Maternal-fetal thyroid hormone relationships and the fetal brain

Thyroid hormones are transferred from the mother to the fetus. Thus, despite the deiodinating enzymes of the placenta (26), some T4 and T3 is transferred, both before and after onset of fetal thyroid function, at least in those cases where fetal thyroid function is impaired. It is also possible that transfer occurs under normal conditions. Maternal to fetal transfer of T3 and T4 is partially limited. But it might be enough to mitigate severe fetal T4 and T3 deficiencies. However, the mitigating effects of both hormones are not equivalent for all fetal tissues. 1) Maternal T4 mitigates T4 and T3 deficiency of most fetal tissues, the brain included. 2) Maternal T3 mitigates T3 deficiency only in some fetal tissues, the brain being excluded. It does not mitigate cerebral T3 deficiency even at doses which are toxic for the mother, and it does not depress fetal plasma TSH. 3) Normal maternal thyroid function is important for fetal development. Maternal hypothyroxinemia is damaging to the developing fetal brain early in gestation. It might also later have adverse effects in gestation, if the fetal thyroid is impaired. Normal maternal T3 levels might avoid overt hypothyroidism of some fetal tissues, but is of no benefit to the brain.