A 38-year-old woman from rural Maharashtra presents with a 3-month history of progressive weakness, anorexia, and weight loss of 8 kg. She reports darkening of her skin, particularly in sun-exposed areas and skin creases. On examination, blood pressure is 92/58 mmHg (previously 120/75), and there is generalized hyperpigmentation including the buccal mucosa. Laboratory investigations show: sodium 128 mEq/L, potassium 5.8 mEq/L, fasting glucose 62 mg/dL, and cortisol at 8 AM is 3 µg/dL (normal >10). ACTH level is 156 pg/mL (normal <46). Chest X-ray is normal. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Primary Adrenal Insufficiency (Addison Disease) ### Key Clinical Features Present **Key Point:** The combination of hyperpigmentation (including buccal mucosa), hyponatremia, hyperkalemia, hypoglycemia, and elevated ACTH with low cortisol is pathognomonic for primary adrenal insufficiency. ### Laboratory Interpretation | Parameter | Finding | Interpretation | |-----------|---------|----------------| | Cortisol (8 AM) | 3 µg/dL (low) | Inadequate cortisol production | | ACTH | 156 pg/mL (elevated) | Pituitary attempting to stimulate failing adrenal glands | | Sodium | 128 mEq/L (low) | Aldosterone deficiency → sodium wasting | | Potassium | 5.8 mEq/L (high) | Aldosterone deficiency → potassium retention | | Glucose | 62 mg/dL (low) | Loss of cortisol's gluconeogenic effect | ### Pathophysiology of Hyperpigmentation 1. Low cortisol fails to suppress CRH and ACTH 2. Elevated ACTH stimulates melanocytes via melanocyte-stimulating hormone (MSH) 3. MSH is a byproduct of POMC cleavage (same precursor as ACTH) 4. Hyperpigmentation is **most prominent in sun-exposed areas and skin creases** (pressure points) **High-Yield:** Hyperpigmentation in Addison disease is a **cardinal distinguishing feature** from secondary adrenal insufficiency, where ACTH remains low. ### Mechanism of Electrolyte Abnormalities **Key Point:** Both cortisol AND aldosterone are deficient in primary adrenal insufficiency: - **Aldosterone deficiency** → sodium loss, potassium retention, hypotension - **Cortisol deficiency** → hypoglycemia, weakness, inability to maintain blood pressure ### Clinical Pearl The **elevated ACTH with low cortisol** is the diagnostic hallmark that distinguishes primary (Addison) from secondary adrenal insufficiency. In secondary insufficiency, both ACTH and cortisol are low. ### Diagnostic Confirmation The **short Synacthen test** (ACTH stimulation test) would show: - Baseline cortisol <3 µg/dL - Post-ACTH cortisol <18 µg/dL (failure to respond) [cite:Harrison 21e Ch 375]