[A thyrotoxic myopathy accompanied with unusual muscle symptoms and MRI muscle findings].

We report a patient with thyrotoxic myopathy associated with unusual muscle symptoms. A 29-year-old man developed hyperhidrosis, diarrhea, increase in appetite, and excitability in July, 1999. In August, he experienced muscle stiffness in bilateral lower extremities after maintaining postures such as driving a car or sitting on a chair. He was admitted to our hospital, in January, 2000. On physical examination, goiter was noted. Neurological examination was normal except for proximal muscle weakness. Laboratory test showed elevated free T3 and free T4, decreased TSH. TSH receptor antibody was increased. MRI of lower extremities revealed atrophy of bilateral biceps femoris. Muscle strength increased gradually after an oral administration of thiamazole 30 mg/day, and muscle stiffness disappeared. The clinical features of this patient and differential diagnosis were discussed.