Hyponatremia

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Differential Diagnosis

Investigations

  • Serum and urine osmolality and electrolytes (prior to treatment)
  • TSH and AM cortisol

Diagnosis by Lab Criteria

  • Serum Na <134?
  • Measure serum osmolality and urea level
    • Sosm ≥ 280: normotonic/hypertonic
      • Ddx includes pseudohyponatremia (from hyperlipidemia, hyperparaproteinemia, etc.), or presence of osmotically active substances (glucose, mannitol)
    • Sosm <280 mOsm/kg: true hyponatremia
      • Measure urine osmolality
      • Uosm <100 mOsm/kg: normal water excretion
        • Primary polydipsiea or reset osmostat syndrome
      • Uosm ≥ 100 mOsm/kg: impaired water excretion
        • Exclude hypothyroidism and adrenal insufficiency with morning cortisol and TSH
        • Measure urine sodium
        • UNa < 20 mmol/L: hypovolemia, including EABV depletion (heart failure, cirrhosis, nephrotic syndrome)
        • UNa > 40 mmol/L: SIADH, reset ostmostat, renal salt wasting
          • Salt supplementation and water restriction
          • Normal serum urate and reduced FEurate: SIADH
          • Hypouricemia and unchanged FEurate: renal salt wasting
        • UNa between 20 and 40 mmol/L
          • Bolus 2L/day normal saline for 2 days and trend sodium
          • If SNa increases by ≥ 5 mmol/L: hypovolemia
          • If SNa increases by <5 mmol/L: SIADH or reset ostmostat
            • FEurea >55%, serum urate <0.24, and FEurate >10%: SIADH
            • Oral or IV water-loading test: reset osmostat syndrome

Further Reading