Iopanoic acid‐induced decrease of circulating T3 causes a significant increase in GH responsiveness to GH releasing hormone in thyrotoxic patients

Abstract

The results show that an acute reduction in serum T3 levels elicits an increase in GH responsiveness to GHRH in hyperthyroidism. Although the mechanisms involved in this process are still unknown, it is possible that T3 influences GH responsiveness to GHRH via hypothalamic somatostatin release. Alternatively, T3 could have a direct effect at the pituitary somatotroph, modulating GHRH intracellular pathways.

Effects of ghrelin, GH-releasing peptide-6 (GHRP-6) and GHRH on GH, ACTH and cortisol release in hyperthyroidism before and after treatment

“…In untreated thyrotoxic patients, a blunted GH response to ghrelin, GHRP-6 and GHRH was seen, as previously described [12,13,[15][16][17][18].…”

“…In thyrotoxicosis, GH responses to different provocative stimuli, including hypoglycemia and GHRH, have been reported to be normal [9] or diminished [10][11][12][13][14][15][16][17][18]. We have demonstrated that ghrelin-and GHRP-6-induced GH release are also blunted [18].…”

“…We have demonstrated that ghrelin-and GHRP-6-induced GH release are also blunted [18]. Interestingly, an acute reduction in serum circulating T3 levels enhances, but does not normalise the GH response to GHRH and GHRP-6 plus GHRH in thyrotoxicosis [17,19]. The few studies about the effects of antithyroid therapy have shown lack of change [20] or enhancement/restoration of GHRH-induced GH release [12,13] in hyperthyroid patients.…”

“…Impairment of hypothalamic GHRH secretion [21,22] and a direct effect of thyroid hormone excess at the pituitary might be involved [23], while an increase in somatostatin tone is unlikely [21,24]. Although controversial [15,17,25], increased IGF-I levels [13] could also be related to the impaired GH responses after stimulation in thyrotoxicosis.…”

The insulin-like growth axis in patients with autoimmune thyrotoxicosis: effect of antithyroid drug treatment

“…Hypothyroidism is associated with growth impairment, and hyperthyroidism with the development of a hypercatabolic state and skeletal muscle wasting. Thyroid hormones influence GH synthesis and secretion, and impaired GH responses to many pharmacological stimuli, including GH releasing hormone (GHRH), which has been found in patients with thyrotoxicosis [19][20][21][22][23][24][25][26][27][28]. In hypothyroidism IGF-I and IGF-II have been found decreased [28][29][30][31], while elevated IGF-I and/or IGFBP-3 have been demonstrated in thyrotoxicosis in several studies [24,[30][31][32][33][34][35][36], but not in all [20,[28][29][30][31]34,37,38].…”