The effect of 3,5,3'-triiodothyronine on phosphoenolpyruvate carboxykinase, fatty acid synthetase and malic enzyme activity of liver and brown fat of fetal and neonatal rats.

Pregnant rats were injected with triiodothyronine (T3) on the 19th and 20th days of pregnancy. This elevated fetal T3 plasma values and caused a large rise in fetal liver phosphoenolpyruvate carboxykinase (PEPcK) activity. At the same time PEPcK activity in fetal brown fat was inhibited. Essentially the same effect of T3 was also noted when injected into postnatal rats. T3 injections also elevated fatty acid synthetase activity in brown fat of weanling rats but, in contrast to corticosteroids, had no effect earlier in life. No effect was noted in the liver, except a slight decrease in the fetus. T3 injections to suckling rats elevated plasma levels of insulin and glucagon within 24 h.