Medullary thyroid carcinoma (MTC) commonly metastasizes locally to the cervical lymph nodes and distantly to the liver, lungs and bones. Metastatic involvement of the muscles is extremely rare. We reported an unusual case of undiagnosed MTC presenting with symptoms related to metastatic lesions of the brachoradialis and the gluteus medius muscles. A 53-year-old man consulted for a painful mass of the right forearm and atrophy of the quadriceps. Ultrasonography revealed a heterogeneous collection in the forearm. Computed tomographic scan showed a mass in the right lung, an enlargement of mediastinal lymph nodes and solid masses in the right gluteus medius and the left iliopoas muscle extending to the left iliac bone. Pulmonary biopsies displayed findings consistent with metastatic MTC. Fine needle aspiration cytology from the right arm swelling revealed a lesion with the same calcitonin immunostaining patterns as the lung metastasis. The diagnosis of multiple metastases (lung, muscle and bone) of MTC is established. The patient has received local radiation therapy and was planned for chemotherapy. Muscular metastases from MTC are rare and although their prognosis is poor, local treatment may be worthwhile.