Hyperplacentosis: a novel cause of hyperthyroidism.

Human chorionic gonadotropin (hCG), which is capable of thyrotropic activity, is believed responsible for the hyperthyroidism of gestational trophoblastic disease and hyperemesis gravidarum. Hyperplacentosis is a condition of heightened trophoblastic activity characterized by increased placental weight and circulating hCG levels higher than those associated with normal pregnancy. We report the first case of hyperthyroidism associated with hyperplacentosis. Correction of the hyperthyroidism occurred after hysterotomy and correlated with declining hCG levels. Hyperplacentosis should be included among the causes of hCG-mediated hyperthyroidism.