In evaluating a patient with suspected acromegaly, which dynamic test provides the most reliable physiological confirmation, and what is the expected result in an affected individual?
- A Oral glucose tolerance test — GH fails to suppress below 1 ng/mL (nadir ≥1 ng/mL on current ultrasensitive assays) after 75 g oral glucose ✓
- B Insulin tolerance test — GH fails to rise above 3 ng/mL after hypoglycaemia, confirming GH excess
- C TRH stimulation test — GH paradoxically rises >1 ng/mL; this paradoxical rise is pathognomonic of active acromegaly
- D Glucagon stimulation test — GH rises above 9 ng/mL, indicating pituitary somatotroph hyperplasia
Explanation
The gold-standard confirmatory test for acromegaly is the 75 g OGTT: in normal individuals, glucose suppresses GH below 0.4 ng/mL (ultrasensitive assay nadir cut-off); in acromegaly, GH fails to suppress and nadir remains ≥1 ng/mL. Option C (paradoxical GH rise with TRH) is a useful confirmatory finding in active acromegaly because somatotroph adenoma cells aberrantly express TRH receptors, but it is not the gold-standard confirmatory test. The insulin tolerance test is a GH stimulation test used to diagnose GH deficiency, not excess. Glucagon stimulation also tests for GH deficiency.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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