A 33-year-old man has elevated IGF-1 and a pituitary macroadenoma. During a 75 g oral glucose tolerance test, growth hormone is measured serially. Which result confirms autonomous growth hormone secretion and establishes acromegaly?
- A Suppression of GH below 0.05 µg/L throughout the test
- B A rise in GH above 10 ng/mL exactly one hour after glucose load
- C Failure of GH to fall below 0.4 µg/L at any point, with possible paradoxical rise ✓
- D A flat GH curve between 2 and 3 ng/mL regardless of glucose
Correct answer: C. Failure of GH to fall below 0.4 µg/L at any point, with possible paradoxical rise
Explanation
Normal subjects suppress growth hormone to very low nadirs, below 0.4 µg/L with modern sensitive assays, after an oral glucose load. In acromegaly, somatotroph adenomas fail to suppress and may even show a paradoxical rise, a response also classically seen in some ectopic GHRH states. Option D describes partial suppression into the 2 to 3 ng/mL range, which older assays might call normal but still exceeds the modern nadir criterion, so it does not confirm the diagnosis.
Reference: Williams Textbook of Endocrinology, 14th ed.
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