Fetus deiodination

The concentrations of T3 are controlled by deiodinases type 1 (Dl), 2 (D2), and 3 (D3). D2 transformsT4 intoT3, whereas D3 transforms T4 and T3 into inactive products, reverse T3 and 3, 3 -diiodothyronine, respectively (Bernal et al, 2003). Development regulates the expression and local activity of D2 and D3 in the brain (Kaplan and Yaskoski, 1981). In addition, despite the restricted access of molecules fi-om the blood to the brain parenchyma due to the blood-brain barrier, small amounts of T4 and T3 may enter the brain in the fetus through specific transporters (Sugiyama etaL, 2003). T3 is formed by deiodination ofT4 and defivered... 

Table 1 outlines the experimental design which we used. Methimazole (MMI) was used to block maternal and fetal thyroid function. Some of the MMI-treated dams received a replacement dose of T4 given by constant infusion, as described in detail elsewhere23. The main findings are summarized in Fig. 5, which shows the T4 and T3 concentrations in fetal carcass and brain. Comparison of the hormone levels in fetuses from C + MMI + T4 (or T + MMI + T4) dams with those from C + MMI (or T + MMI) mothers shows that infusion of T4 into the mothers not only ameliorates fetal deficiency of T4, but also of T3. This occurs without an increase of fetal plasma T3, suggesting that the fetal brain derived its T3 from local deiodination of T4. 

The myxoedematous form of endemic cretinism, which is still prevalent in many parts of the world (CoNTEMPRE et al. 1991), is not only due to the lack of iodine, but also of selenium. The selenoprotein type 1 5 -deiodinase catalyses the deiodination of the prohormone L-thyroxine (3,3 ,5,5 -tetraiodo-L-thyronine, T4) to the biologically active form 3,3 ,5-triiodo-L-thyronine (L-T3). During fetal development the maternal organism contributes at least minimal amounts of thyroid hormone for the fetus. After birth, the baby then slowly reaches the condition of thyroid hormone deficiency, because contin-... 

The deiodination of thyroxine in peripheral tissues results in the formation of 3,3, 5-tri-iodothyronine (T3) or 3,3 5 -tri-iodothyronine (reverse T3). Unlike T3, which is more physiologically active than thyroxine itself, reverse T3 has no physiological activity. It is possible that the ratio of T3 to reverse T3 may vary in various physiological and pathological conditions. For example, reverse T3 is found in high concentrations in amniotic fluid. After birth, the serum reverse T3 level falls while the level of normal T3 increases. It is therefore possible that reverse T3 may be a marker of congenital hypothyroidism in fetuses. Specific radioimmunoassays are available for the determination of reverse T3. 

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