Physiologic Approach to Hypotonic Hyponatremia
The next time you have a case of hypotonic hyponatremia, give the physiologic approach a try. It can be easier to remember than the normal volume-based algorithm
Urine Na+K / serum Na >1 is predictive of poor response to volume restriction alone in SIADH

RAAS Inactive or Ineffective (UNa >30)
 • ADH Present (Uosm ≥100): SIADH, Renal Sodium loss (diuretics, mineralocorticoid deficiency, Post-AKI diuresis can be high or low Uosm), Hypothyroidism, Adrenal Insufficiency, Cerebral Salt Wasting
 • ADH Not Present (Uosm <100): Renal Losses (AKI, Post-AKI diuresis)
RAAS Active (or too little relative solute intake, UNa <20)
 • ADH Present (Uosm ≥100): Hypovolemia, Hypervolemia with poor renal perfusion (Heart failure, Cirrhosis, Nephrotic syndrome)
 • ADH Not Present (Uosm <100): Excessive Free water intake (Primary polydipsia), Low solute intake (Beer potomania, malnutrition), Reset Osmostat

Stanford Internal Medicine Chiefs @StanfordChiefs

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