A 38-year-old woman presents with progressive central obesity, proximal muscle weakness, and hypertension. Investigations reveal elevated 24-hour urinary free cortisol and loss of normal circadian rhythm of cortisol. High-dose dexamethasone suppression test (HDDST) shows suppression of cortisol to <50% of baseline. Which single feature best distinguishes Cushing syndrome due to pituitary adenoma from ectopic ACTH syndrome?

Explanation

## Distinguishing Pituitary from Ectopic ACTH Cushing Syndrome ### Clinical Context Both pituitary adenoma (Cushing disease) and ectopic ACTH-secreting tumors cause ACTH-dependent Cushing syndrome. The key difference lies in ACTH responsiveness to high-dose dexamethasone. ### High-Dose Dexamethasone Suppression Test (HDDST) **Key Point:** The HDDST is the gold standard discriminator between pituitary and ectopic ACTH sources. | Feature | Pituitary Adenoma | Ectopic ACTH | |---------|-------------------|---------------| | **HDDST response** | Suppression (cortisol <50% baseline) | No suppression (cortisol remains elevated) | | **Mechanism** | Pituitary tumor retains some glucocorticoid sensitivity; high-dose dexamethasone suppresses residual ACTH secretion | Ectopic tumor is autonomous; insensitive to feedback inhibition | | **Plasma ACTH** | Elevated but usually <200 pg/mL | Often >200 pg/mL (very high) | | **Clinical severity** | Often milder onset | Often acute, severe presentation | ### Why HDDST is Superior **High-Yield:** The pituitary corticotroph retains some sensitivity to negative feedback from glucocorticoids. When dexamethasone dose is increased 8-fold (from 2 mg to 8 mg daily), it suppresses the pituitary tumor's ACTH secretion. Ectopic ACTH-secreting tumors (lung, thymus, pancreas) are completely autonomous and do NOT respond to any dose of dexamethasone. **Clinical Pearl:** In the patient described, the HDDST showing suppression confirms pituitary origin. If cortisol had remained elevated despite HDDST, ectopic ACTH would be the diagnosis. ### Mnemonic **HDDST = Hierarchy Determines Diagnosis (Suppression = pituitary)** — The pituitary sits in the hypothalamic-pituitary-adrenal hierarchy and retains feedback sensitivity; ectopic tumors are outside this axis and ignore feedback. ### Other Features Are Non-Discriminatory - **Elevated ACTH:** Present in both pituitary and ectopic ACTH Cushing syndrome (rules out primary adrenal disease, but does not distinguish between the two ACTH-dependent forms). - **Hypokalemic metabolic alkalosis & elevated UFC:** Both occur in severe ACTH-dependent Cushing, regardless of source. [cite:Harrison 21e Ch 387]