Inhibition of hepatic monooxygenases in neonatal rats by thyroxine

The effect of thyroxine administration on the activities of hepatic microsomal monooxygenases were studied in neonatal rats. While there were no sexual differences in the activities of hepatic hexobarbital hydroxylase, cytochrome P-450 and cytochrome b5 in 8 day old rats, thyroxine administration significantly reduced the activities of these monooxygenases in both male and female neonates. The results suggest that the liver of the neonatal rat has not developed the ability to respond to the enzyme-stimulating effects of thyroxine as reported for adults.