Hyponatremia

Differential Diagnosis

• Pseudohyponatremia from lab error
• Translational hyponatremia from mannitol or hyperglycemia
• Hypovolemic
  • UNa >20: Renal losses, including mineralocorticoid deficiency
  • UNa <10: Non-renal losses
• Euvolemic
  • Uosm >100: SIADH, hypothyroidism, glucocorticoid deficiency
  • Uosm <100: Primary polydipsia, low solute intake
  • Uosm variable: Reset osmostat
• Hypervolemic
  • UNa <10: CHD, cirrhosis, nephrosis
  • UNa >20: Renal failure

Investigations

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

Further Reading

• Milionis HJ, Liamis GL, and Elisaf MS. The hyponatremic patient: a systematic approach to laboratory diagnosis. CMAJ. 2002;166(8):1056-1062.