A patient is always thirsty, urinating large volumes of dilute urine. This suggests ADH (antidiuretic hormone) deficiency — diabetes insipidus. ADH, made by the hypothalamus and released from the posterior pituitary, tells kidney tubules to reabsorb water. Without it, the kidneys can't concentrate urine — liters of dilute urine, unquenchable thirst. It is named for resembling diabetes's urination, but blood sugar is normal; the fault is hormonal, not metabolic.