Bipolar Affective Disorder — Practice

A 55-year-old man with bipolar affective disorder has been on lithium for 10 years with good mood stabilisation. He now presents with polyuria, polydipsia, and fine resting tremor. Serum lithium level is 0.8 mEq/L (therapeutic range 0.6–1.2 mEq/L). Which investigation should be done next to evaluate the most likely complication of lithium in this patient?