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Low Dose Hexarelin and Growth Hormone (GH)-Releasing Hormone as a Diagnostic Tool for the Diagnosis of GH Deficiency in Adults: Comparison with Insulin-Induced Hypoglycemia Test¹

Department of Endocrinology, University of Pisa (M.G., G.S., C.C., E.M.), 56124 Pisa; and the Division of Endocrinology, Department of Internal Medicine, University of Turin (G.A., G.C., E.A., E.G.), 10126 Turin, Italy

Address all correspondence and requests for reprints to: Dr. M. Gasperi, Dipartimento di Endocrinologia, Ospedale Cisanello, Via Paradisa 2, 56124 Pisa, Italy. E-mail: mgasperi{at}endoc.med.unipi.it

GH deficiency (GHD) in adults must be shown by provocative testing of GH secretion. Insulin-induced hypoglycemia (ITT) is the test of choice, and severe GHD, treated with recombinant human GH replacement, is defined by a GH peak response to ITT of less than 3 µg/L. GHRH plus arginine (ARG) is a more provocative test and is as sensitive as ITT provided that appropriate cut-off limits are assumed. GH secretagogues are a family of peptidyl and nonpeptidyl GH-releasing molecules that strongly stimulate GH secretion and, even at low doses, truly synergize with GHRH. Our aim was to verify the diagnostic reliability of the hexarelin (HEX; 0.25 µg/kg, iv) and GHRH (1 µg/kg, iv) test for the diagnosis of adult GHD. To this goal, in the present study we 1) defined the normal ranges of the GH response to GHRH+HEX in a group of normal young adult volunteers (NS; n = 25; 18 men and 7 women; age, 28.5 ± 0.6 yr) and in 11 of them verified its reproducibility in a second session, and 2) compared the GH response to GHRH+HEX with that to ITT in a group of normal subjects (n = 33; 12 men and 21 women; age, 34.1 ± 1.5 yr) and hypopituitaric adults with GHD (n = 19; 10 men and 9 women; age, 39.9 ± 2.2 yr; GH peak <5 µg/L after ITT). The GH response to GHRH+ARG was also evaluated in all GHD and in 77 normal subjects (40 men and 37 women; age, 28.1 ± 0.6 yr). The mean GH peak after GHRH+HEX in NS was 83.6 ± 4.5 µg/L; the third and first percentile limits of the normal GH response were 55.5 and 51.2 µg/L, respectively). The GH response to GHRH+HEX in NS showed good intraindividual reproducibility. In GHD the mean GH peak after GHRH+HEX (2.6 ± 0.7 µg/L) was similar to that after GHRH+ARG (3.6 ± 1.0 µg/L), and both were higher (P < 0.001) than that after ITT (0.6 ± 0.1 µg/L); the GH responses to GHRH+HEX were positively associated with those to ITT and GHRH+ARG. Analyzing individual GH responses, 100% had severe GHD after ITT (GH peak, <3 µg/L). After GHRH+HEX all GHD had GH peaks below the third percentile limit of normality appropriate for this test (i.e. 55.5 µg/L). Thirteen of 19 (68.4%) GHD subjects had GH peaks below 3 µg/L after GHRH+HEX but all 19 (100%) had GH peaks below the first percentile limit of normality (i.e. 51.2 µg/L). The GH responses to GHRH+HEX were highly concordant with those after GHRH+ARG. In conclusion, the present results define normal limits of the GH response to stimulation with low dose HEX+GHRH in normal adults and show that this test is as sensitive as ITT for the diagnosis of adult GHD provided that appropriate cut-off limits are considered.

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