Delivery of maternal thyroid hormones to the fetus.

Thyroid hormones (THs) play an essential role in ensuring normal fetal development, particularly that of the central nervous system. Before 16 weeks gestation, the fetus relies solely on transplacental delivery of maternal T(4), and clinical studies suggest that even mild maternal thyroid hormone deficiency adversely affects the intellectual function of offspring. Maternofetal TH transfer is regulated by trophoblast cell membrane transporters, which mediate influx and efflux of THs, placental deiodinases D3 and D2, which control intraplacental TH levels, and TH-binding proteins (transthyretin), which provide transport roles in the placenta. This review discusses new information about mechanisms of transplacental delivery of T(4) to the fetus, providing insight into complex processes that are vitally important for normal fetal development.