A 21-year-old male was involved in a motorcycle accident and sustained a closed head injury. He is waking up and interacting with his family and medical team. He complained of thirst that doesn’t seem to go away no matter how much water he drinks. The nurses note that he has had 3500 cc of pale-yellow urine in the last 24 hours. Urine was sent for osmolality which was reported as 122 mOsm/L. A diagnosis of probable neurogenic diabetes insipidus was made

Diabetes insipidus (DI) is defined as the passage of large volumes (>3 L/24 hr) of dilute urine (< 300 mOsm/kg). There are 2 major forms: Central (neurogenic, pituitary, or neurohypophyseal) DI, characterized by decreased secretion of antidiuretic hormone (ADH; also referred to as arginine vasopressin [AVP]). Nephrogenic DI characterized by decreased ability to concentrate urine because of resistance to ADH action in the kidney.

AVP is the primary determinant of free water excretion in the body. Its main target is the kidney, where it acts by altering the water permeability of the cortical and medullary collecting tubules. Water is reabsorbed by osmotic equilibration with the hypertonic interstitium and returned to the systemic circulation.