MATERNAL THYROXINE AND 3,5,3'-TRI-IODOTHYRONINE KINETICS IN NEAR-TERMPREGNANT RATS AT 2 DIFFERENT LEVELS OF HYPOTHYROIDISM

Thyroid hormones are extremely important for development of the fetal central nervous system. Thyroidectomy results in severe hythyroidism. In this study two levels of maternal hypothyroidism were reached by ad ministration of different amounts of thyroxine (T-4) and 3,5,3'-tri-io dothpronine (T-3) to thyroidectomized pregnant rats. We examined the p roduction, distribution and transport of T-4 and Tg by performing a ki netic experiment (three-compartment analysis) with intact and thyroide ctomized near-term pregnant rats which received either very low (Tx lowTH) or normal (Tx + TH) doses of T-4 and Tg. Despite administration of normal doses of thyroid hormones, plasma TSH was still elevated in the Tx + TH rats, meaning that these rats were still mildly hypothyro id. The Tx + lowTH rats were markedly hypothyroid, the plasma T-4 and T-3 levels being very low. In the mildly hypothyroid rats the transpor t of T-4 from plasma to the fast pool and vice versa was decreased com pared with intact near-term pregnant rats. This could imply that much less T-4 is transported to the fete-placental compartment. Liver type I deiodinase was decreased, resulting in lowered plasma Tg values. In the markedly hypothyroid rats all pools and rates of transport of T-4 and T-3 were greatly decreased. In conclusion, even mild hypothyroidis m,despite normal plasma T-4 values, results in significant changes, es pecially in maternal T-4 transport. We suggest that even mild maternal hypothyroidism will have a negative effect on the availability of mat ernal T-4 for fetuses.