Human Fetal and Cord Serum Thyroid Hormones: Developmental Trends and Interrelationships

Thyroid hormone is essential for fetal and neonatal development in particular of the brain, but little is known about regulation of fetal thyroid hormone levels throughout human gestation. The purpose of this study was to clarify developmental trends and interrelationships among T4, free T4 (FT4), thyroxine-binding globulin (TBG), TSH, T3, rT3, and T4 sulfate (T4S) levels in cord and fetal blood sera (n = 639, 15–42 wk gestation) and correlate infant levels (23–42 wk gestation) to maternal values (n = 428, 16–45 yr) and those of nonpregnant women (n = 233, 16–46 yr). In cord and fetal serum, T4, T3, and TBG levels increase with gestation until term; TSH, FT4, T4S, and rT3 levels increase and peak in the late second/early third trimester and then decline to term; T4/TBG ratios increase until late second trimester and plateau to term. Term cord sera TSH, TBG, and all iodothyronine levels, except T3, are higher than nonpregnant women. In the third trimester, cord serum FT4, TSH, rT3, and T4S levels are also ...