Effects of neonatal exposure to TRH on development of the pituitary-thyroid axis in rats.

TRH (10 and 1000 micrograms/kg body weight) was administered ip daily to neonatal rats from day 0 to 9 after birth (Neo-TRH rats) and their pituitary-thyroid axis was examined on days 4, 10, 21 and 90. The pituitary TSH content in Neo-TRH rats was significantly smaller than in controls on days 4 and 10. The serum TSH levels in Neo-TRH rats were significantly lower than those in controls on days 4 (male group only), 10 and 21 (only 10 micrograms/kg group). The serum T4 levels in Neo-TRH rats were lower than in controls on day 10. The reduced pituitary TSH content and serum TSH and T4 were restored to control levels on day 90. However, the response of serum TSH to exogenous TRH (10 micrograms/kg/ip) was blunted in Neo-TRH rats on days 10, 21 and 90. It is concluded that repetitive administration of TRH during the neonatal period suppresses the pituitary-thyroid axis in neonatal life, even after the basal hormone level has been restored to normal.