The clinical course of an 18-yr-old woman with metastatic choriocarcinoma, goiter and overt physical and laboratory manifestations of hyperthyroidism is presented. A remission of both the neoplastic disease and the hyperthyroidism was achieved by oncolytic chemotherapy. The fall in gonadotropin excretion and plasma thyroid hormone concentrations occurred concomitantly during therapy. Ultimately, entirely normal thyroid function and regulatory mechanisms were restored, and the thyroid gland returned to normal size. The clinical course and response to therapy suggest that the thyroid abnormalities in this patient were secondary to the secretion of large quantities of a thyrotropic substance by the trophoblastic tumor.