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

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Abstract
Rats receiving large doses of thyroxine [T4] (30 .mu.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 [growth hormone] content at least until day 22. GH 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 .mu.g/100 g body wt per 3 doses or 2.5 .mu.g/100 g body wt per 8 doses, to 70 day old rats, and 5 .mu.g/100 g body wt per 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 .mu.g T4/100 g body wt, which produced the same increase in pituitary GH in both groups of animals. Changes in pituitary GH content of neo-T4 rats are not due to hypothyroidism. 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.

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