A 40-year-old woman presents with thirst, polydipsia, and polyuria for 2 months, with a urine output of 5000-7000ml/d. The first investigation to perform is:
A 40-year-old woman presents with thirst, polydipsia, and polyuria for 2 months, with a urine output of 5000-7000ml/d. The first investigation to perform is: