Effects of hypothyroidism, tri-iodothyronine and glucocorticoids on growth hormone responses to growth hormone-releasing hormone and His-d-Trp-Ala-Trp-d-Phe-Lys-NH2

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Abstract
The aim of this study was to investigate the role of thyroid hormones and glucocorticoids on GH secretion. Secretion of GH in response to GH-releasing hormone (GHRH) (5 μg/kg) was markedly (P < 0·001) decreased in hypothyroid rats in vivo (peak GH responses to GHRH, 635 ± 88 μg/l in euthyroid rats vs 46 ±15 μg/l in hypothyroid rats). Following treatment with tri-iodothyronine (T3; 20 μg/day s.c. daily for 2 weeks) or cortisol (100 pg/day s.c. for 2 weeks) or T3 plus cortisol, a marked (P 3 vs 133+19 μg/l after cortisol vs 283 ± 35 μg/l after cortisol plus T3). In contrast, none of these treatments modified GH responses to GHRH in euthyroid animals. Hypothyroidism was also associated with impaired GH responses to the GH secretagogue, Hisd-Trp-Ala-Trp-d-Phe-Lys-NH2 (GHRP-6). Secretion of GH in response to GHRP-6 in vivo was reduced (P Journal of Endocrinology (1989) 121, 31–36