Levels of L-T3 in maternal and foetal compartments following experimental modifications of the maternal thyroid state in rats.

In pregnant female rats, concentrations of tri-iodo-L-thyronine in maternal serum, amniotic fluid and placental tissue after 15, 18 and 20 days of gestation were measured by homologous radioimmune-analysis. The three experimental groups of pregnant rats were: 1) euthyroid (or control), 2) hypothyroid, provoked by iodine-deficient diet for two months before conception and during gestation, 3) hyperthyroid, provoked by subcutaneous injections of L-thyroxine during gestation. Maternal serum L-T3 was measured in order to check the thyroid state. Significant decreases in L-T3 concentrations were found at all stages of gestation in the amniotic fluid of hypothyroid group. The hormonal concentrations in the placental tissues were correlated with the different treatments (decreased in hypothyroid state and increased in hyperthyroid state). This could suggest that the transfer of maternal iodothyronines to the foetus influences its foetal thyroid development.