Amelioration of Some Metabolic Effects Produced by Hyperthyroidism in Late Pregnant Rats and their Fetuses

Summary

We have studied the effect of 40-45 days administration of 1 mg/kg thyroxine on protein and lipid metabolism in liver, heart, lungs, kidneys and adrenal glands of virgin and 21-day pregnant rats and their fetuses and placentae. The chronic administration of thyroid hormone produced significant increases in serum T₃ and T₄ in both groups as well as in organ weights and protein concentrations in virgin rats, but much smaller modifications in pregnant ones. Hyperthyroidism decreased the weight of fetal livers and increased that of placentae; protein content was increased in all fetal organs. Hyperthyroidism induced increases in phospholipid concentrations in all the organs and in total lipids only in liver and heart of adult rats, which were not counteracted by pregnancy. Pregnant rats had increases in total lipids in liver and kidneys and in adrenal phospholipids. In hyperthyroid fetuses there was an increase in hepatic total lipids and no changes in phospholipids. Hepatic lipogenesis (measured by in vivo incorporation of ³H₂O into lipids) was increased by hyperthyroidism in virgin and pregnant rats, but the increase was significantly smaller in the pregnant hyperthyroid rats compared with the virgin ones. Fetal lipogenesis in liver and lung was not changed. In addition, an increase was observed in lipogenic enzyme (fatty acid synthetase and glucose-6-phosphate dehydrogenase) activities in hyperthyroid virgin rats which was prevented by pregnancy. In fetuses only pulmonary glucose-6-phosphate dehydrogenase was increased when expressed in terms of tissue weight. Our results indicate that the metabolic effect of hyperthyroidism is attenuated in pregnant rats and their fetuses, when compared with adult virgin rats, in most of the parameters studied. The reduced effects of hyperthyroidism in fetuses could be due to a reduced transplacental passage of thyroid hormones as well as to a reduced fetal response to these hormones, since thyroid hormone receptors appear late in fetal life.

Key words

Hyperthyroidism - Pregnancy - Fetus - Lipids - Proteins - Organ Hypertrophy