Intrasubject reproducibility of growth hormone-releasing hormone-stimulated growth hormone in older women, older men, and younger men.

Intrasubject reproducibility of growth hormone (GH) response to growth hormone-releasing hormone (GHRH) was studied in healthy older women (n = 9), older men (n = 8), and younger men (n = 10). Subjects received IV injections of 0.1 ml/kg saline, 1 micrograms/kg GHRH, and 2 micrograms/kg GHRH, three times each, and blood was sampled at 0, 15, 30, 45, 60, and 120 min for GH concentration. There was no significant difference in peak GH response between the 1- and 2-micrograms/kg GHRH dosages. GH responsiveness, group variance of peak GH, and intrasubject variability were greatest for younger men, less for older men, and least for older women at both dosages of GHRH. Because of the large intrasubject variability observed in this study, it appears necessary to test subjects more than once to obtain a valid characterization of GH responsiveness.