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Cushing Disease Diagnosis — RACP Adult Medicine MCQ

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ModerateEndocrinology & DiabetesCushing Disease DiagnosisRACP Adult Medicine

A 55-year-old woman presents with a 6-month history of progressive weight gain (15 kg), facial plethora, proximal myopathy, purple striae, and easy bruising. 24-hour urinary cortisol is markedly elevated. ACTH is elevated at 120 pg/mL. High-dose dexamethasone (8 mg) suppresses cortisol by 70%. What is the most likely source?

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Correct answer: A — Ectopic CRH secretion

ACTH-dependent Cushing syndrome (elevated ACTH) with cortisol suppression on high-dose dexamethasone (>50% reduction) is consistent with a pituitary source (Cushing disease) – ~90% sensitivity. Ectopic ACTH sources (SCLC, carcinoid) typically do NOT suppress with high-dose dexamethasone. However, ~10% of ectopic ACTH tumours (particularly bronchial carcinoids) can suppress, making inferior petrosal sinus sampling (IPSS) the gold standard for confirming pituitary vs ectopic source. MRI pituitary should be performed – microadenomas <6 mm may be missed. Transsphenoidal surgery is first-line treatment for Cushing disease.

Reference: eTG – 2025 – Endocrinology; Endocrine Society – 2015 – Cushing Syndrome Guidelines