Increase in plasma thyrotropin-releasing hormone in normal human pregnancy.

Maternal thyroid regulation in pregnancy was examined by measuring the concentrations of TRH, T4, T3, free T4 (by RIA), T4-binding globulin, TSH, PRL, and hCG in the sera of 30 normal pregnant women, 10 puerperally lactating women, and 10 normal pregnant women, 10 puerperally lactating women, and 10 normal nonpregnant female controls. Results showed that serum T4, T3, T4-binding globulin, PRL, and hCG, but not free T4 (by RIA), increased significantly during pregnancy. The plasma level of TRH was significantly higher (P less than 0.01) in the second trimester and significantly lower (P less than 0.05 1 month post partum than those values in nonpregnant controls. No significant correlations, however, were observed between the serum level of TRH and those of the thyroid hormones TSH, PRL, and hCG in pregnancy. The TRH-degrading activity of the plasma in the second trimester was normal. These results indicate that TRH secretion may be increased in the second trimester of pregnancy.