DIFFERENT GROWTH-HORMONE (GH) RESPONSES TO GH-RELEASING PEPTIDE AND GH-RELEASING HORMONE IN HYPERTHYROIDISM

Altered GH responses to several pharmacological stimuli, including GHR H, have been found in hyperthyroidism. The mechanisms underlying these disturbances have not been fully elucidated. GH-releasing peptide-6 ( GHRP-B) is a synthetic hexapeptide that specifically stimulates GH rel ease both in vitro and in vivo. The mechanism of action of GHRP-6 is u nknown, but it probably acts by inhibiting the effects of somatostatin on GH release. The aim of this study was to evaluate the effects of G HRP-6 on GH secretion inpatients with hyperthyroidism (n = 9) and in c ontrol subjects (n = 9). Each subject received GHRP-B (1 mu g/kg, iv), GHRK (100 mu g, iv), and GHRP-6 plus GHRH on 3 separate days. GH peak values (mean +/- SE; micrograms per L) were significantly lower in hy perthyroid patients compared to those in control subjects after GHRH a lone (9.0 +/- 1.3 vs. 27.0 +/- 5.2) and GHRP-6 plus GHRH (22.5 +/- 3.5 vs. 83.7 +/- 15.2); a lack of the normal synergistic effect of the as sociation of both peptides was observed in thyrotoxicosis. However, a similar GH response was seen in both groups after isolated GHRP-6 inje ction (31.9 +/- 5.7 vs. 23.2 +/- 3.9). In summary, we have shown that hyperthyroid patients have a normal GH response to GHRP-6 together wit h a blunted GH responsiveness to GHRH. Our data suggest that thyroid h ormones modulate GH release induced by these two peptides in a differe ntial way.