Because of the pulsatile nature of GH secretion, plasma GH levels vary widely in normal subjects. It follows that a single GH measurement is not diagnostic for an abnormality in the GH axis. Therefore, dynamic testing of the response of plasma GH to standardized provocative or inhibitory tests is mandatory for the evaluation of GH disorders. Table 12-2 lists the dynamic tests in clinical use. This topic has been comprehensively reviewed.31 Among the provocative tests, the insulin-tolerance test is considered the “gold standard,” as it is a potent and reliable stimulus for GH release. Its disadvantage is that induction of hypoglycemia can be risky, and, therefore, the test must be closely supervised. For the test to be valid, a drop in blood glucose by at least 50% from the starting level must be achieved. GHRH testing has not proven to be as useful as anticipated because the GH response is highly variable, presumably because of differences in prevailing somatostatin tone. Clonidine is used successfully in children, but in adults it is a relatively weak stimulus for GH release. In normal subjects, the highest serum GH levels are seen after combined GHRH-GHRP stimulation. However, the clinical utility of this potent test in the diagnosis of GH deficiency is not yet known.