Delay in natural decline of lactase activity in the small intestine of prematurely weaned rats as related to changes in their thyroid status.

Premature weaning of suckling rats on the 15th day of life delayed the natural reduction of lactase activity in the small intestine and reduced the level of T4 in serum. Preliminary treatments (from day 12 to 14 of life) with exogenous T4 (1 mg/kg BW) or exposure to cold (10 degrees C) significantly increased the level of T4 in serum on the 15th day of life, and sharply accelerated the reduction of lactase activity. It is concluded that the delay in lactase reduction in prematurely weaned rats may be related to a decrease in serum thyroxine level. Cold as well as exogenous thyroxine may be considered as a means for regulation of the functional maturation of the small intestine.