Advances in hormone testing made in 2001 have made accurate measurement of growth hormone (GH) levels possible. This is accomplished by measuring it directly in a 24-hour sample of urine. It is expected to replace IGF1 testing, which is an indirect measure of GH and less accurate.
Low levels of IGF-1 are usually indicative of significant adult growth hormone deficiency, but it is not a very sensitive test marker and will miss up to 60% of growth hormone deficient patients aged over 40. Many endocrinologists will test to diagnose adult GH deficiency by stimulating a GH response to a combination of GHRH and an inhibitor of somatostatin tone such as pyridostigmine, arginine, clonidine or insulin. Endocrinologists generally consider the insulin tolerance test (ITT) to be the most useful test to evaluate the overall GH secretion in subjects with possible hypopituitary disease. However, ITT is not suitable in elderly or in patients with cardiovascular disease or seizure disorders. Furthermore, the GH response to ITT may be normal in "physiologic" GH deficiency, as it measures the overall capacity of the stress-axis rather than the physiological secretion of GH.
A comparison of ITT, the pyridostigmine plus GHRH (PD + GHRH) test, the clonidine plus GHRH (CLO+GHRH) test, and insulin-like growth factor I (IGF-I) in diagnosing GH deficiency has recently been reported. The peak GH response was significantly higher during the PD+GHRH test than during the ITT. IGF-I levels were subnormal in only 42% of the patients. It was recommended that adults with suspected GH deficiency and a normal IGF-I level should undergo two different stimulation tests. In patients with a subnormal IGF-I value, a single stimulation test would suffice to confirm the presence of GH deficiency.
Direct GH testing is currently offered by Genova Diagnostics.