A 40-year-old woman experienced psychological stress 3 months ago and has had poor sleep since. She suffers from unbearable thirst, requiring large amounts of water intake (4-5 thermos flasks daily), preferring cold water. Urine output is significantly increased, averaging once per hour, with nocturia over 5 times nightly and total daily urine output reaching 9L. Since onset, she has had poor mental state, irritability, weight loss, and headache for the past 2 weeks. Which test should be performed next to identify the cause?