Decreased ghrelin-induced GH release in thyrotoxicosis: comparison with GH-releasing peptide-6 (GHRP-6) and GHRH

In thyrotoxicosis GH response to several stimuli is impaired, but there is no data on ghrelin-induced GH release in these patients. Ghrelin is a potent GH secretagogue and it also increases glucose levels in men. The aim of this study was to evaluate the effects of ghrelin (1 μg/kg), GHRP-6 (1 μg/kg) and GHRH (100 μg), i.v., on GH levels in 10 hyperthyroid patients and in 8 controls. Glucose levels were also measured during ghrelin and GHRP-6 administration. In control subjects and hyperthyroid patients peak GH (μg/l; mean ± SE) values after ghrelin injection (controls: 66.7 ± 13.6; hyper: 19.3 ± 2.4) were significantly higher than those obtained after GHRP-6 (controls: 26.7 ± 5.1; hyper: 12.6 ± 1.3) and GHRH (controls: 13.5 ± 4.3; hyper: 5.3 ± 1.3). There was a significant decrease in GH responsiveness to ghrelin, GHRP-6 and GHRH in the hyperthyroid group compared to controls. In control subjects and hyperthyroid patients basal glucose (mmol/l) values were 4.5 ± 0.1 and 4.7 ± 0.2, respectively. There was a significant increase in glucose levels 30 min after ghrelin injection (controls: 4.9 ± 0.1; hyper: 5.2 ± 0.2), which remained elevated up to 120 min. When the two groups were compared no differences in glucose values were observed. GHRP-6 administration was not able to increase glucose levels in both groups. Our data shows that GH release after ghrelin, GHRP-6 and GHRH administration is decreased in thyrotoxicosis. This suggests that thyroid hormone excess interferes with GH-releasing pathways activated by these peptides. Our results also suggest that ghrelin’s ability to increase glucose levels is not altered in thyrotoxicosis.