A 42-year-old woman with central obesity, proximal myopathy and wide violaceous striae undergoes an overnight 1 mg dexamethasone suppression test. Serum cortisol next morning is 12 micrograms/dL. What is the correct interpretation?
- A Normal result, excluding Cushing syndrome
- B Failure of suppression, consistent with Cushing syndrome and requiring further localisation ✓
- C Exaggerated suppression, indicating primary adrenal hyperplasia
- D Result valid only if performed during the follicular phase of the menstrual cycle
Correct answer: B. Failure of suppression, consistent with Cushing syndrome and requiring further localisation
Explanation
In healthy individuals dexamethasone suppresses ACTH, and next-morning serum cortisol falls below about 1.8 micrograms/dL (50 nmol/L). Failure to suppress below this cutoff indicates autonomous cortisol production or pseudo-Cushing states and mandates further testing with midnight salivary cortisol or 24-hour urinary free cortisol before imaging. The test relies on negative feedback integrity, not menstrual cycle phase, and a value of 12 micrograms/dL is clearly abnormal rather than normal.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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