A 38-year-old man from Mumbai presents with 6 months of progressive fatigue, muscle weakness, and weight gain despite anorexia. He has central obesity, proximal myopathy, and hypertension (162/104 mmHg). Laboratory findings: morning cortisol 32 µg/dL, 24-hour urinary free cortisol 320 µg/day, ACTH 68 pg/mL (normal 10–46), and plasma potassium 3.1 mEq/L (normal 3.5–5.0). A low-dose dexamethasone suppression test (LDDST) shows cortisol of 22 µg/dL. A high-dose dexamethasone suppression test (HDDST) shows cortisol of 8 µg/dL. Pituitary MRI is normal. What is the most likely diagnosis?

Explanation

## Diagnostic Algorithm for ACTH-Dependent Cushing Syndrome **Key Point:** The combination of elevated ACTH (68 pg/mL), failure to suppress cortisol at low-dose dexamethasone (22 µg/dL), but **suppression at high-dose dexamethasone** (8 µg/dL = 75% reduction from baseline of 32 µg/dL), and a normal pituitary MRI is most consistent with **Cushing disease (pituitary adenoma)** — specifically a radiologically occult microadenoma (~20% of cases). ### Step-by-Step Diagnostic Logic #### 1. Confirm Hypercortisolism - Morning cortisol 32 µg/dL (elevated; normal 5–25) - 24-hour urinary free cortisol 320 µg/day (markedly elevated; normal <50) - LDDST cortisol 22 µg/dL (fails to suppress; normal <1.8 µg/dL after 1 mg dexamethasone) - **Cushing syndrome is confirmed.** #### 2. Determine ACTH Dependence - ACTH 68 pg/mL (elevated; normal 10–46) - **This is ACTH-dependent Cushing syndrome** (pituitary or ectopic source) #### 3. Differentiate Pituitary vs. Ectopic ACTH | Feature | Cushing Disease (Pituitary) | Ectopic ACTH | |---------|---------------------------|--------------| | **ACTH level** | Mildly–moderately elevated (20–200) | Often markedly elevated (>200), but can overlap | | **HDDST suppression** | Cortisol suppresses **>50%** from baseline | No suppression (<50% reduction) | | **Pituitary MRI** | Adenoma visible in 70–80%; **occult in ~20%** | Normal | | **Hypokalemia** | Mild or absent | Severe (often <3.0 mEq/L) | | **Onset** | Gradual (months–years) | Often rapid and severe | **Clinical Pearl:** The **HDDST is the key discriminator** in ACTH-dependent Cushing syndrome: - **Pituitary adenoma (Cushing disease):** Cortisol suppresses **>50%** from baseline — this patient's cortisol fell from 32 to 8 µg/dL, a **75% reduction**, meeting the classic criterion for pituitary-dependent disease. - **Ectopic ACTH:** Cortisol does NOT suppress >50% (remains >50% of baseline), because ectopic tumors lack glucocorticoid-receptor feedback. ### Why Cushing Disease (Option A) is Most Likely 1. **HDDST shows >50% suppression** — cortisol fell from 32 → 8 µg/dL (75% reduction). This is the hallmark of pituitary-dependent Cushing disease. Ectopic ACTH characteristically does NOT suppress on HDDST. 2. **Normal pituitary MRI does NOT exclude Cushing disease** — approximately 15–20% of corticotroph adenomas are microadenomas below MRI resolution (Harrison's Principles of Internal Medicine, 21st ed., Ch. 379). 3. **ACTH of 68 pg/mL** — this is a mild-to-moderate elevation, typical of pituitary adenoma. Ectopic ACTH usually produces markedly higher ACTH levels (often >200 pg/mL), though overlap exists. 4. **Hypokalemia (3.1 mEq/L)** — mild hypokalemia can occur in Cushing disease; severe hypokalemia (<3.0) is more characteristic of ectopic ACTH. 5. **Gradual onset over 6 months** — consistent with the slower progression typical of pituitary adenoma. ### Why Other Options Are Incorrect - **Option B (Ectopic ACTH):** The HDDST shows >50% suppression (75%), which argues strongly against ectopic ACTH. Ectopic sources typically show no suppression on HDDST. ACTH of 68 pg/mL is also lower than typically seen in ectopic ACTH. - **Option C (Adrenal carcinoma):** Adrenal carcinoma causes ACTH-independent Cushing syndrome — ACTH would be suppressed (<5 pg/mL), not elevated at 68 pg/mL. - **Option D (PBAH):** Primary bilateral adrenal hyperplasia is ACTH-independent; ACTH would be low or suppressed, not elevated. ### Next Steps in Management 1. **Inferior petrosal sinus sampling (IPSS):** Gold standard to confirm pituitary source when MRI is negative. A central-to-peripheral ACTH gradient >2 (basal) or >3 (post-CRH) confirms pituitary origin (Harrison's, 21st ed.). 2. **CRH stimulation test:** ACTH and cortisol rise in pituitary disease; no rise in ectopic ACTH. 3. **Repeat/dedicated pituitary MRI** with gadolinium and thin cuts (3 mm) to detect occult microadenoma. 4. **Transsphenoidal surgery** is the definitive treatment for Cushing disease. **High-Yield:** A normal pituitary MRI does NOT equal ectopic ACTH. The HDDST suppression pattern (>50% reduction) is the biochemical key — always interpret MRI findings in the context of dynamic testing.