Effects of L-thyroxine treatment on pituitary GH content of adult rats with neonatal thyrotoxicosis.
Rats receiving large doses of thyroxine (30 micrograms/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 micrograms/100 g body weight/3 doses or 2.5 micrograms/100 g body weight/8 doses, to 70 day old rats, and 5 micrograms/100 g 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 micrograms 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-thyroid axis but other pituitary functions as well.
D000818:Animals; D000831:Animals, Newborn; D004305:Dose-Response Relationship, Drug; D013006:Growth Hormone / Q000378:metabolism*; D006980:Hyperthyroidism / Q000378:metabolism*; D007030:Hypothalamo-Hypophyseal System / Q000187:drug effects; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013961:Thyroid Gland / Q000187:drug effects; D013965:Thyroidectomy; D013974:Thyroxine / Q000494:pharmacology*
D013006:Growth Hormone; D013974:Thyroxine