Pituitary reserve is completely assessed by using a combined pituitary function test: this combines an insulin tolerance test (ITT), TRH and GnRH tests. In practice the TRH and GnRH tests provide no clinically useful information above that provided by basal hormone measurements: TFTs and prolactin; gonadal steroids and gonadotropins. Moreover, there are case reports of pituitary apoplexy associated with the use of both TRH and GnRH.
The ITT involves precipitating hypoglycaemia with subcutaneous insulin injection, inducing a rise in cortisol and GH in normal individuals. However it is contraindicated in individuals over age 70, and those with ischaemic heart disease or epilepsy, and is unpleasant in all cases.
Alternatives are the glucagon stimulation test, which is a central test of GH and cortisol reserve, and thus comparable to the ITT, or the short Synacthen test, which has the disadvantage of only testing adrenal reserve and may give a false positive result if performed soon after pituitary surgery/damage.
- Burke CW. The pituitary megatest: outdated? Clin Endocrinol (Oxf) 1992;36:133-134.