A 38-year-old woman presents with weight gain, proximal myopathy, violaceous striae, hypertension, and amenorrhoea. 24-hour urinary free cortisol is elevated. Low-dose dexamethasone suppression test (1 mg overnight) shows a cortisol of 7.2 mcg/dL. The NEXT step to establish aetiology is:
- A CRH stimulation test
- B Plasma ACTH measurement ✓
- C MRI pituitary
- D Bilateral inferior petrosal sinus sampling (BIPSS)
Correct answer: B. Plasma ACTH measurement
Explanation
Once Cushing's syndrome is confirmed biochemically (failure to suppress on LDDST), the next step is to determine ACTH-dependency: a suppressed plasma ACTH (<5 pg/mL) points to a primary adrenal source, whereas elevated ACTH indicates ACTH-dependent disease (pituitary Cushing's disease or ectopic ACTH). Imaging and BIPSS are reserved for after ACTH-dependency is established. CRH stimulation is used later to distinguish pituitary from ectopic ACTH after BIPSS.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.