## Distinguishing SIADH from Ectopic ADH Secretion **Key Point:** Both SIADH and ectopic ADH secretion present with identical biochemistry (low serum osmolality, elevated urine osmolality, hyponatremia). The discriminating feature is **volume status and urine sodium concentration**. ### Comparison Table | Feature | SIADH (True) | Ectopic ADH (Paraneoplastic) | |---------|-------------|-----------------------------| | **Volume Status** | Euvolemic | Euvolemic | | **Urine Na+ (mEq/L)** | >40 (high) | <10 (low) | | **Mechanism** | Neurogenic (CNS/lung irritation) | Tumor-secreted ADH | | **Extracellular Fluid** | Expanded (mild) | Normal | | **Fluid Restriction Response** | Excellent | Poor | | **Hypertonic Saline** | Worsens hyponatremia | May improve | **High-Yield:** In paraneoplastic ectopic ADH secretion, the **urine sodium is characteristically LOW (<10 mEq/L)** because the expanded intracellular volume triggers renal sodium wasting via natriuretic peptides. In true SIADH (from CNS disease or medications), urine sodium remains **high (>40 mEq/L)** because the primary defect is ADH dysregulation, not volume expansion. **Clinical Pearl:** The **urine sodium concentration** is the single best discriminator. Ectopic ADH from malignancy causes volume depletion at the renal tubule level, leading to avid sodium reabsorption and low urine Na+. True SIADH maintains normal sodium handling. **Mnemonic:** **SIADH = Sodium In ADH-dysregulation** (urine Na+ stays high); **Ectopic ADH = Excessive ADH-driven volume expansion** (urine Na+ drops low). ### Why This Matters Both conditions are common in small cell lung cancer. Recognizing the urine sodium pattern guides treatment: - **Low urine Na+ (ectopic ADH):** Fluid restriction + hypertonic saline - **High urine Na+ (SIADH):** Fluid restriction alone; hypertonic saline contraindicated [cite:Robbins 10e Ch 7] 
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