A new test for the diagnosis of growth hormone deficiency due to primary pituitary impairment: combined administration of pyridostigmine and growth hormone-releasing hormone

Abstract

The diagnosis of growth hormone (GH) deficiency (GHD) is currently based on failure to increase plasma GH levels to an arbitrary cutoff point of 7 or 10 μg/l in response to two provocative stimuli. False negative responses to these tests, however, frequently occur thus reducing their diagnostic reliability. The aim of this study was to assess a combination of pyridostigmine (PD) and GH-releasing hormone (GHRH) (60 mg oral PD 60 min before 1 μg/Kg GHRH iv) as a reliable test probing pituitary somatotropic function. In fact PD, an acetylcholinesterase inhibitor, strikingly potentiates GH response to GHRH likely by inhibiting somatostatin release. The combination PD + GHRH was tested in normal children and adolescents (NS, n = 27) and in a large group of short children classified as having familial short stature (FSS, n = 24), constitutional growth delay (CGD, n = 34) and GH deficiency (organic, oGHD, n = 6; idiopathic, iGHD, n = 10). In all groups results obtained by PD + GHRH were compared with those obtained by testing with GHRH, Clonidine (CLON) and PD alone and by studying spontaneous nocturnal GH secretion over 8 hours. Assuming 7 μg/l as minimum normal GH peak, a positive response occurred in only 18/24, 11/12 and 12/13 NS for GHRH, CLON, and PD, respectively. In contrast even assuming a minimum normal GH peak as high as 20 μg/l, PD + GHRH induced a positive response in 27/27 NS all having a nocturnal GH mean concentration (MC) ≥ 3 ug/l. Therefore PD + GHRH test gave no false negative responses and this was true not only in NS but even in all FSS and CGD having a GH MC ≥ 3 μg/l. On the other hand, PD + GHRH induced a negative GH response in all oGHD and in 8/10 iGHD patients. In the remaining two iGHD patients, PD + GHRH demonstrated a normal pituitary GH reserve in spite of a GH MC < 3 μg/l and low IGF-I level, thus pointing to a hypothalamic pathogenesis for the GHD. Considering FSS and CGD children having a GH MC < 3 μg/l, PD + GHRH showed a primary pituitary GH deficiency in 3/12 CGD with low plasma IGF-I levels. In conclusion, in slowly growing children PD + GHRH test is the most reliable provocative test for the diagnosis of primary pituitary GH deficiency being capable to discriminate between an unequivocally normal and impaired somatotropic function. The study of spontaneous GH secretion is mandatory only for those patients having low height velocity but a normal GH response to PD + GHRH in order to look for the existence of a GHD due to hypothalamic dysfunction.