complication of thionamide treatment mimicking thyrotoxic myopathy and points to the therapeutic necessity of making such a differentiation.