A 72-year-old man presents to the emergency department with a one-week history of progressively worsening confusion, profound lethargy, and severe constipation. His family reports that he has been extremely thirsty, drinking large amounts of water, and urinating frequently. His medical history is significant for squamous cell carcinoma of the lung, diagnosed six months ago, for which he is currently receiving palliative chemotherapy. He has a 40-pack-year smoking history. He does not take any over-the-counter vitamin D or calcium supplements. He appears lethargic, profoundly dehydrated with dry mucous membranes, and is slow to respond to questions. He is afebrile with a blood pressure of 90/60 mmHg, a heart rate of 112 bpm, and a respiratory rate of 18 breaths per minute. He is lethargic but arousable and oriented to person only. His mucous membranes are dry and his skin turgor is poor.

A total serum calcium is 14.6 mg/dL.