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?

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