As a review, growth hormone is secreted by the pituitary gland. Some pituitary tumors secrete too much GH which causes gigantism in children and acromegaly in adults. However, on the flip side, some tumors suppress the pituitary and too little is secreted. Even if the tumor does not do that, surgery to remove a tumor may cause the pituitary to quit or lessen it's secretion of GH. Radiation is also used on pituitary tumors that cannot be totally removed or if there is hyperplasia, and it, too, can damage the pituitary.
Adult Growth Hormone Deficiency (GHD) is a very real problem. GH maintains a healthy balance of muscle, bones, and fat and if an adult is deficient, her body composition changes. The body has less muscle, visceral fat is deposited around the abdomen, and bones weaken. Other fats in the body are affected. "Good" cholesterol (HDL) decreases but "bad" cholesterol (LDL) increases. This is very hard on the cardiovascular system (remember, the heart is a muscle) and the cerebrovascular system.
In Diagnosis of adult GH deficiency [V. Gasco, et al, Pituitary (2008) 11:121–128], the authors state:
Adults with growth hormone deficiency (GHD) have impaired health, which improves with GH replacement. GHD in adults leads to impairment in body composition and function, as well as to deranged lipoprotein and carbohydrate metabolism and increased cardiovascular morbidity. Based on evidence that GHD in adults is a
new syndrome which may benefit from GH replacement, health authorities in many countries have approved the therapeutic use of GH in hypopituitaric patients with severe GHD.
Ok, all of that to get to the testing part! Whew!
Growth hormone secretion is pulsatile which means random measurements of GH levels are not helpful or diagnostic. Since insulin-like growth factor-1(IGF-1) is stimulated by GH but does not fluctuate during the day like GH, it is useful in monitoring GH levels. Low levels are an excellent indication of a GHD problem. However, normal levels do not mean there is no deficiency.
...the patient with objective evidence of hypothalamic–pituitary disease (e.g., on imaging or after irradiation), who may present with organic isolated GHD as the first hormonal deficiency...may account for up to 25% of cases of GHD in the adult.
Consensus guidelines for the diagnosis and treatment of adults with GH deficiency II[European Journal of Endocrinology (2007) 157 695–700]
Not all patients suspected of having GHD,however, require a GH stimulation test for diagnosis.Patients with three or more pituitary hormone deficiencies and an IGF-I level below the reference range have >97% chance of being GHD, and therefore do not need a GH stimulation test.
Numerous pharmacologic agents can be used to assess GH production and secretion
by the pituitary in adults (Table 3). These include insulin, arginine, levodopa
(L-dopa), arginine plus L-dopa, arginine plus GHRH, and the glucagon test. None
display perfect sensitivity and specificity; however, the insulin tolerance test
(ITT) and arginine-GHRH are excellent tests.