Q. A 35-year-old woman with a five-year history of bipolar disorder treatment using lithium comes to the clinic for her routine check-up. She complains of a weight gain of 5 lbs in the past 3 months. Her temperature is 98.6°F (37°C), pulse is 86 beats per minute, blood pressure is 130/70 mmHg and respiratory rate is 14 per minute. On examination, her skin is dry with coarse hair, and she has mild periorbital puffiness. The rest of the examination is normal. Given her presentation and medication history, what is the most appropriate next step in her management?