Unraveling the Role of Maternal Thyroid Hormones on Fetal Development

Over the past four decades, a substantial body of evidence has emerged demonstrating the permeability of the placenta to thyroid hormones. Maternal thyroid hormones cross the placental barrier, becoming present in embryonic tissues well before the onset of thyroid gland function in both rodents and humans. This raises a fundamental question regarding the extent to which certain early developmental processes rely on maternal hormonal influence. While this concept is firmly supported by robust experimental data in rodents, the situation in humans is more nuanced. Numerous clinical observations suggest that a reduction in T4 levels in the blood of otherwise euthyroid pregnant women, a condition known as hypothyroxinemia, may have adverse effects on fetal development. However, clinical trials aimed at assessing the impact of treating maternal hypothyroxinemia with T4 have yielded disappointing results thus far, leaving the matter unresolved.