Temporary depression of lactase activity by thyroxine in suckling rats.

The present study was performed in order to clarify controversies concerning the effect of thyroxine on lactase activity in young mammals. Lactase activity was determined in suckling rats with two reference systems (protein and DNA) after one and three thyroxine injections (0.2 mg/100 g body weight). In comparison with control animals lactase activity was decreased after thyroxine administration. The thyroxine effect was dosage-dependent: three injections caused a more profound and prolonged depression of lactase activity. In both thyroxine-treated groups lactase activity returned to age-specific normal levels before the low lactase activity of adult rats was attained. The transient thyroxine-induced changes of lactase activity were similar to the reference parameters protein and DNA.