Despite recent descriptions of dramatic changes in the thyroidal economy in pregnancy, the precise mechanisms of these alterations have remained obscure. Measurements of total and free triiodothyronine (T3) and thyroxine (T4) and thyroxine binding capacity (TBG) were made in 120 serum samples from 42 first trimester, 55 second trimester, and 23 third trimester pregnant women. Sixteen paired term and cord blood samples were also analyzed. Mean total serum T4 rose progressively from 9.8 in the first trimester to 11.5 μg/100 ml in the third trimester. Mean TBG capacity also increased from 38.4, first trimester, to 56.2 μg/100 ml in the third trimester. Mean total T3 concentrations were significantly elevated in all trimesters (p < 0.02-0.001) when compared to normal nonpregnant controls. Total T3, T4 and TBG levels were all significantly higher than in normal nonpregnant women in all trimesters (p < 0.001). TSH levels remained unchanged in all three trimesters; values were 2.2, 2.0, and 2.0 μU/ml, respectively. Free T4 levels were at the lower limits (2.4, 2.5, and 2.2 ng/100 ml) and free T3 levels were below normal (0.29, 0.27, and 0.24 ng/100 ml). In addition, fetal T3 concentrations were strikingly lower in cord blood than in the maternal circulation at term. Maternal and cord free T3 and free T4 levels were not significantly different. These data underscore the need for multiple measurements of thyroid function in pregnancy and in the newborn in the evaluation of the thyroid gland activity.