Urine Studies

Urine Osmolality Urine Sodium
Hypovolemia CSW > 100 mOsm/kg > 40 mEq/L
Low solute intake/GI losses < 100 mOsm/kg < 40 mEq/L
Euvolemia SIADH > 100 mOsm/kg > 40 mEq/L
Polydipsia < 100 mOsm/kg < 40 mEq/L
Hypervolemia CHF/Cirrhosis > 100 mOsm/kg < 40 mEq/L
AKI < 100 mOsm/kg > 40 mEq/L

Impact of Diuretics

  • Administration of loop or thiazide diuretics will alter interpretation of UNa
  • Expect UNa to be high after diuretic dosing
  • Fractional excretion of urea can be helpful to interpret sodium handling in the presence of diuretics
    • Urea re-absorbed proximal to site of action of diuretics
    • Low FeUrea (<55%) suggests appropriate sodium retention
    • High FeUrea (>55%) suggests inappropriate sodium retention


Severe Symptoms

  • Seizures, coma, altered mental status possible when sodium very low (< 120 mEq/L)

Give 3% hypertonic saline regardless of etiology

Can bolus 100-250 mL over 15 minutes or start infusion at 25-50 mL/hr

Hypovolemic Hyponatremia

  • Isotonic saline functions both as solute replacement and volume replacement
  • Normal saline contains 154 mEq/L of sodium, equating to osmolality of 308 mOsm/L
    • Administration will replace sodium and volume losses
  • Eventually, volume will be restored and volume-related ADH release will be turned off
    • Important to correct at a safe level, 6-8 mEq/L/day

Volume resuscitated after 2 liters                                                       Volume resuscitated after 5 liters

Hypervolemic Hyponatremia

  • Volume restriction
  • Symptoms very rare, so aggressive volume restrictions often not needed
    • Progression to significant hyponatremia often a very chronic process
  • Combination of loop diuresis and ACE inhibitor effective at improving sodium

Other Less Common Causes of Hyponatremia

  • Adrenal insufficiency
    • Suspect in hypotensive, hypokalemic, hyponatremic patients
    • Hydrocortisone supplementation
  • Hypothyroidism
    • Rare, responds well to LT4
  • Hypokalemia
    • Reduction in total body potassium will lead to osmotic shift of sodium
    • Potassium repletion always indicated when treating hyponatremia