A 52-year-old woman is referred for evaluation of Cushing syndrome. The overnight 1 mg dexamethasone suppression test (DST) shows cortisol of 5.8 mcg/dL (normal suppression <1.8 mcg/dL). Her ACTH is undetectable. The most likely etiology is:

• A Pituitary corticotroph adenoma (Cushing disease)
• B Primary adrenocortical adenoma ✓
• C Ectopic ACTH secretion from small cell lung cancer
• D Pseudo-Cushing syndrome due to depression

Explanation

Undetectable ACTH (<5 pg/mL) in the setting of hypercortisolism indicates an ACTH-independent (primary adrenal) etiology — most commonly an adrenocortical adenoma. The high cortisol suppresses pituitary ACTH production. Cushing disease (pituitary adenoma) and ectopic ACTH syndrome both show detectable or elevated ACTH. Pseudo-Cushing from depression typically shows partial suppression on DST but ACTH levels remain measurable.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.