A 38-year-old man with Cushing syndrome undergoes 1 mg overnight dexamethasone suppression test (DST), which shows no suppression of cortisol. High-dose dexamethasone suppression test (8 mg overnight) also shows no suppression. Which finding best distinguishes adrenal Cushing syndrome from ectopic ACTH syndrome in this patient?

Explanation

## Dexamethasone Suppression Test: Distinguishing Adrenal from Ectopic ACTH Cushing ### The Diagnostic Principle **Key Point:** The **high-dose dexamethasone suppression test (8 mg overnight)** differentiates between adrenal Cushing syndrome and ectopic ACTH syndrome based on the **suppressibility of cortisol and the ACTH level**. ### Interpretation of DST Results #### Adrenal Cushing Syndrome (ACTH-Independent) - **Plasma ACTH:** Suppressed (< 5 pg/mL) — autonomous adrenal tumor - **Low-dose DST (1 mg):** No suppression of cortisol - **High-dose DST (8 mg):** No suppression of cortisol (because ACTH is already suppressed; further dexamethasone cannot suppress an autonomous adrenal source) - **Mechanism:** The adrenal tumor produces cortisol independently of ACTH; dexamethasone cannot suppress a non-ACTH-driven source #### Ectopic ACTH Syndrome (ACTH-Dependent) - **Plasma ACTH:** Elevated (often markedly, > 100 pg/mL in ectopic sources) - **Low-dose DST (1 mg):** No suppression of cortisol - **High-dose DST (8 mg):** Usually **no suppression** (unlike pituitary Cushing, which suppresses at high-dose) - **Mechanism:** The ectopic ACTH source is not responsive to dexamethasone feedback; it continues to drive ACTH and cortisol secretion ### Comparison Table: DST Responses in Cushing Syndrome | Cushing Type | Plasma ACTH | 1 mg DST | 8 mg DST | Distinguishing Feature | | --- | --- | --- | --- | --- | | **Adrenal (ACTH-Independent)** | **Suppressed** | No suppression | No suppression | **Suppressed ACTH + non-suppressible cortisol** | | Pituitary (ACTH-Dependent) | Elevated | No suppression | **Suppression** | Suppression at high-dose DST | | Ectopic ACTH (ACTH-Dependent) | **Elevated** | No suppression | No suppression | **Elevated ACTH + non-suppressible cortisol** | **High-Yield:** The combination of **suppressed ACTH + non-suppressible cortisol on both DST levels** is **pathognomonic for adrenal Cushing syndrome**. Conversely, **elevated ACTH + non-suppressible cortisol on high-dose DST** points to ectopic ACTH (or rarely, aggressive pituitary disease). ### Clinical Pearl In this case, the patient shows no suppression on both low-dose and high-dose DST. To distinguish adrenal from ectopic ACTH, you **must measure plasma ACTH**: - If ACTH is **suppressed** → Adrenal Cushing (adrenalectomy) - If ACTH is **elevated** → Ectopic ACTH (search for source: lung, thymus, pancreas, etc.) **Mnemonic:** **"Adrenal = Absent ACTH; Ectopic = Elevated ACTH"** — Both fail high-dose DST suppression, but ACTH level is the discriminator. ### Why Option 2 (Elevated ACTH + Non-Suppressible Cortisol) Is NOT the Answer Here Option 2 describes ectopic ACTH, not adrenal Cushing. The question asks what **distinguishes adrenal from ectopic**. The answer must describe the **adrenal pattern**, not the ectopic pattern.