The role of the placenta in thyroid hormone delivery to the fetus.

The transplacental passage of thyroid hormones from the maternal circulation to the fetal circulation within the human hemochorial placenta is important for normal fetal development, particularly the development of the central nervous system. The role of maternal thyroid hormones is particularly important in the first half of pregnancy, before the onset of endogenous thyroid hormone production in the fetus. The human placenta regulates the quantity and composition of different forms of transported thyroid hormones to ensure that the requisite levels are present in the fetus for each stage of development. Transplacental thyroid hormone supply to the fetus is modulated by several factors, including the following proteins: plasma membrane transporters, which regulate the passage of thyroid hormones in and out of cells; iodothyronine deiodinases, which metabolize thyroid hormones; and proteins within trophoblast cells, which bind thyroid hormones. In pathological situations of either maternal or fetal thyroid hormone deficiency during pregnancy, the placenta seems to lack the full compensatory mechanisms necessary to optimize maternal-fetal transfer of thyroid hormones. Inadequate passage of thyroid hormones can lead to suboptimal fetal thyroid hormone levels, which might contribute to the neurodevelopmental delay associated with such conditions. Thus, maintaining normal maternal thyroid hormone status is likely to be the primary factor in ensuring adequate transplacental thyroid hormone passage and appropriate iodide supply to the fetus.