A 38-year-old woman presents with progressive fatigue, weight loss, and hyperpigmentation of skin creases and oral mucosa. Serum sodium is 128 mEq/L and potassium is 5.8 mEq/L. Which investigation is most appropriate to confirm the diagnosis of primary adrenal insufficiency?

Explanation

## Diagnostic Approach to Addison Disease ### Clinical Suspicion The patient presents with the classic triad of Addison disease: - **Hyperpigmentation** (due to elevated ACTH stimulating melanocytes) - **Hyponatremia and hyperkalemia** (aldosterone deficiency) - **Fatigue and weight loss** (cortisol deficiency) ### Investigation Strategy **Key Point:** Confirmation of primary adrenal insufficiency requires demonstrating an inadequate cortisol response to ACTH stimulation. The **high-dose (250 μg) ACTH stimulation test** is the **gold standard** for this purpose. ### The High-Dose ACTH Stimulation Test (250 μg) | Feature | Details | |---------|----------| | **Gold standard test** | Standard confirmatory test for primary adrenal insufficiency | | **Mechanism** | Supraphysiologic dose of synthetic ACTH (cosyntropin) maximally stimulates adrenal cortex | | **Normal response** | Cortisol rises to ≥18–20 μg/dL at 30–60 min | | **Addison disease** | Cortisol fails to rise adequately (remains <18 μg/dL) | | **Advantage** | Robust, reproducible, widely validated; recommended by Endocrine Society guidelines | | **Timing** | Ideally performed in the morning | **High-Yield:** Per Harrison's Principles of Internal Medicine and Endocrine Society Clinical Practice Guidelines (2016), the **250 μg ACTH (cosyntropin) stimulation test** is the recommended gold standard for confirming primary adrenal insufficiency. A peak cortisol <18 μg/dL at 30–60 minutes is diagnostic. ### Why Not the Other Options? - **Option A – 9 AM serum cortisol:** A useful screening test (cortisol <3 μg/dL is highly suggestive; >15 μg/dL makes insufficiency unlikely), but it is **not confirmatory** on its own due to overlap with normal values in the 3–15 μg/dL range. - **Option B – Low-dose (1 μg) ACTH test:** More sensitive for detecting subtle or early secondary adrenal insufficiency (where the adrenal gland may be partially atrophied from ACTH deprivation), but it is **not the standard gold standard** for primary Addison disease. It is less reproducible and not universally available. - **Option D – Plasma ACTH level:** Markedly elevated in primary Addison disease (often >100 pg/mL) due to loss of cortisol-mediated negative feedback. However, it **cannot confirm** adrenal insufficiency by itself — it only helps differentiate primary (high ACTH) from secondary (low/normal ACTH) insufficiency after the diagnosis is established. ### Diagnostic Algorithm ``` Clinical suspicion → 9 AM cortisol (screening) → If <3 μg/dL: highly likely primary AI → If >15 μg/dL: unlikely AI → If 3–15 μg/dL: perform 250 μg ACTH stimulation test (gold standard) → Peak cortisol <18 μg/dL → Primary adrenal insufficiency confirmed → Then measure plasma ACTH to confirm primary vs. secondary ``` **Clinical Pearl:** In established primary Addison disease, plasma ACTH is markedly elevated (>100 pg/mL), but the adrenal glands still cannot produce adequate cortisol despite high endogenous ACTH — this is precisely what the 250 μg stimulation test demonstrates. **Reference:** Harrison's Principles of Internal Medicine, 21st edition; Endocrine Society Clinical Practice Guideline on Adrenal Insufficiency (2016).