EFFECTS OF THYROID-HORMONES ON LIVER BINDING-SITES FOR HUMAN GROWTH-HORMONE, AS STUDIED IN THE RAT

Several weeks after thyroidectomy (t̄), female rats stopped growing, and their pituitary GH content had decreased to less than 2‐3% of the values found for age‐matched controls (C). The liver membranes of such animals were explored with human GH (hGH). It was found that in the severely hypothyroid t̄ rat, the number, but not the affinity, of the lactogenic binding sites was markedly reduced. Treatment of these rats for 3 weeks with 1.75 μg or T4 or 0.5 μg T3/100 g body weight/ day restored growth, increased pituitary GH content and restored the number of liver lactogenic binding sites practically to normal. As regards the lactogenic binding sites, similar results were obtained when the severely hypothyroid rats were treated with a much lower T4 dose (0.2 μg/lOO g/day): this dose was clearly growth promoting, and restored to normal both the low circulating GH levels and the pituitary PRL content of the severely hypothyroid rat. The changes in plasma PRL were not clear. The lactogenic binding sites on liver membranes from rats which were both thyroidectomized and hypophysectomized were decreased in number. Treatment with 0.5 μg T3/100 g/day for 30 days (but not for 12 days) resulted in an increase in the number of lactogenic binding sites, though it did not affect growth or the undetectable plasma CH levels. The effect on the lactogenic binding sites was less marked than in t̄ rats with an intact pituitary. It would appear that minute amounts of thyroid hormones are needed for maintenance of liver lactogenic binding sites; it is possible that this not only occurs through mechanism(s) Correspondence: Prof. S. Duran‐Garcia, Hospital Universitario, Planta 7a, Sevilla, Spain. Copyright © 1979, Wiley Blackwell. All rights reserved