Is there an undiscovered neurocircuit for regulating GH secretion? -Pitfalls of GHRP-2 and ITT as GH provocative tests-.

GH secretion is mainly regulated at the hypothalamus by a dual interplay between growth hormone releasing hormone (GHRH) and somatostatin, which are modulated by various factors. We examined the regulatory mechanism of GH secretion in an apparently healthy young man without decreased IGF-1 concentration and nocturnal GH secretion, but who showed low responses to insulin tolerance (ITT) and to GHRP-2 tests. The patient also had no GH response to acute aerobic exercise. However, he had normal secretion of pituitary hormone based on hypothalamic releasing hormone tests combined with CRH, GRH as GHRH, LH-RH and TRH. In addition, he had a GH response without paradoxical secretion to TRH stimulation as well as an ACTH response to subcutaneous glucagon stimulation, and AVP secretion responded to 5% hypertonic saline infusion, though it was not adequately stimulated by ITT. MRI showed no structural abnormalities in the hypothalamus-pituitary gland. These findings indicate that this subject may have an undiscovered neurocircuit for regulating GH secretion, as well as other neurohormones, to maintain homeostasis, even though there were low responses of the hormones to ITT and GHRP-2 stimuli, probably via altered secretion of hypothalamic hormones.