A 45-year-old man with progressive shoe-size increase and coarse facial features has an IGF-1 level three times the age-matched upper limit. During a 75 g oral glucose tolerance test, his growth hormone nadir is 4.5 ng/mL. Which interpretation is correct?

• A This is a normal response, excluding acromegaly
• B The result reflects impaired hepatic IGF-1 generation rather than pituitary disease
• C Oral glucose should raise GH in health, so a low value indicates pathology
• D Failure to suppress GH below the diagnostic nadir confirms autonomous somatotroph secretion consistent with acromegaly ✓

Explanation

In healthy subjects, oral glucose suppresses growth hormone to very low levels, typically below 1 ng/mL with modern assays. In acromegaly, the somatotroph adenoma secretes GH autonomously, so suppression fails, and a paradoxical rise may even occur. An elevated IGF-1 with a nonsuppressed GH nadir establishes the diagnosis and directs pituitary imaging. Impaired hepatic IGF-1 generation would lower IGF-1, not elevate it.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.