Altered GH responses to several pharmacological stimuli, including GHRH, have been found in
hyperthyroidism. The mechanisms underlying these disturbances have not been fully elucidated.
GH-releasing peptide-6 (GHRP-6) is a synthetic hexapeptide that specifically stimulates GH release both in vitro and in vivo. The mechanism of action of
GHRP-6 is unknown, but it probably acts by inhibiting the effects of
somatostatin on GH release. The aim of this study was to evaluate the effects of
GHRP-6 on GH secretion in patients with
hyperthyroidism (n = 9) and in control subjects (n = 9). Each subject received
GHRP-6 (1 microg/kg, iv), GHRH (100 microg, iv), and
GHRP-6 plus GHRH on 3 separate days. GH peak values (mean +/- SE; micrograms per L) were significantly lower in
hyperthyroid patients compared to those in control subjects after GHRH alone (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 association of both
peptides was observed in
thyrotoxicosis. However, a similar GH response was seen in both groups after isolated
GHRP-6 injection (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 with a blunted GH responsiveness to GHRH. Our data suggest that
thyroid hormones modulate GH release induced by these two
peptides in a differential way.