A 52-year-old woman with hyponatremia (Na+ 125 mEq/L) is being evaluated. In hyponatremia due to primary polydipsia, the urine osmolality is typically <100 mOsm/kg. In contrast, hyponatremia from SIADH shows urine osmolality >200 mOsm/kg. Which feature most reliably distinguishes primary polydipsia from SIADH?

Explanation

## Distinguishing Primary Polydipsia from SIADH ### Pathophysiology **Primary Polydipsia:** - Excessive water intake due to abnormal thirst sensation (psychiatric, CNS lesion, or idiopathic) - Kidneys are **normal** and respond appropriately to dilution - When plasma osmolality drops, ADH is suppressed → urine becomes dilute (<100 mOsm/kg) - Urine osmolality **can be reduced further** with water restriction (paradoxically, because the problem is intake, not retention) - Polyuria is prominent **SIADH:** - Inappropriate ADH secretion despite low plasma osmolality - Kidneys **cannot dilute urine** adequately even when plasma osmolality is low - Urine osmolality remains >200 mOsm/kg (inappropriately concentrated) - Water restriction **does not lower urine osmolality** significantly; instead, it worsens hyponatremia - Polyuria is absent or mild ### Diagnostic Approach: The Water Restriction Test **High-Yield:** The **water restriction test** is the gold standard discriminator: 1. **In Primary Polydipsia:** Water restriction → urine osmolality drops to <100 mOsm/kg (kidneys respond normally by suppressing ADH) 2. **In SIADH:** Water restriction → urine osmolality remains >200 mOsm/kg (ADH cannot be suppressed; kidneys remain inappropriately concentrated) ### Comparison Table | Feature | Primary Polydipsia | SIADH | |---------|-------------------|-------| | **Baseline Urine Osmolality** | <100 mOsm/kg | >200 mOsm/kg | | **After Water Restriction** | Drops further (<50 mOsm/kg) | Remains >200 mOsm/kg | | **Plasma Osmolality** | <270 mOsm/kg | <270 mOsm/kg | | **ADH Suppressibility** | Yes (normal response) | No (inappropriate) | | **Polyuria** | Prominent | Absent/mild | | **Thirst Mechanism** | Abnormal (excessive) | Normal | **Key Point:** The ability to dilute urine to <100 mOsm/kg is **normal kidney function**. In primary polydipsia, kidneys work perfectly—the problem is the patient drinks too much. In SIADH, kidneys **cannot dilute urine** appropriately, even when they should. ### Clinical Pearl **Clinical Pearl:** If a hyponatremic patient can produce dilute urine (<100 mOsm/kg), their kidneys are functioning normally. The diagnosis is primary polydipsia (or another cause of excessive intake). If they **cannot** produce dilute urine despite low plasma osmolality, suspect SIADH or renal disease. **Mnemonic: SIADH = Stuck ADH** — ADH cannot be suppressed, so urine stays concentrated. **Primary Polydipsia = Persistent Polyuria** — kidneys dilute urine normally because ADH is suppressed, but the patient keeps drinking.