A 40-year-old woman experienced emotional stress 3 months ago and has had poor sleep since then. She suffers from extreme 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 (>5 times/night) and total daily urine output reaching 9L. Since onset, she has had poor mental state, irritability, weight loss, and developed headache in the past 2 weeks. After admission, she caught a cold on day 2, developed fever, worsening headache, reduced water intake, and confusion. BP: 90/60 mmHg, dry lips, poor skin turgor; no neck stiffness or papilledema. What should be considered first?