We report a case of a man with thyrotoxicosis due to excess production of human chorionic gonadotropin (hCG) by metastatic choriocarcinoma, followed by alterations of his thyroid function tests by nonthyroidal illness. All reported cases of thyrotoxicosis due to high hCG levels in male patients are reviewed. Patients with this syndrome usually have widespread choriocarcinoma and relatively few symptoms of thyrotoxicosis. Typically, if the patient survives the metastatic germ cell tumor, the thyrotoxicosis resolves as the hCG levels decrease after chemotherapy directed at the choriocarcinoma. Only rarely are specific antithyroid medications required. The hCG molecule directly stimulates the thyroid gland, and these patients appear to have in the serum a predominance of acidic variants of hCG with greater intrinsic thyroid-stimulating activity than the hCG secreted during a normal pregnancy. In general, these patients have a poor prognosis due to the usually widespread nature of the germ cell tumor at the time of diagnosis.