Growth hormone responses to GH-releasing peptide (GHRP-6) in hypothyroidism

Clin Endocrinol (Oxf). 1997 Mar;46(3):295-300. doi: 10.1046/j.1365-2265.1997.1270942.x.


Objective: Both spontaneous and stimulated GH secretion are reduced in patients with hypothyroidism. The mechanisms involved in these alterations are not yet fully understood. GHRP-6 is a synthetic hexapeptide that releases GH both in vivo and in vitro. Its mechanism of action is unknown, but there is evidence that this peptide acts as a functional somatostatin antagonist at pituitary level. The aim of this study was to evaluate the GH response to GHRP-6 in patients with primary hypothyroidism and in normal controls.

Design: Patients with hypothyroidism and normal controls were randomly submitted to 3 tests with GHRH (100 micrograms i.v.), GHRP-6 (1 microgram/kg i.v.) and GHRH + GHRP-6, on separate days.

Patients: Eleven patients with primary hypothyroidism were compared with 10 control subjects.

Measurements: GH, TSH and free T4 were measured by immunofluorometric assay and IGF-1 by radioimmunoassay.

Results: Hypothyroid patients had markedly lower peak GH values (mean +/- SE micrograms/l) after GHRH administration (4.1 +/- 0.9) compared to control subjects (24.9 +/- 5.1). After GHRP-6 injection hypothyroid patients had a significantly higher GH release (12.6 +/- 1.9) than that obtained with GHRH, while in control subjects GH values were similar (22.1 +/- 3.6). No significant differences in peak GH responses were observed following the administration of either GHRP-6 alone (controls 22.1 +/- 3.6; patients 12.6 +/- 1.9) or in combination with GHRH (controls 77.4 +/- 15.0; patients 52.8 +/- 10.9), despite the trend to smaller responses in hypothyroid patients.

Conclusion: We have shown that patients with primary hypothyroidism have higher GH responses to GHRP-6 than to GHRH, which are markedly blunted. When GHRP-6 was associated with GHRH, a significant increase in the GH response was observed in these patients, which could suggest a role for somatostatin in this process. Our data suggest that thyroid hormones modulate GH release induced by GHRH and GHRP-6 through different mechanisms. However, additional studies are necessary to further elucidate this hypothesis.

Publication types

  • Clinical Trial
  • Comparative Study
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Female
  • Growth Hormone / blood
  • Growth Hormone / metabolism*
  • Growth Hormone-Releasing Hormone / pharmacology*
  • Hormones / pharmacology
  • Humans
  • Hypothyroidism / blood*
  • Hypothyroidism / physiopathology
  • Male
  • Middle Aged
  • Oligopeptides / pharmacology*
  • Thyrotropin / blood
  • Thyroxine / blood


  • Hormones
  • Oligopeptides
  • growth hormone releasing hexapeptide
  • Thyrotropin
  • Growth Hormone
  • Growth Hormone-Releasing Hormone
  • Thyroxine