A 52-year-old man from Delhi presents with progressive fatigue, weight loss of 8 kg over 3 months, and recurrent episodes of hypoglycemia. On examination, he appears cachectic with generalized hyperpigmentation of the skin and oral mucosa. Blood pressure is 95/60 mmHg. Laboratory investigations reveal: sodium 128 mEq/L, potassium 5.8 mEq/L, fasting blood glucose 62 mg/dL, and cortisol at 8 AM is 3.2 µg/dL (normal 10–20 µg/dL). ACTH level is 156 pg/mL (normal 10–50 pg/mL). A high-dose dexamethasone suppression test (8 mg overnight) shows cortisol remains at 3.1 µg/dL. What is the most likely diagnosis?

Explanation

## Clinical Presentation & Key Findings **Key Point:** The combination of low cortisol with elevated ACTH (>100 pg/mL) is pathognomonic for primary adrenal insufficiency (Addison's disease). ### Diagnostic Reasoning This patient exhibits the classic triad of primary adrenal insufficiency: 1. **Low cortisol** (3.2 µg/dL) — adrenal glands cannot produce adequate hormone 2. **Elevated ACTH** (156 pg/mL) — pituitary responds to low cortisol by increasing ACTH secretion in an attempt to stimulate the failing adrenal glands 3. **Loss of negative feedback** — the normal inhibitory effect of cortisol on ACTH is absent ### Clinical Features Supporting Addison's Disease | Feature | Mechanism | Present in Patient | |---------|-----------|--------------------| | Hyperpigmentation | ACTH stimulates melanocytes via melanocyte-stimulating hormone (MSH) | Yes (skin & oral mucosa) | | Hypotension | Loss of aldosterone & cortisol → Na⁺ wasting | Yes (95/60 mmHg) | | Hyponatremia | Aldosterone deficiency → sodium loss | Yes (128 mEq/L) | | Hyperkalemia | Loss of aldosterone → K⁺ retention | Yes (5.8 mEq/L) | | Hypoglycemia | Loss of cortisol → impaired gluconeogenesis | Yes (fasting 62 mg/dL) | | Weight loss & fatigue | Cortisol deficiency | Yes | ### High-Dose Dexamethasone Suppression Test (HDDST) **High-Yield:** In primary adrenal insufficiency, the adrenal glands are already maximally stimulated and cannot produce more cortisol. Therefore, even high-dose dexamethasone (8 mg) does NOT suppress ACTH or increase cortisol output — cortisol remains low (3.1 µg/dL in this case). **Key Point:** HDDST helps differentiate the cause of elevated ACTH: - **Primary adrenal insufficiency:** No suppression (cortisol stays low) - **Secondary/tertiary insufficiency:** ACTH and cortisol are both low (HDDST not needed) - **Ectopic ACTH:** No suppression (cortisol stays high) ### Feedback Loop Physiology ```mermaid flowchart TD A[Hypothalamus]:::action --> B[CRH release]:::action B --> C[Anterior Pituitary]:::action C --> D[ACTH release]:::action D --> E[Adrenal Cortex]:::action E --> F[Cortisol production]:::outcome F --> G{Cortisol level adequate?}:::decision G -->|Yes| H[Negative feedback inhibits CRH & ACTH]:::action G -->|No| I[Loss of inhibition → ACTH rises]:::urgent I --> J[Attempt to stimulate failing adrenals]:::action J --> K[Hyperpigmentation from excess ACTH/MSH]:::outcome ``` **Clinical Pearl:** The hyperpigmentation in Addison's disease is a direct consequence of the feedback loop failure — when cortisol cannot suppress ACTH, ACTH levels rise dramatically and stimulate melanocytes via the melanocortin-1 receptor. ## Why This Is Addison's Disease (Not Other Forms of Insufficiency) **Mnemonic: ACTH Response Pattern** — **P**rimary (high ACTH), **S**econdary (low ACTH), **T**ertiary (low ACTH) - **Primary:** Adrenal gland failure → cortisol ↓, ACTH ↑ (loss of negative feedback) - **Secondary:** Pituitary failure → cortisol ↓, ACTH ↓ (no ACTH to stimulate adrenals) - **Tertiary:** Hypothalamic failure → cortisol ↓, ACTH ↓ (no CRH to trigger ACTH) This patient has the **elevated ACTH** that is diagnostic of primary disease. [cite:Harrison 21e Ch 379]