A 55-year-old man has central obesity, hypertension, and a morning cortisol of 650 nmol/L following a 1 mg overnight dexamethasone suppression test (normal <50 nmol/L). ACTH is 2 pmol/L (low). The most appropriate next investigation is:
- A CRH stimulation test
- B CT scan of the adrenal glands ✓
- C Inferior petrosal sinus sampling (IPSS)
- D High-dose dexamethasone suppression test
Correct answer: B. CT scan of the adrenal glands
Explanation
A low ACTH with failed dexamethasone suppression indicates ACTH-independent (adrenal) Cushing's syndrome. CRH stimulation and IPSS are indicated only when ACTH is elevated (ACTH-dependent, to differentiate pituitary from ectopic source). CT adrenal is the next step to identify an adrenal adenoma, carcinoma, or bilateral hyperplasia. High-dose DST would not be required in ACTH-independent disease.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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