A 45-year-old female with bipolar I disorder has been stable on lithium 900 mg twice daily for 3 years. She presents with new-onset coarse tremor, persistent vomiting, ataxia, and confusion. Her lithium level drawn 12 hours post-dose returns at 2.1 mEq/L. In addition to holding lithium and providing IV hydration, which of the following laboratory tests is most critical to monitor in this patient?