A 38-year-old woman presents with progressive fatigue, hypotension (90/60 mmHg), and hyperpigmentation of skin and mucous membranes. Laboratory findings show hyponatraemia (Na+ 128 mEq/L), hyperkalaemia (K+ 5.8 mEq/L), and elevated ACTH (450 pg/mL). Which single feature best distinguishes primary adrenal insufficiency (Addison disease) from secondary adrenal insufficiency?

## Distinguishing Primary from Secondary Adrenal Insufficiency **Key Point:** Hyperpigmentation is the hallmark discriminator between primary (Addison disease) and secondary adrenal insufficiency. ### Mechanism of Hyperpigmentation in Addison Disease In primary adrenal insufficiency, the loss of cortisol feedback causes: 1. Unopposed elevation of ACTH (and pro-opiomelanocortin [POMC] derivatives) 2. ACTH stimulates melanocortin-1 receptors on melanocytes 3. Increased melanin synthesis → bronze or chocolate-coloured skin and oral mucosa pigmentation In secondary adrenal insufficiency, ACTH is LOW or normal → no melanocyte stimulation → **absence of hyperpigmentation**. ### Comparative Table: Primary vs Secondary Adrenal Insufficiency | Feature | Primary (Addison) | Secondary | | --- | --- | --- | | **ACTH level** | ↑↑ (>100 pg/mL, often >400) | ↓ or normal | | **Hyperpigmentation** | **Present (bronze skin)** | **Absent** | | **Hyponatraemia** | Yes (aldosterone loss) | Yes (cortisol loss) | | **Hyperkalaemia** | Yes (aldosterone loss) | No (aldosterone preserved) | | **Hypotension** | Yes | Yes | | **Fatigue** | Yes | Yes | **High-Yield:** The presence of hyperpigmentation + hyponatraemia + hyperkalaemia = **Primary adrenal insufficiency**. The absence of hyperpigmentation despite low cortisol = **Secondary adrenal insufficiency**. ### Why Hyperpigmentation is the Best Discriminator - Hyperpigmentation is **pathognomonic for primary adrenal insufficiency** when combined with adrenal insufficiency signs - Hyponatraemia, hyperkalaemia, hypotension, and fatigue occur in BOTH primary and secondary forms - ACTH elevation is a feature of primary disease, but the question asks for the **distinguishing feature** — the clinical sign that separates them - Hyperpigmentation is the only finding that is present in primary but absent in secondary **Clinical Pearl:** A patient with adrenal insufficiency who lacks hyperpigmentation should raise suspicion for secondary (pituitary/hypothalamic) disease, prompting investigation of TSH, prolactin, and LH/FSH levels.