A 34-year-old man has enlargement of hands and feet, coarse facial features, and headache. Random growth hormone levels measured on three separate days are 2.1, 4.8, and 1.6 ng/mL, values overlapping with the reference range. IGF-1 is clearly above the age-matched upper limit. Why is an oral glucose tolerance test required to establish the diagnosis?

• A Glucose paradoxically stimulates GH release in acromegaly, confirming autonomy
• B IGF-1 rises sharply during glucose loading in pituitary adenomas, providing a second confirmatory marker
• C OGTT distinguishes acromegaly from pseudocardiomegaly caused by growth hormone excess
• D GH secretion is pulsatile so random values are unreliable; failure of GH to suppress below 0.4 ng/mL during OGTT confirms autonomous secretion ✓

Explanation

Growth hormone is secreted in pulses, so a healthy person can show random levels well above baseline, making isolated random values uninterpretable. The dynamic test exploits the normal negative feedback of hyperglycaemia on GH: in acromegaly, GH fails to suppress, with a nadir above 0.4 ng/mL on ultrasensitive assays (about 1 ng/mL on older assays), confirming autonomous adenomatous secretion. Glucose actually suppresses, not stimulates, GH in health, and IGF-1 does not rise during the test.

Reference: Williams Textbook of Endocrinology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.