Effects of l-thyroxine treatment on pituitary GH content of adult rats with neonatal thyrotoxicosis

Abstract

Rats receiving large doses of thyroxine (30 \ g=m\ g/ 5 doses) during their first days of life develop an apparently permanent alteration of the hypothalamus-pituitary-thyroid complex. This neonatal thyrotoxicosis has been called neo-T4 syndrome. A state of permanent but not very severe hypothyroidism seems to be induced, accompanied by a decrease in pituitary GH content at least until day 22. In this work, growth hormone content has been measured by a specific radioimmunoassay in the anterior pituitary of 45 and 78 day old neo-T4 and control (saline-injected) rats. GH content of the adult neo-T4 treated animals was significantly lower than that of the adult controls. Administration of different doses of T4 (1.7 \g=m\g/100g body weight/3 doses or 2.5 \g=m\g/100g body weight/8 doses, to 70 day old rats, and 5 \g=m\g/100g body weight/3 doses to 42 day old rats) to adult neo-T4 rats did not alter these decreased pituitary GH levels.This differs from hypothyroid rats, in which T4 administration has been shown to increase pituitary GH content. A third approach was to thyroidectomize neo-T4 and control rats and administer 5 \g=m\g T4/100 g body weight, which produced the same increase in pituitary GH in both groups of animals. These results seem to indicate that changes in pituitary GH content of neo-T4 rats are not due to hypothyroidism. Thus, it would appear that treatment with large T4 doses during the early perinatal period not only deranges the hypothalamic-pituitary\x=req-\ thyroid axis but other pituitary functions as well.It has repeatedly been observed that large doses of thyroxine produce an alteration of the hypothalaRequests of reprints:

Influence of Hormones and Undernutrition on Brain Development in Newborn Rats

“…Both hypotheses seem improbable [6,25]. Nevertheless, neo-T4 rats seem to have an alteration in pituitary TSH synthesis and a lack of pituitary re sponse to TRH [3,6],…”