A 48-year-old male with bipolar I disorder has been maintained on lithium 900 mg daily for the past eight months with good symptom control. At his routine follow-up, his lithium level drawn 12 hours post-dose is 1.3 mEq/L. He reports new onset of persistent nausea, coarse hand tremor, and three episodes of diarrhea over the past two days. He also notes increased thirst and more frequent urination over the past several weeks. His most recent creatinine was 1.1 mg/dL three months ago. The PMHNP is evaluating these findings in the context of lithium monitoring. Which of the following represents the most accurate clinical evaluation of this presentation?