A 28-year-old woman presents with weight gain, fatigue, and a serum cortisol of 820 nmol/L at 8 AM after a 1 mg overnight dexamethasone suppression test. Late-night salivary cortisol is elevated on two occasions. ACTH is undetectable. The MOST likely diagnosis is:

• A Adrenal adenoma secreting cortisol ✓
• B Cushing's disease (pituitary ACTH adenoma)
• C Ectopic ACTH syndrome
• D Pseudo-Cushing's syndrome (alcohol-related)

Explanation

Undetectable ACTH (<1.1 pmol/L) in the setting of biochemically confirmed hypercortisolism (failed low-dose DST, elevated late-night salivary cortisol) points to ACTH-independent Cushing's syndrome, most commonly an adrenal adenoma in this age group. ACTH is suppressed by autonomous adrenal cortisol secretion. Pituitary Cushing's disease and ectopic ACTH syndrome both feature elevated or inappropriately normal ACTH. Pseudo-Cushing's states also typically show some ACTH suppression but have a clinical context (alcoholism, depression).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.