To clarify the role of various thyroid stimulators in normal human pregnancy, we measured serum TSH, chorionic TSH (hCT), hCG, bioassayable thyroid-stimulating activity, T4, T3, T3 uptake, free T4 and free T3 indexes, free T4, and free T3 by dialysis in 339 serum samples from pregnant women at various intervals of pregnancy and in 40 normal female controls. Serum T4 and T3 and free T4 and free T3 indexes were significantly elevated throughout pregnancy in comparison with controls. Free T4 concentration was elevated after 10 weeks of pregnancy and free T3 concentration was elevated at 13--20 weeks. Bioassayable thyroid-stimulating activity was elevated from 9--16 weeks when serum hCG concentrations were highest. Serum TSH levels were significantly lower at 9--12 weeks compared with the rest of pregnancy. hCT was detected in only 35% of sera tested; the mean detectable value was 0.60 +/- 0.04 (SE) microU/ml; only 15% of the detectable values exceeded 1 microU/ml. The level of hCG correlated with bioassayable thyroid-stimulating activity (P less than 0.01). The data indicate that hCT is not a significant thyroid stimulator. We propose that hCG, as a weak thyroid stimulator, causes a modest rise in free thyroid hormone levels early in pregnancy which in turn causes a modest reduction in pituitary TSH secretion.