Does human chorionic gonadotropin have human thyrotropic activity in vivo?

Current evidence indicates that thyroid cells are sensitive to human chorionic gonadotropin (hCG) stimulation. In turn, thyroid hormones appear to influence ovarian and endometrial physiology and reproductive function. Our studies addressed the possible effect of endogenous and exogenous hCG on in vivo thyroid function in normal pregnancy and controlled ovarian hyperstimulation, respectively. Circulating concentrations of hCG in pregnant women during gestation were positively correlated with serum free thyroxine (r = 0.43, p = 0.02) and negatively correlated with thyrotropin levels in the same patients (r = 0.42, p = 0.02). By contrast, exogenous administration of hCG to effect follicular maturation in non-pregnant patients undergoing ovarian hyperstimulation resulted in lower circulating hCG concentrations than seen in pregnancy and failed to alter free thyroxine or thyrotropin levels (p > 0.22). Endogenous isoforms of hCG in early pregnancy appear to have thyrotropic activity in vivo. However, the results indicate that, under clinical conditions of controlled ovarian hyperstimulation for assisted reproduction, exogenous hCG does not affect the hypothalamic-pituitary-thyroid axis.