Osmolality and ADH Testing

Arginine vasopressin (AVP), commonly called antidiuretic hormone (ADH), is released from the posterior pituitary when plasma becomes more concentrated. It is also released when the body senses threatened circulation: hypovolemia, hypotension, pain, nausea, stress, and the postoperative state can all stimulate AVP even when osmolality is not elevated. AVP acts at V2 receptors in the renal collecting ducts. The result is insertion of aquaporin-2 water channels, allowing water to move back into the circulation. AVP retains water, not sodium. That distinction explains why a patient with excessive AVP activity can become hyponatremic despite having normal or even increased total-body sodium. Serum sodium is best understood as a ratio: total body water relative to total body solute. A low sodium concentration usually means there is too much water for the available solute. A high sodium concentration usually means there is too little water. When AVP is appropriately suppressed, the kidney can produce very dilute urine—approximately 50 mOsm/kg in a healthy person. Inappropriately concentrated urine during hypotonic hyponatremia means that AVP is still active when it should be turned off.