A 52-year-old woman with a 6-month history of progressive proximal muscle weakness, hypertension, and hirsutism is found to have elevated 24-hour urinary free cortisol and suppressed ACTH. Imaging reveals a 2 cm left adrenal mass. Which single feature best distinguishes adrenocorticotropic hormone (ACTH)-independent Cushing syndrome from ACTH-dependent Cushing syndrome in this patient?

Explanation

## Distinguishing ACTH-Dependent from ACTH-Independent Cushing Syndrome ### The Key Discriminator: Plasma ACTH Level **Key Point:** The single most reliable discriminating feature between ACTH-independent and ACTH-dependent Cushing syndrome is the **plasma ACTH concentration**. In ACTH-independent disease (adrenal source), ACTH is suppressed due to negative feedback from elevated cortisol. In ACTH-dependent disease (pituitary or ectopic ACTH), ACTH is elevated or inappropriately normal. ### Pathophysiology of ACTH Suppression In adrenocorticotropic hormone-independent Cushing syndrome: 1. The adrenal tumor produces cortisol autonomously 2. High cortisol exerts negative feedback on the anterior pituitary corticotrophs 3. ACTH secretion is suppressed → plasma ACTH typically **< 5 pg/mL** (normal range 10–50 pg/mL) 4. The dexamethasone suppression test shows **no suppression** of cortisol (unlike pituitary Cushing, which suppresses with high-dose dexamethasone) In ACTH-dependent Cushing syndrome: - Pituitary adenoma or ectopic ACTH source drives ACTH secretion - ACTH is elevated or high-normal (typically > 20 pg/mL) - Cortisol remains elevated despite feedback ### Comparison Table: ACTH-Independent vs. ACTH-Dependent | Feature | ACTH-Independent (Adrenal) | ACTH-Dependent (Pituitary/Ectopic) | | --- | --- | --- | | **Plasma ACTH** | **Suppressed (< 5 pg/mL)** ✓ | Elevated or high-normal (> 20 pg/mL) | | 24-h UFC | Elevated | Elevated | | Hypokalemia | Common | Common | | Metabolic alkalosis | Common | Common | | Dexamethasone suppression (8 mg) | No suppression | Suppression (if pituitary source) | | Imaging | Adrenal mass | Pituitary adenoma or ectopic source | **High-Yield:** ACTH measurement is the **first-line discriminator** after confirming hypercortisolism. It divides the diagnostic pathway into two distinct branches with different imaging and management strategies. ### Clinical Pearl In this case, the suppressed ACTH with elevated cortisol and an adrenal mass on imaging confirms **adrenal Cushing syndrome** (ACTH-independent). The next step would be adrenalectomy, not further pituitary or ectopic source workup. **Mnemonic:** **"ACTH-I = Adrenal, ACTH suppressed"** — Independent Cushing = Adrenal source = ACTH is Inhibited by feedback. ### Why Other Features Are Non-Discriminatory - **Hypokalemic metabolic alkalosis, elevated UFC, rapid symptom onset:** These occur in both ACTH-dependent and ACTH-independent disease and cannot distinguish between them. - The severity and speed of presentation depend on the tumor burden and cortisol secretion rate, not the source of ACTH drive.