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Certain pathologic conditions related to pregnancy have been associated with biochemical and clinical hyperthyroidism. These conditions include trophoblastic tumors (molar gestation and choriocarcinoma) and hyperemesis gravidarum. More recently, it has been recognized that multiple gestation and some otherwise normal-appearing singleton gestations will result in hyperthyroidism. In this article the authors review the natural history and clinical significance of these conditions and conclude with an analysis of the lines of evidence that suggest that the cause of the hyperthyroidism is human chorionic gonadotropin secreted by the trophoblast.