Acute decrease in circulating T3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis.

Abstract

In thyrotoxicosis there is an impaired GH response to GHRH, normal GH responsiveness to GHRP-6 and lack of synergistic GH response after simultaneous administration of both peptides. We have previously shown that the GHRH-induced GH release in these patients increases after an acute reduction of circulating T3 values with administration of iopanoic acid, a compound that inhibits peripheral conversion of T4 to T3. We have now studied the effect of a decrease in serum T3 levels on the GH response to GHRP-6 (1 microg/kg) plus GHRH (100 microg) in 9 hyperthyroid patients before and after 15 days of treatment with iopanoic acid (3 g every 3 days) and propylthiouracil (600 mg/day). Nine normal subjects were also studied. In all hyperthyroid patients iopanoic acid induced a rapid decrease and normalisation of serum T3 levels. In these subjects peak GH (microg/l; mean +/- SE) and AUC (microg/l x 120 min) values after GHRP-6 plus GHRH were significantly higher on day 15 compared to pretreatment values (peak, 18.3 +/- 3.0 vs 13.4 +/- 1.9; AUC, 1227.9 +/- 212.9 vs 968.5 +/- 160.4; p<0.05). Despite the significant enhancement of the GH responsiveness to GHRP-6 plus GHRH after treatment with iopanoic acid, this response remained significantly blunted when compared to controls both in terms of peak GH (18.3 +/- 3.0 vs 83.7 +/- 15.2; p<0.05) and AUC values (1227.9 +/- 212.9 vs 4956.5 +/- 889.3; p<0.05). In conclusion, our results show that an acute decrease of circulating T3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis. This could suggest that circulating T3 does not have a major role in the mechanisms involved in the synergistic effect of these peptides.

• Authors: S O Nascif, M H Senger, J C Ramos-Dias, A M J Lengyel
• Journal: Journal of endocrinological investigation (J Endocrinol Invest) Vol. 26 Issue 8 Pg. 733-7 (Aug 2003) ISSN: 0391-4097 [Print] Italy
• PMID: 14669827 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Antithyroid Agents
• Oligopeptides
• Triiodothyronine
• Human Growth Hormone
• growth hormone releasing hexapeptide
• Propylthiouracil
• Growth Hormone-Releasing Hormone
• Iopanoic Acid

Topics

• Adolescent
• Adult
• Antithyroid Agents (therapeutic use)
• Area Under Curve
• Female
• Fluorescent Antibody Technique
• Growth Hormone-Releasing Hormone (pharmacology)
• Human Growth Hormone (blood)
• Humans
• Hyperthyroidism (complications, drug therapy)
• Iopanoic Acid (therapeutic use)
• Male
• Oligopeptides (pharmacology)
• Propylthiouracil (therapeutic use)
• Thyrotoxicosis (blood, drug therapy)
• Triiodothyronine (blood)