A 42-year-old woman with central obesity, proximal myopathy and striae undergoes an overnight 1 mg dexamethasone suppression test. Serum cortisol the next morning is 8 micrograms per decilitre. How should this result be interpreted?
- A Normal suppression, excluding Cushing syndrome
- B Diagnostic of primary adrenal insufficiency
- C Failure of suppression, consistent with Cushing syndrome and warranting further evaluation ✓
- D Diagnostic of exogenous steroid intake without further testing
Correct answer: C. Failure of suppression, consistent with Cushing syndrome and warranting further evaluation
Explanation
In healthy individuals, 1 mg of dexamethasone given at night suppresses the next morning cortisol below 1.8 micrograms per decilitre. A value of 8 micrograms per decilitre represents failure of suppression, which is consistent with Cushing syndrome but not diagnostic by itself, since depression, obesity and alcohol use can cause false positives. Further testing with midnight salivary cortisol, 24-hour urinary free cortisol and ACTH levels is needed to confirm and localise the cause.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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