FIGURE 1. Maternal concentrations of serum TSH and hCG as a function of gestational age. The decrease in serum TSH at approximately 10 week’s gestation may be due to thyrotropic effects of hCG. ( From Glinoer D, de Nayer P, Bourdoux P, et al. Regulation of maternal thyroid during pregnancy. J Clin Endocrinol Metab 1990; 71:276. )