Acute decrease in circulating T-3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis

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Nascif, S. O. [UNIFESP]
Senger, M. H.
Ramos-Dias, J. C.
Lengyel, AMJ [UNIFESP]
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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 T-3 values with administration of iopanoic acid, a compound that inhibits peripheral conversion of T-4 to T3. We have now studied the effect of a decrease in serum T3 levels on the GH response to GHRP-6 (1 mug/kg) plus GHRH (100 mug) 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 T-3 levels. In these subjects peak GH (mug/l; mean+/-SE) and AUC (pg/l(.)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 T-3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis. This could suggest that circulating T-3 does not have a major role in the mechanisms involved in the synergistic effect of these peptides. (C) 2003, Editrice Kurtis.
Journal Of Endocrinological Investigation. Milan: Editrice Kurtis S R L, v. 26, n. 8, p. 733-737, 2003.