Effects of iodine deficiency on thyroid hormone metabolism and the brain in fetal rats: the role of the maternal transfer of thyroxin.

Thyroid hormones, thyroxin (T4) and 3,5,3'-triiodothyronine (T3), of maternal origin, are available to the mammalian embryo early in development. However, after the onset of fetal thyroid function, they are of both fetal and maternal origin. Maternal T4 has a protective effect on the fetal brain in cases of congenital hypothyroidism. In severe iodine deficiency, maternal T4 is low, although T3 is normal; the developing embryo is markedly T4-deficient; and T3 deficiency increases with gestational age. In contrast to mechanisms in the hypothyroid fetus from a normal mother, the low T4 of the iodine-deficient mother prevents any protective effects on the fetal brain. Thyroid hormone deficiency of the iodine-deficient fetus, including the brain, is more severe and prolonged than it is in the cases of maternal or fetal thyroid failures. These findings may help to explain the relationship between severe maternal hypothyroxinemia and the severe central nervous system damage of the neurological endemic cretin.