Acute decrease in circulating T3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis.
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.
Nascif
S O
SO
Division of Endocrinology, Universidade Federal de São Paulo, UNIFESP/EPM, São Paulo, Brazil.
Senger
M H
MH
Ramos-Dias
J C
JC
Lengyel
A M J
AM
D000293:Adolescent; D000328:Adult; D013956:Antithyroid Agents / Q000627:therapeutic use*; D019540:Area Under Curve; D005260:Female; D005455:Fluorescent Antibody Technique; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D019382:Human Growth Hormone / Q000097:blood*; D006801:Humans; D006980:Hyperthyroidism / Q000150:complications / Q000188:drug therapy; D007480:Iopanoic Acid / Q000627:therapeutic use; D008297:Male; D009842:Oligopeptides / Q000494:pharmacology*; D011441:Propylthiouracil / Q000627:therapeutic use; D013971:Thyrotoxicosis / Q000097:blood* / Q000188:drug therapy*; D014284:Triiodothyronine / Q000097:blood*
D013956:Antithyroid Agents; D009842:Oligopeptides; D014284:Triiodothyronine; D019382:Human Growth Hormone; C041048:growth hormone releasing hexapeptide; D011441:Propylthiouracil; D013007:Growth Hormone-Releasing Hormone; D007480:Iopanoic Acid