Clinical investigations from the RACP

Hyponatraemia: more salt or less water?

Mark Thomas



An underlying cause should be sought in all patients with hyponatraemia. Assessment of volume status and, in some patients, serum and urine osmolality and osmolar gap can help direct initial treatment.

Key Points

  • Acute hyponatraemia is a medical emergency that can cause permanent brain damage, coma and death.
  • Chronic hyponatraemia is common and can cause forgetfulness, ataxia, asterixis and drowsiness but is usually asymptomatic.
  • History and examination can help determine the patient’s volume status and guide initial treatment.
  • In all cases, an underlying cause should be sought and if possible corrected.
  • Water-overloaded (hypervolaemic) and euvolaemic patients need fluid restriction, and dry (hypovolaemic) patients need intravenous saline to correct hyponatraemia.
  • Measurement of serum and urine osmolality and calculation of the osmolar gap can clarify some cases.
  • In most patients, hyponatraemia is multifactorial and resolves after assessment of volume status, medication review and treatment of the underlying condition.