**3. Maternal-fetal thyroid in normal state**

THs are essential for normal neonatal development in both humans and rodents [3,23,101-104] and the experimental work indicated that THs are transported from the mother to the fetus, albeit in limited amounts, and that the fetal brain is exposed to THs before initiation of fetal TH synthesis [1]. In addition, the maternal TH regulates early fetal brain development in human and animal models [2]. The TH of maternal origin can cross the placenta and reach the fetus [2,105,106] and that TRs are expressed in the fetal rat brain before the onset of fetal thyroid function [107]. Thus, the THs are essential for brain maturation from early embryonic stages onward [103,104,108]. However, TH-dependent stages of fetal brain development remain to be characterized. Notably, the maternal thyroid is the only source of T4 and T3 for the brain of the fetus because its thyroid gland does not start contributing to fetal requirements until midgestation in man, and days 17.5–18 in rats [109]. Therefore, the amount of maternal T4 that the fetus receives early in pregnancy will determine TH action in its brain because it depends on maternal T4 for its intracellular supply of the active form of the hormone, T3. However, fetal brain total T3 levels are low (ca. 100 pM) at this time [1], but receptor occupancy approximates 25% since free T3 concentrations are high in the nucleus relative to the cytosol [110]. In general, materno-fetal transfer of THs has been demonstrated in early fetal stages [111] and continues, at least in the case of fetal inability, to produce sufficient TH until term [44]. Actually, brain cells can protect themselves against higher fetal T4 and T3 values by decreasing DII and increasing DIII activity [2]. Taken together, thyroid activity undergoes many changes during normal pregnancy including [1,112-115]: (a) a significant increase in serum thyroxine-binding globulin, thyroglobulin, total T4, and total T3; (b) an increase in renal iodide clearance; and (c) stimulation of the thyroid by human chorionic gonadotropin (hCG). These changes can make diagnosis of thyroid dysfunction during pregnancy difficult.
