A 48-year-old woman with bipolar I disorder has been stable on lithium 750 mg daily (serum level 0.9 mEq/L) for 18 months. She presents to psychiatry clinic with complaints of polyuria (3–4 L/day), polydipsia, and a 4 kg weight gain over 3 months. Serum sodium is 138 mEq/L (normal 135–145), fasting glucose 95 mg/dL, TSH 2.1 mIU/L (normal), and urine osmolality is 180 mOsm/kg (low). Water deprivation test shows no rise in urine osmolality. What is the most appropriate next step in management?