A 34-year-old Caucasian woman was seen and evaluated for hyperemesis and abnormal thyroid function tests, consistent with hyperthyroidism, during her second pregnancy. Her hyperemesis and hyperthyroidism both resolved during the second trimester of this pregnancy. She recounted very similar symptoms of similar duration in her first pregnancy. She was again monitored in a third pregnancy during which hyperemesis and hyperthyroidism were once more documented, resolving during the second trimester. The recurrence of hyperemesis in three consecutive singleton pregnancies, with documentation of transient hyperthyroidism in two of these pregnancies, is suggestive of human chorionic gonadotropin (hCG)-mediated hyperthyroidism. Either the trophoblasts synthesized hCG of high thyrotropic grade or, alternatively, hCG could be modified in the maternal tissues, resulting in molecules with greater thyrotropic activity. These potential mechanisms are discussed.