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Endocrinology
Endocrinology

Approach to Hyponatraemia

A clean three-step framework for hyponatraemia that gets you from a low sodium on a routine blood test to a working diagnosis and a safe plan.

Published
15 May 2026
Read time
3 min read
Differential DiagnosisFoundation / Junior Doctor

Hyponatraemia is the most common electrolyte disturbance on the inpatient board. The reflex is to "give some sodium" — which is wrong about half the time. A structured approach gets to the right answer in five minutes.

Step 1 — is this real?

Before any workup, exclude pseudohyponatraemia:

  • Hyperglycaemia — every 5.5 mmol/L rise in glucose lowers measured sodium by ~2.4 mmol/L. Correct the glucose first.
  • Severe hyperlipidaemia or hyperproteinaemia — older flame photometry methods give falsely low sodium. Less common with modern analysers but still seen.

Confirm with paired serum and urine osmolality; serum osmolality should be low (<275 mOsm/kg) in true hypotonic hyponatraemia.

Step 2 — assess volume status

The volume-status question is the single most important step. It is also the step most often done badly. Use the full toolkit:

  • Mucous membranes and skin turgor
  • JVP / postural BP and pulse
  • Peripheral oedema, ascites
  • Urine output and urinary sodium
  • Recent fluid balance and drug chart
Volume statusSuggests
HypovolaemicGI losses, diuretics, third-space loss, adrenal insufficiency
EuvolaemicSIADH, hypothyroidism, glucocorticoid deficiency, primary polydipsia
HypervolaemicHeart failure, cirrhosis, nephrotic syndrome, AKI/CKD

Clinical pearl

Urinary sodium splits hypovolaemic causes neatly. UNa <20 mmol/L points to extra-renal loss (vomiting, diarrhoea, sweat). UNa >20 mmol/L points to renal loss (diuretics, salt-wasting, adrenal insufficiency).

Step 3 — confirm with urine osmolality

  • Urine osmolality <100 mOsm/kg — appropriately dilute urine. The kidney is responding correctly to the low sodium. Think primary polydipsia, low solute intake (tea-and-toast, beer potomania).
  • Urine osmolality >100 mOsm/kg — inappropriately concentrated urine. ADH is on. Now the volume status determines the cause.

If you're stuck between SIADH and hypovolaemia in a euvolaemic-looking patient, the cleanest test is a trial of 0.9% saline — SIADH typically does not correct (or worsens slightly); hypovolaemia does.

Treatment — match the rate to the symptoms, not the number

Red flag

Acute severe hyponatraemia (<125 mmol/L developing in <48 hours) with seizures, reduced GCS, or vomiting needs hypertonic 3% saline — 150 mL bolus over 10 minutes, repeat to a max of three, aiming for a 5 mmol/L rise. Escalate to critical care.

For asymptomatic chronic hyponatraemia, correct slowly — no more than 10 mmol/L in the first 24 hours, 18 mmol/L in 48 hours. Overcorrection causes osmotic demyelination, especially in alcohol-dependent, malnourished, hypokalaemic, or liver-disease patients.

  • Hypovolaemic — replace volume with isotonic saline. The sodium follows.
  • Euvolaemic / SIADH — fluid restrict (usually 800-1000 mL/24h). Consider tolvaptan only in specialist settings.
  • Hypervolaemic — treat the underlying disease and fluid-restrict; loop diuretic if congestion.

Key points

  • Exclude pseudohyponatraemia before anything else.
  • Volume status is the diagnostic hinge.
  • Urinary sodium splits hypovolaemia; urine osmolality clarifies SIADH.
  • Correct slowly in chronic hyponatraemia. Treat symptoms, not the number.

References

  1. [1]Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1-G47. Link
  2. [2]Verbalis JG, Goldsmith SR, Greenberg A, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-S42. Link

About the author

Dr. Sara HassanVerifiedVerified medical practitioner

MBBS, MRCP

Endocrinology· Oxford University Hospitals NHS Foundation Trust

Endocrinology specialist registrar with an interest in inpatient diabetes and adrenal disease. Writes the endocrinology pearls for MedSchool.

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