Haddow JE et al. (2008) Variability in thyroid stimulating hormone suppression by human chorionic gonadotropin during early pregnancy. J Clin Endocrinol Metab 93: 3341–3347

An inverse relationship has been described between maternal serum TSH and human chorionic gonadotropin (hCG) levels during early pregnancy. TSH levels reach a nadir at around 10 weeks' gestation, at which time hCG levels peak. In addition, free T4 levels might be elevated, although other features of hyperthyroidism are usually absent.

Haddow and colleagues evaluated the associations between TSH, free T4 and hCG levels at 11–18 weeks' gestation in a cohort of 9,562 pregnant women enrolled in the multicenter FaSTER (First and Second Trimester Evaluation of Risk for Aneuploidy) trial. Biochemical analysis was performed on paired first and second trimester serum samples collected between 1999 and 2002. Strong associations (P ≤0.0005) were detected in both trimesters between maternal covariates and serum hormone levels. These associations included BMI (hCG and free T4), tobacco smoking (hCG, TSH and free T4), gravidity (hCG, TSH and free T4), ethnic background (hCG and TSH), and maternal age (free T4). The correlations between TSH and hCG levels and between hCG and free T4 levels were weak in both trimesters. Of note, however, the TSH percentile seemed to determine susceptibility to suppression by hCG, as TSH concentrations in the lower percentiles became sharply and selectively lower at higher hCG levels.

The authors speculate that hCG might be the primary factor responsible for stimulation of the thyroid gland during early pregnancy.