A 72-year-old woman with small-cell lung cancer (SCLC) is admitted with lethargy, nausea, and a single episode of seizure. Serum sodium 118 mEq/L, serum osmolality 252 mOsm/kg, urine osmolality 680 mOsm/kg, urine sodium 78 mEq/L, normal renal and adrenal function. Chest imaging shows a 4 cm left hilar mass. What is the most appropriate initial management for her electrolyte abnormality?