Intra- and inter-individual variability of growth hormone (GH) responsiveness to repetitive GH-releasing hormone stimulation test in man

Normal subjects show a wide range of growth hormone (GH) responses to growth hormone-releasing hormone (GHRH) stimulation, but it is uncertain whether this variability reflects differences among individuals or whether it would also be observed on repeated tests of the same subject. To clarify this, we tested nine normal men repeatedly with iv bolus doses of 1 mug/kg GHRH(1-44)NH(2). Most subjects showed wide variations in their GH responses on repeated testing, and the intra-individual variability was nearly as great as the inter-individual variability in responses, accounting for about two-thirds of the overall variance. A minority of subjects had lower and less variable responses. Ultradian fluctuations in hypothalamic somatostatin secretion may account for this marked intra-individual variability.