A 62-year-old man is evaluated in the ER for drowsiness and generalized weakness for the past 4 weeks. He is brought to the hospital by EMS after he experienced a tonic clonic seizure in a local restaurant. His medical history is positive for HTN and gout, for which he takes metoprolol and allopurinol. He has smoked 1 pack per day for nearly 40 years, with no alcohol or recreational drug abuse. He noticed nearly 20 lb weight loss in past 2 months. Physical examination is normal. Laboratory tests reveal hemoglobin 10.2 gm/dL, leukocyte count 9500/mL, and platelet 171,000/mL. Serum sodium is 122 mEq/L, potassium 3.8 mEq/L, blood urea nitrogen (BUN) 26 mg/dL, and creatinine 0.9 mg/dL. Additional laboratory findings include serum osmolality 258 mOsm/kg, urine osmolality 300 mOsm/kg, and urine sodium 48 mmol/L. TSH and cortisol levels are normal.