Tests of Pituitary Function: GH Deficiency

 

From Dr. Lewis Blevins.  Growth Hormone levels, by themselves, are not a good indicator of GH sufficiency. Levels are undetectable 50% of the time during the day in normal persons. Thus, when a diagnosis of GH deficiency is suspected, a stimulation test must be done to assess pituitary reserve and ability to produce GH on demand. The “gold standard” test is an insulin-induced hypoglycemia test. This is, however, a cumbersome and sometimes dangerous test so we do not use it much at all. Good surrogates in adult patients include the glucagon stimulation test and the arginine stimulation test.

In children exercise, l-dopa, and clonidine all can stimulate GH release. In these tests, GH levels usually rise to above 5 in normal adult persons. The cutoffs for children are different and sometimes vary depending on test and site. Obesity may produce a blunted response and some argue that the cutoff should be much lower in obese people. Regardless, GH an abnormal stimulation test is usually required for GH to be covered by insurance companies and approved by the FDA.

Some insurance companies require two abnormal tests but one is sufficient in those with pituitary diseases and other pituitary hormone deficiencies. IGF-I produced by the liver in response to GH is only 75% accurate in the diagnosis of GH deficiency. Low levels are easy…but some patients have low normal levels and are still GH deficient.

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