Effect of Hypothyroidism and Thyroxine Replacement on Growth Hormone in the Rat*

Abstract

Thyroidectomy is known to decrease the amount of GH in the pituitary and serum of rats. The present experiments were undertaken to assess GH levels in the serum of thyroidectomized (Tx) rats using RIA and biological criteria simultaneously. In addition to its ability to sustain growth, the biological activity of circulating GH was assessed by measuring its ability to induce refractoriness to the early insulin-like response of epididymal fat to a test dose of GH added in vitro. This paradoxical desensitization of tissue to some of its own actions constitutes a highly specific and sensitive biological index of GH action. Serum concentrations of T4 declined rapidly after Tx and reached barely detactable levels by the 4th postoperative day. Pituitary concentrations of GH remained unchanged through the 4th postoperative day. Pituitary concentrations of GH remained unchanged through the 4th postoperative day and thereafter declined slowly, reaching 50% of control values by the 7th day and 15% by the 10th day an...