A 48-year-old man with bipolar disorder has been stable on lithium carbonate 1200 mg daily (serum level 0.9 mEq/L) for 5 years. He is now referred for routine monitoring. On examination, he has mild tremor, normal mental status, and no focal neurological signs. Laboratory investigations show: serum creatinine 1.3 mg/dL (baseline 0.8 mg/dL), eGFR 58 mL/min/1.73 m², TSH 6.2 mIU/L (normal <4.5), free T4 8.5 pg/dL (normal 8.5–18), fasting glucose 118 mg/dL, and 24-hour urine output 2.8 L (normal 1–2 L). What is the most appropriate next step in management?