A 48-year-old man with bipolar I disorder has been stable on lithium carbonate 1000 mg daily (serum level 0.9 mEq/L) for 18 months. He presents to the psychiatry clinic for routine follow-up. His wife reports he has been increasingly forgetful, and he complains of persistent polyuria and polydipsia. Physical examination reveals no acute distress, normal mental status, and no tremor. Urinalysis shows low specific gravity (1.008). Serum creatinine is 1.3 mg/dL (baseline 0.8 mg/dL), serum sodium 138 mEq/L, and serum osmolality 295 mOsm/kg. What is the most appropriate next step in management?