A 52-year-old woman with bipolar I disorder stabilized on lithium 900 mg daily presents to the outpatient clinic reporting new onset hand tremor, nausea, and diarrhea over the past week. Her most recent lithium level was 0.8 mEq/L two months ago. She mentions that her primary care provider recently started her on ibuprofen 600 mg three times daily for chronic knee pain. Her vital signs are within normal limits. Which of the following is the most appropriate next step in management?