Growth Hormone Stimulation Tests in Assessing Adult Growth Hormone Deficiency

Adult growth hormone deficiency (GHD) is a clinical syndrome that can manifest either as isolated or associated with additional pituitary hormone deficiencies. Its clinical features are subtle and nonspecific, requiring GH stimulation testing 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 perform a GH stimulation test 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 remains the gold standard test, while the glucagon stimulation test and macimorelin test are reasonable alternatives to the insulin tolerance test, whereas the arginine test is no longer recommended because arginine is a poor GH secretagogue that requires a very low peak GH cut-point of 0.4 μg/L. In this chapter, we discuss 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 based on the clinical suspicion of having adult GHD, and summarize current knowledge and change of status of availability of the oral macimorelin test in the United States. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text, WWW.ENDOTEXT.ORG.