A 52-year-old man with a 6-month history of progressive weakness, hypertension, and hypokalemia is found to have elevated plasma cortisol (28 µg/dL; normal 5–25 µg/dL at 08:00 h) and suppressed ACTH (2 pg/mL; normal 10–50 pg/mL). A low-dose dexamethasone suppression test (LDDST) shows cortisol remains elevated at 18 µg/dL (normal suppression to <5 µg/dL). Regarding the pathophysiology of this patient's condition, all of the following are true EXCEPT:

Explanation

## Differential Diagnosis of Cushing Syndrome **Key Point:** The dexamethasone suppression test (DST) is used to distinguish between ACTH-dependent (pituitary/ectopic) and ACTH-independent (primary adrenal) Cushing syndrome. ### Interpretation of DST in Cushing Syndrome | Test Result | ACTH-Dependent (Pituitary/Ectopic) | ACTH-Independent (Primary Adrenal) | | --- | --- | --- | | **Low-dose DST (0.5 mg × 4 doses)** | Cortisol suppresses to <5 µg/dL | Cortisol does NOT suppress (<5 µg/dL) | | **High-dose DST (8 mg × 4 doses)** | Cortisol suppresses to <5 µg/dL (pituitary) OR does not suppress (ectopic) | Cortisol does NOT suppress (<5 µg/dL) | | **ACTH level** | Elevated or normal (>10 pg/mL) | Suppressed (<5 pg/mL) | | **Mechanism** | Pituitary/ectopic ACTH drives adrenal cortisol; high-dose dex suppresses pituitary ACTH | Autonomous adrenal cortisol production; dexamethasone cannot suppress | **High-Yield:** In primary adrenal Cushing syndrome (adenoma/carcinoma), cortisol is produced autonomously and is **NOT suppressed** by either low-dose or high-dose dexamethasone. The suppressed ACTH confirms the adrenal source. ## Clinical Presentation and Pathophysiology ### Autonomous Cortisol Production **Key Point:** An adrenal adenoma or carcinoma produces cortisol independently of ACTH stimulation. The elevated cortisol exerts negative feedback on the hypothalamus and pituitary, suppressing CRH and ACTH to low levels. ### Hypokalemia Mechanism **Clinical Pearl:** Although cortisol is primarily a glucocorticoid, at high concentrations it can activate mineralocorticoid receptors in the distal nephron (collecting duct), causing sodium retention and potassium wasting. This is the mechanism of hypokalemia in Cushing syndrome. **Mnemonic:** **Cortisol at HIGH levels** → **M**ineralocorticoid **R**eceptor **A**ctivation → **K** loss (hypokalemia). ### Why HDDST Does NOT Suppress in Primary Adrenal Disease **Key Point:** Dexamethasone suppresses the HPA axis by negative feedback on the pituitary and hypothalamus. In primary adrenal disease, the adrenal tumor produces cortisol autonomously and is **not responsive** to dexamethasone feedback. Therefore, HDDST will NOT suppress cortisol in primary adrenal Cushing syndrome.