**1. Introduction**

Life is getting complicated in the world of local thyroid hormones (THs) regulation. On account of TH action can be controlled in individual cells through selective TH uptake and intracellular TH metabolism, the placenta is an important link in the maternal-fetal communication network for THs which are essential for the normal development and differentiation of the fetus [1-3]. Generally, intracellular activation or inactivation of Lthyroxine (T4) and 3,5,3'-triiodothyronine (T3) in turn is determined by three types of iodothyronine deiodinases (Ds), namely DI, DII, and DIII [4-7]. The placenta transports and metabolizes maternal THs, and mainly expresses DIII, which inactivates T4 and other iodothyronines and thus limits the transfer of maternal active THs to the fetus in the late pregnancy [8]. DII is also active in the placenta and locally provides active T3 from the maternal prohormone T4 for placental metabolic functions [1,2]. The placental expression of DI, which also activates T4 to T3, is still controversial. Because of the lipophilic nature of THs, it was thought that they traversed the plasma membrane by simple diffusion [9,10]. The transport of T4 and T3 in and out of cells is controlled by several classes of transmembrane TH-transporters (THTs) [11], including members of the organic anion transporter family (OATP), L-type amino acid transporters (LATs), Na+/Taurocholate cotransporting polypeptide (NTCP), and monocarboxylate transporters (MCTs) [10,12]. Particularly, monocarboxylate transporter 8 (MCT8) has recently been identified as an active and specific TH transporter. Also, placental membranes are also involved in 4'-OH-sulfation reactions of iodothyronines [8]. Sulfation (S) plays a role in TH metabolism by interacting between the deiodination and sulfation pathways of TH [13]. Interestingly, placental cells express high affinity, stereo-specific, energy-dependent uptake systems for T4 and T3. On the other hand, the cellular activity of THs is usually classified as genomic (nuclear) and non-genomic (initiated either at cytoplasm or at membrane TH receptors) [14-21]. Binding of T3 to its nuclear thyroid receptors (TRs) directly affects transcription of many genes that are important in development [22].

© 2012 Ahmed, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In general, pregnancy is accompanied by profound alterations in the thyroidal economy (hypo- or hyper-thyroidism), resulting from a complex combination of factors specific to the pregnant state, which together concur to stimulate the maternal thyroid machinery [1,23]. Also, clinical studies showed that maternal TH deficiency during the first trimester of pregnancy can affect the outcome of human neurodevelopment [24,25]. Experiments in rats showed that early maternal TH deficiency affects neuronal migration in the cortex [26], while maternal hyperthyroidism too can disturb fetal brain development [27]. Experimental data on the mechanisms regulating intracellular TH availability and action prior to the onset of fetal TH secretion, however, remain scarce. Thus, in this chapter will be aware about the significant roles of THs, their metabolism by Ds and sulfotransferases, their transport by THTs and their binding to TRs from the mother via the placenta to the fetal compartment especially during the gestation period in both human and animals.

Maternal-Fetal Thyroid Interactions 127

however, fetuses with total thyroid agenesis but with evidence of circulating maternal TH have normal placental DIII activity, suggesting that there might be other factors modulating T4 access to the deiodinases, such as intracellular protection of TH by THbinding protein (THBP) [40,44]. Collectively, express placental Ds (II, III) may play a critical role in delivery of TH to the fetus as summarized in figure1 [2,45-47] and table 1

**Figure 1.** Summary about the interactions of maternal, placental and fetal thyroid metabolism. I, II and III denote deiodinases type 1 (DI), type two (DII) and type three (DIII). SO4 is a sulfation pathway and –SO4 is a desulfation pathway. CNS is central nervous system, TRH is thyroid releasing

diiodothyronine, T3 is triiodothyronine, rT3 is reverse triiodothyronine, T4 is thyroxine, T2S is diiodothyronine sulfate, T3S is triiodothyronine sulfate, T4S is thyroxine sulfate, rT3S is reverse triiodothyronine sulfate, MCT8 is monocarboxylate transporter 8, OATP4A1 is organic anion

hormone, M-TRH is maternal thyroid releasing hormone, TSH is thyrotrophin, T2 is

transporter 4A1, TBG is thyroxin binding globulin, TTR is transthyretin, ACTH is

adrenocorticotrophin and hCG is human chorionic gonadotrophin.

[1,2,31,48].
