A 28-year-old man with bipolar I disorder started on lithium 600 mg daily 1 week ago is brought to the clinic by his wife with complaints of polyuria (8–10 L/day), polydipsia, and nocturia ×5. Serum lithium level is 0.8 mEq/L (therapeutic). Serum sodium is 145 mEq/L, osmolality is 310 mOsm/kg, and urine osmolality is 200 mOsm/kg despite high serum osmolality. What is the most likely diagnosis and the best management?