Measures:
(1) random fasting or random growth hormone level
(2) nadir growth hormone during an oral glucose tolerance test (OGTT, 75 gram, with glucose measurements every 30 minutes up to 2 hours)
(3) serum insulin growth factor I (IGF-I) levels
(4) mean integrated 24 hours growth hormone level
Criteria for exclusion of acromegaly using biochemical tests:
(1) random serum growth hormone concentration < 0.4 µg/L AND serum IGF-1 within normal reference range for age and gender
(2) nadir serum growth hormone concentration during the OGTT is < 1.0 µg/L AND serum IGF-1 within normal reference range for age and gender
(3) mean integrated 24-hour growth hormone level < 2.5 µg/L
where:
• The mean integrated 24-hour growth hormone level is usually not performed since the oral glucose tolerance test is more cost effective.
• The oral glucose tolerance test is used to suppress growth hormone release.
The recommended sequence of testing is:
(1) Measure random GH and IGF-1 levels. If both normal then acromegaly is excluded.
(2) If the random GH is elevated, then GH suppression with the oral GTT is performed. If criteria 2 is met then acromegaly is excluded.
(3) If acromegaly is not excluded then additional studies are warranted.
Conditions associated with a failure for growth hormone nadir during OGTT (false positive test):
(1) diabetes mellitus
(2) liver disease
(3) renal disease
(4) adolescence
(5) anorexia nervosa
Occasional patients with acromegaly may show a nadir in the growth hormone to < 1 µg/L during the OGTT, but the IGF-I levels will be elevated and there may be clinical findings suggestive of acromegaly.
