Porterfield S P, Hendrich C E
Department of Physiology and Endocrinology, Medical College of Georgia, Augusta 30912.
Endocrinology. 1992 Jul;131(1):195-200. doi: 10.1210/endo.131.1.1611997.
Some investigators have reported that there is minimal placental transport of thyroid hormones in humans and rats. Consequently, it was thought that thyroid hormones were not present in the fetal brain before fetal thyroid hormone synthesis and, hence, were not important for brain development before fetal thyroid hormonogenesis. Recently, however, thyroid hormones have been detected by 14 days postconception (dpc) in the rat fetus and by 11 dpc in the rat embryotrophoblast. Thyroid hormone receptors have been shown in the fetal rat by 14 dpc. The present experiments were designed to determine if T4, T3, and their metabolites can be detected in rat fetuses at 13 and 16 dpc and if iodothyronines are selectively accumulated in fetal brain and liver. Furthermore, one group of dams was radiothyroidectomized before breeding to ascertain the effect of maternal hypothyroxinemia on fetal tissue iodothyronine concentrations. Tissue iodothyronines were extracted and measured by HPLC. T4, T3, rT3, and 3,5-diiodothyronine were well within the limits of detection by this procedure at both fetal ages. The only possible source of these hormones is the mother. In addition, if maternal serum T4 levels are low, fetal tissue T4 and T3 levels are low. The presence of high intracellular T3 levels, even at 13 dpc, shows that 5'-monodeiodination occurs in the midgestational fetus. Intracellular hormone measurements show that T3, rather than rT3, is the predominant intracellular iodothyronine in the rat fetus. Both brain and liver selectively accumulate T4 and T3, supporting the observations of others that fetal thyroid hormone receptors are present in midgestation. The presence of thyroid hormones in fetal rat brain by 13 dpc coupled with the observation that hormone receptors are present by 14 dpc suggests that thyroid hormones do play a role in midgestational brain development. These data show that normal maternal serum thyroid hormone levels are important during midgestation to provide adequate thyroid hormones to the fetus.
一些研究人员报告称，在人类和大鼠中，甲状腺激素的胎盘转运极少。因此，人们认为在胎儿甲状腺激素合成之前，胎儿大脑中不存在甲状腺激素，所以在胎儿甲状腺激素生成之前，甲状腺激素对大脑发育并不重要。然而，最近在大鼠胎儿受孕后14天（dpc）以及大鼠胚胎滋养层细胞受孕后11天检测到了甲状腺激素。在大鼠胎儿中，到受孕后14天已显示出甲状腺激素受体。本实验旨在确定在受孕后13天和16天的大鼠胎儿中是否能检测到T4、T3及其代谢产物，以及碘甲状腺原氨酸是否在胎儿脑和肝脏中选择性蓄积。此外，一组母鼠在繁殖前接受放射性甲状腺切除术，以确定母体甲状腺素血症对胎儿组织碘甲状腺原氨酸浓度的影响。通过高效液相色谱法提取并测量组织中的碘甲状腺原氨酸。在这两个胎儿年龄阶段，T4、T3、反T3（rT3）和3,5 - 二碘甲状腺原氨酸均在该检测方法的检测限范围内。这些激素唯一可能的来源是母体。此外，如果母体血清T4水平较低，胎儿组织中的T4和T3水平也较低。即使在受孕后13天，细胞内T3水平较高，这表明在妊娠中期胎儿中发生了5'-单脱碘作用。细胞内激素测量结果显示，在大鼠胎儿中，T3而非rT3是主要的细胞内碘甲状腺原氨酸。脑和肝脏均选择性蓄积T4和T3，这支持了其他人的观察结果，即在妊娠中期胎儿中存在甲状腺激素受体。受孕后13天大鼠胎儿脑内存在甲状腺激素，再加上在受孕后14天观察到存在激素受体，这表明甲状腺激素在妊娠中期脑发育中确实发挥作用。这些数据表明，在妊娠中期，正常的母体血清甲状腺激素水平对于为胎儿提供足够的甲状腺激素很重要。
