The clinical features of adult GH deficiency (GHD) are nonspecific, and GH stimulation testing is often required to arrive at a correct diagnosis. However, diagnosing adult GHD can be challenging due to the episodic and pulsatile endogenous GH secretion, concurrently modified by age, gender, and body mass index. Hence, a GH stimulation test is often required to establish the diagnosis, and should only be considered if there is a clinical suspicion of GHD and the intention to treat if the diagnosis is confirmed. Currently, there is no ideal stimulation test and the decision to consider performing a GH stimulation test to diagnose adult GHD must factor in the validity of the chosen test, the appropriate GH cut-points, and the availability of local resources and expertise. For now, the insulin tolerance test should still be considered as the gold standard GH stimulation test, and the glucagon stimulation test and macimorelin test are reasonable alternatives to the insulin tolerance test, whereas the arginine test is no longer recommended by the 2019 AACE Clinical Practice Guidelines because this test has insufficient diagnostic accuracy and requires a very low peak GH cut-point of 0.4 μg/L to make the diagnosis. In this chapter, we discuss recently published evidence of the GH stimulation tests used in the United States and the inherent caveats and limitations of each individual test. We propose utilizing the lower GH cut-point to 1μg/L for the glucagon stimulation test to improve its diagnostic accuracy in some overweight and all obese patients, and summarize current knowledge of the new oral macimorelin test as the recently and only approved diagnostic test for adult GHD by the United States Food and Drug Administration and the European Medicines Agency. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text,
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