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ACTH-dependent Cushing's syndrome — SCE Endocrinology MCQ

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HardAdrenalACTH-dependent Cushing's syndromeSCE Endocrinology

A 49-year-old woman has easy bruising, proximal weakness and new diabetes. Overnight 1 mg dexamethasone suppression test shows cortisol 248 nmol/L. ACTH is 18 ng/L and pituitary MRI shows a 5 mm lesion. What is the most appropriate next investigation?

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Correct answer: C — Inferior petrosal sinus sampling if ACTH-dependent Cushing's remains uncertain

Inferior petrosal sinus sampling if ACTH-dependent Cushing's remains uncertain is best because a small pituitary lesion can be incidental, so IPSS is used when ACTH-dependent Cushing's localisation remains uncertain. The alternatives are less appropriate because MRI alone can mislead; synacthen assesses adrenal insufficiency; water deprivation tests polyuria; calcitonin assesses medullary thyroid cancer. The SCE teaching point is to integrate the clinical pattern, biochemistry and context rather than treating an isolated result.

Reference: Endocrine Society Cushing's syndrome guideline