GH-releasing hormone and GH-releasing peptide-6 for diagnostic testing in GH-deficient adults

Background The diagnosis of growth hormone (GH) deficiency in adults is bas ed on provocative testing of GH secretion. The insulin tolerance test (ITT) , currently the favoured test for this diagnosis, has been criticised for p oor reproducibility and inconvenience. Since the combined administration of GH-releasing hormone (GHRH) plus GH-releasing peptide-6 (GHRP-6) is the mo st potent stimulus of GH secretion, we did a multicentre study comparing GH peaks elicited by ITT with those elicited by the GHRH/GHRP-6 rest in healt hy controls and GH-deficient individuals (cases), Methods 125 adult patients with organic pituitary disease and 125 healthy i ndividuals were studied. All cases and controls were given GHRH 1 mu g per kg bodyweight intravenously plus GHRP-6 1 E-g per hg intravenously at 0 min and blood samples were obtained during a subsequent 120 min period. 27 con trols and all cases had an ITT. Inclusion criteria were severe GH deficienc y-ie, a GH peak after ITT of less than or equal to 3 mu g/L Results of the GHRH/GHRP-6 test were analysed by receiver-operating characteristic curve m ethodology. Findings GH peaks seen after the GHRH/GHRP-6 test did not result in any sid e-effects and were not affected by age, sex, amount of adipose tissue, or b y the GH assay system used. The GH mean peak after the GHRH/GHRP-6 test was 59.2 mu g/L (SD 2.2) for controls and 4.1 mu g/L (0.3) for cases, whereas after ITT the mean peak was 14.3 mu g/L (1.7) and 0.5 mu g/L (0.06), respec tively. The differential peak responses of controls and cases was greater ( p<0.001), for GHRH/GHRP-6 test than for ITT, When individually analysed GH peaks were a continuum, from 139.0 mu g/L to 0.01 mu g/L, with a cut-off po int of 15.0 mu g/L. The GHRH/GHRP-6 test performed well under the ROC curve analysis. For clinical utility, it is then proposed that values greater th an or equal to 20.00 mu g/L be considered normal and less than or equal to 10.00 mu g/L as GH deficient Interpretation The GHRH/GHRP-6 test is a convenient, safe and reliable test for adult GH deficiency and is not confounded by clinical factors known to alter GH secretory patterns. An evoked GH concentration of greater than or equal to 15.0 mu g/L accurately distinguishes between healthy and GH-defic ient adults.