A 28-year-old woman with severe hyponatremia (Na+ 118 mEq/L) and euvolemia is admitted. Urine osmolality is 580 mOsm/kg and urine sodium is 95 mEq/L. Thyroid and adrenal function are normal. Which finding best distinguishes Syndrome of Inappropriate ADH secretion (SIADH) from Primary Polydipsia in this patient?

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