Gestational thyrotoxicosis is now widely believed to result from excessive thyroidal stimulation by hCG. Our clinical impression has been that this condition is more frequent in Asian women than in those of European origin. To assess this possibility further, we analyzed thyroid hormone levels in the sera of 294 Asian women, obtained as part of a screening program at 15-16 weeks of pregnancy, and compared these with 292 sera from age- and parity-matched European women at a similar time of gestation. TSH levels were significantly lower in the Asian group (P < 0.001). Suppressed TSH levels (< 0.35 mIU/L) were found in 15.7% of Asian women and 4.8% of European women (P < 0.001). In both groups of women with suppressed TSH values, hCG and hCG beta levels were higher than in the women with normal TSH levels. Free T4 levels in the Asian women were significantly higher in those with suppressed TSH (P < 0.001), but this was not found in the European women. There was also a significant increase in the free T3 index in the Asian women with suppressed TSH compared to that in an age-matched group of Asian women with normal TSH levels (P < 0.02), but this was not observed in European women with suppressed TSH. None of the women with suppressed TSH had thyroid-stimulating antibodies. These results show that Asian women more frequently develop biochemical evidence of thyrotoxicosis at the beginning of the second trimester of pregnancy than those of European origin and are, therefore, likely to be at greater risk of clinically apparent gestational thyrotoxicosis and hyperemesis gravidarum. Genetically determined differences in the production or metabolism of hCG isoforms may account for this increased risk.