Biochemical evidence of hyperthyroidism is frequently encountered in hyperemesis gravidarum, but its relationship to the cause of hyperemesis is unknown. We studied the relationship of serum hCG, thyroid function, and severity of vomiting among 57 hyperemesis patients and 57 controls matched for gestational age. TSH was suppressed in 60% of hyperemesis patients and 9% of controls. hCG correlated directly with free T4(r = 0.45, P < 0.001) and inversely with TSH (r = -0.48, P < 0.001). Hyperemesis patients had significantly greater mean serum hCG, free T4, total T3, and estradiol, and lesser serum TSH compared to controls. Hyperemesis patients with suppressed TSH had significantly greater free T4 and hCG compared to those with TSH in the normal range. Control and hyperemesis subjects were divided into four groups based on the severity of vomiting. The degree of biochemical hyperthyroidism and hCG concentration varied directly with the severity of vomiting. Unextracted serum was tested for thyrotropic activity by measuring its effect on iodide uptake in cultured FRTL-5 rat thyroid cells. Thyrotropic activity correlated with serum hCG (r = 0.50, P < 0.001). These data show that biochemical hyperthyroidism is a common finding in patients with hyperemesis gravidarum and suggest that hCG is the thyroid stimulator in this state. The increased estradiol concentration in patients with hyperemesis gravidarum may be attributed to the effects of hCG on steroidogenesis.