Comparison of serum pituitary thyrotropin and chorionic gonadotropin concentrations throughout pregnancy.

Concentrations of human pituitary thyrotropin (hTSH) and human chorionic gonadotropin (hCG) were measured by specific radioimmunoassays in serum samples obtained from 243 pregnant women. The hTSH levels demonstrated suppression during the sceond through the fourth months, followed by a linear rise to non-pregnant control levels throughout the remainder of gestation. At the time that the hCG levels were at their lowest concentration. However, a reciprocal relationship between the hCG and hTSH concentrations in individual samples was not found. The inverse relationship between mean levels of hTSH and hCG during early pregnancy suggest that the intrinsic thyroid-stimulating activity of hCG may be important in the control of thyroid function during the first trimester. The failure to confirm this relationship in individual serum samples indicates that other factors also influence maternal thyroid homeostasis during early gestation.