Hypothyroidism Blunts the Growth Hormone (GH) Releasing Effect of Human Pancreatic GH Releasing Factor in the Adult Male Ratin Vivoand in Vitro*

The effect of the 44-amino-acid peptide human pancreatic GH releasing factor (hpGRF-44) upon the secretion of GH was studied in control and hypothyroid adult male rats. In animals rendered hypothyroid by ingestion of propylthiouracil (PTU), basal and hpGRF-44-stimulated secretion of GH was depressed in vivo. Administration of T4 together with PTU prevented the decline in basal and hpGRF-provoked GH secretion in vivo. HpGRF-44-stimulated release of rat GH was also impaired in vitro, an effect partially reversed by administration of low doses of T4 in vivo. The depressed in vitro secretion of GH could not be restored in hypothyroidism pituitaries by incubation of the glands with T3. Thus, hypothyroidism blunts hpGRF-44-stimulated secretion of GH in vivo and in vitro in the hypothyroid adult male rat.