A 44-year-old man has enlarged hands, frontal bossing, and a sellar mass of 14 mm. Random growth hormone is 9 ng/mL and IGF-1 is markedly elevated. Which result on a 75 g oral glucose tolerance test confirms active acromegaly?
- A GH suppressed to less than 1 ng/mL at 120 minutes
- B Rise in GH above baseline within 60 minutes
- C Failure to suppress GH below 0.4 ng/mL (ultrasensitive assay) during the test ✓
- D Fall in IGF-1 by more than 50 percent at 2 hours
Correct answer: C. Failure to suppress GH below 0.4 ng/mL (ultrasensitive assay) during the test
Explanation
Normal physiology suppresses GH during a glucose load; failure to suppress below 0.4 ng/mL on an ultrasensitive assay, together with an elevated IGF-1, confirms acromegaly. IGF-1 elevation itself is strong evidence but the OGTT is the confirmatory dynamic test. Option A describes the normal response, a paradoxical rise occurs in only a minority, and IGF-1 does not fall acutely with glucose.
Reference: Williams Textbook of Endocrinology, 14th ed.
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