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 rT 3 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 higher than corresponding maternal levels, but T4, T3, and TBG levels are lower than maternal values. The late second/early third trimester is a critical transition period in fetal thyroid hormone metabolism, which may be interrupted by preterm birth and contribute to postnatal thyroid dysfunction.,
Journal of Clinical Endocrinology and Metabolism
Department of Neuroscience

Hume, R., Simpson, J., Delahunty, C., van Toor, H., Wu, S.-Y., Williams, F., & Visser, T. (2004). Human fetal and cord serum thyroid hormones: Developmental trends and interrelationships. Journal of Clinical Endocrinology and Metabolism, 89(8), 4097–4103. doi:10.1210/jc.2004-0573