A 38-year-old woman from Delhi presents with progressive weakness, hyperpigmentation of skin and areolae, and postural dizziness for 3 months. On examination, blood pressure is 95/60 mmHg, and there is generalized hyperpigmentation. Laboratory investigations show: sodium 128 mEq/L, potassium 5.8 mEq/L, cortisol (8 AM) 2.2 µg/dL (normal 5–25), ACTH 156 pg/mL (normal 10–50). A high-dose dexamethasone suppression test (8 mg overnight) shows cortisol remains 2.1 µg/dL. Which of the following best explains the elevated ACTH despite low cortisol in this patient?

Explanation

## Pathophysiology of Primary Adrenal Insufficiency (Addison's Disease) ### Clinical Presentation Recognition **Key Point:** The triad of hyperpigmentation + hyponatremia + hypotension in this patient is classic for primary adrenal insufficiency (Addison's disease). ### Cortisol-ACTH Feedback Loop **High-Yield:** In healthy individuals, cortisol exerts strong negative feedback on both the anterior pituitary (direct inhibition of ACTH secretion) and the hypothalamus (CRH suppression). When cortisol levels fall below the feedback threshold (~5 µg/dL), this inhibition is removed, allowing ACTH to rise unchecked. ### Why ACTH is Elevated Despite Low Cortisol 1. Adrenal glands are destroyed (autoimmune in ~90% of Addison's cases in India) 2. Cortisol production collapses → serum cortisol drops to 2.2 µg/dL 3. Loss of negative feedback → pituitary ACTH secretion increases compensatorily 4. High ACTH stimulates melanocytes via melanocyte-stimulating hormone (MSH), a byproduct of POMC cleavage → hyperpigmentation **Mnemonic:** **ACTH-UP** = **A**drenal **C**ortex **T**rophy **H**igh **U**nder **P**rimary insufficiency ### Dexamethasone Suppression Test Interpretation **Clinical Pearl:** The high-dose dexamethasone test (8 mg) fails to suppress cortisol in primary adrenal insufficiency because the adrenal glands are non-functional — no amount of ACTH suppression will restore cortisol production. This distinguishes primary from secondary insufficiency (where high-dose dex *does* suppress ACTH and cortisol rises slightly). ### Diagnostic Summary Table | Feature | Primary Adrenal Insufficiency | Secondary Insufficiency | Ectopic ACTH | |---------|-------------------------------|------------------------|---------------| | **Cortisol** | Low | Low | High | | **ACTH** | High (↑↑) | Low/normal | Very high (↑↑↑) | | **Hyperpigmentation** | Present (MSH ↑) | Absent | Absent | | **Na⁺/K⁺** | Low Na⁺, high K⁺ | Normal (aldosterone intact) | Normal | | **High-dose dex suppression** | No suppression | ACTH ↓, cortisol ↑ | No suppression | ### Why Hyperpigmentation Occurs **Key Point:** POMC (pro-opiomelanocortin) is cleaved into ACTH and β-lipotropin. In primary insufficiency, massive ACTH overproduction stimulates melanocyte receptors (MC1R), causing diffuse hyperpigmentation of skin, areolae, and buccal mucosa. This is pathognomonic for primary insufficiency. [cite:Harrison 21e Ch 375]