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.

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 micrograms/kg) was markedly (P less than 0.001) decreased in hypothyroid rats in vivo (peak GH responses to GHRH, 635 +/- 88 micrograms/l in euthyroid rats vs 46 +/- 15 micrograms/l in hypothyroid rats). Following treatment with tri-iodothyronine (T3; 20 micrograms/day s.c. daily for 2 weeks) or cortisol (100 micrograms/day s.c. for 2 weeks) or T3 plus cortisol, a marked (P less than 0.01) increase in GH responses to GHRH was observed in hypothyroid rats (peak GH responses, 326 +/- 29 micrograms/l after T3 vs 133 +/- 19 micrograms/l after cortisol vs 283 +/- 35 micrograms/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, His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6). Secretion of GH in response to GHRP-6 in vivo was reduced (P less than 0.01) in hypothyroid rats (peak GH responses, 508 +/- 177 micrograms/l in euthyroid rats vs 203 +/- 15 micrograms/l in hypothyroid rats). In-vitro studies carried out using monolayer cultures of rat anterior pituitary cells derived from euthyroid and hypothyroid rats showed a marked impairment of somatotroph responsiveness to both GHRP-6 and somatostatin in cultures derived from hypothyroid rats. In summary, our data suggest that thyroid hormones and glucocorticoids influence GH secretion by modulating somatotroph responsiveness to different GH secretagogues.