Thyroid hormones fetal brain availability

Table 49.2 Physiology of thyroid hormone availability to the fetal brain...

Altogether, these findings indicate not only that a transfer of maternal thyroid hormone, namely of T4, to the fetus is effective from early pregnancy, but also that the systems regulating the availability of T3 to the fetal brain are expressed and likely operating by that time. 

These observations suggested that normal brain development requires the availability of both maternal and fetal thyroid hormones,a suggestion which is at variance with earlier reports (9) (14) (15) (16) that the placenta in many mammalian species is relatively impermeable to thyroid hormones and with the suggestion (17) (18) that early mammalian development takes place normally in the absence of thyroid homrones. The observations do agree however with a more recent report (19) that rat embryonic tissues are provided with T and T3 only four days after uterine implantation and well before the onset of fetal thyroid function at 17 days. y also supported by the work of Woods et al (20) who showed that T and T3, when injected into pregnant rats, entered the rat... 

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