Thyroid cancer is occasionally associated with hyperthyroidism. Papillary thyroid carcinoma is the most frequently reported histologic type followed by follicular thyroid carcinoma. Medullary thyroid carcinoma (MTC) has been rarely described in association with Graves' disease or other forms of hyperthyroidism. To our knowledge, only 14 cases have been described in which MTC was associated with hyperthyroidism. We report a 70-year-old male who had a clinically significant MTC discovered in the course of diagnostic work-up for hyperthyroidism. We also summarize the clinical features of the other 14 reported cases.