Hyponatraemia becomes much easier to approach when you stop treating every low sodium as the same problem.
The key is to work through the pattern systematically: first identify the plasma osmolality, then assess volume status, and then use urinary sodium or urinary osmolality to narrow the cause.
The practical framework: Serum sodium → plasma osmolality → volume status → urinary sodium / urinary osmolality → likely cause → broad management.
1. Start with plasma osmolality
For serum sodium below 135 mmol/L, divide the problem according to plasma osmolality. This is the first branch in any useful causes map and matches the approach summarised in NICE CKS hyponatraemia.
- 280–295 mOsm/kg — isotonic hyponatraemia — think pseudohyponatraemia.
- <280 mOsm/kg — hypotonic hyponatraemia — true hyponatraemia; assess volume status next.
- >295 mOsm/kg — hypertonic hyponatraemia — think translocational causes.
2. Isotonic hyponatraemia
When plasma osmolality is in the isotonic range, the pathway points towards pseudohyponatraemia rather than a true water excess problem.
- Hyperlipidaemia
- Hyperparaproteinaemia
Assess lipid and protein levels, identify the underlying cause and treat that cause.
For hyperlipidaemia, lifestyle measures and consideration of statin therapy may be relevant.
For hyperproteinaemia, the pathway highlights causes such as malignancy / multiple myeloma and treatment of the underlying condition.
3. Hypertonic hyponatraemia
When plasma osmolality is above 295 mOsm/kg, this is hypertonic or translocational hyponatraemia.
- Hyperglycaemia
- Secondary to hypertonic fluid
Assess for hyperglycaemia, identify the cause and treat it.
Diabetes — particularly DKA — is a classic context.
Calculate a corrected serum sodium so you are not misled by the measured value alone.
4. Hypotonic hyponatraemia: assess volume status
If plasma osmolality is below 280 mOsm/kg, you are in true hypotonic hyponatraemia.
Split patients into hypovolaemic, euvolaemic and hypervolaemic groups before interpreting urine sodium or urine osmolality.
Volume status is clinical — look at mucous membranes, JVP, oedema, capillary refill, postural observations and fluid balance, not just the chart label.
5. Hypovolaemic hypotonic hyponatraemia
In the hypovolaemic branch, urinary sodium helps separate renal from extra-renal losses.
- Urine Na >20 mmol/L — renal loss — diuretic use; osmotic diuresis; mineralocorticoid deficiency.
- Urine Na <20 mmol/L — extra-renal loss — GI loss; burns; pancreatitis.
Broad management options:
- Assess the cause of the volume-status abnormality and treat the cause.
- Consider isotonic saline.
- Stop diuretic use where appropriate.
- Steroid replacement therapy for Addison's disease.
6. Euvolaemic hypotonic hyponatraemia
In the euvolaemic branch, urinary osmolality helps narrow the cause.
- Urine osmolality >100 mOsm/kg — SIADH / endocrinopathies — SIADH; glucocorticoid deficiency.
- Urine osmolality <100 mOsm/kg — primary polydipsia — mental health cause; exercise-induced.
Broad management options:
- Assess the cause and treat it.
- Fluid restriction.
- Medication change.
- Treat the underlying cause of SIADH.
SIADH remains a diagnosis of exclusion. Confirm the hypotonic euvolaemic pattern and exclude important alternatives (including adrenal insufficiency and severe hypothyroidism) before locking onto the label — see NICE CKS assessment.
7. Hypervolaemic hypotonic hyponatraemia
The hypervolaemic branch is linked to oedematous disorders, with urinary sodium below 20 mmol/L typically shown in the pathway.
- Heart failure
- Cirrhosis
- Nephrotic syndrome
- Renal failure
Broad management options:
- Assess the cause of the volume-status abnormality and treat the cause.
- Diuresis.
- Fluid restriction.
- Sodium restriction.
8. Causes of SIADH
Important SIADH associations (not an exhaustive list):
- Brain injury — traumatic brain injury; CVA; SAH; meningitis.
- Malignancy — small-cell lung cancer.
- Endocrine — hypothyroidism.
- Infection — cerebral abscess; lung abscess; atypical pneumonia.
- Medications — SSRI; amitriptyline; carbamazepine; lisinopril; levodopa.
9. General management map
Pattern → broad management (see also NICE CKS management):
- Isotonic — assess lipid/protein levels, identify the cause and treat it.
- Hypertonic — assess for hyperglycaemia, identify and treat the cause; corrected serum sodium is highlighted.
- Hypotonic — hypovolaemic — assess and treat cause; consider isotonic saline; stop diuretic use; steroid replacement for Addison's.
- Hypotonic — hypervolaemic — assess and treat cause; consider diuresis, fluid restriction and sodium restriction.
- Hypotonic — euvolaemic — assess and treat cause; consider fluid restriction, medication change and treatment of underlying SIADH.
10. Important safety points
Hypertonic saline: Requires specialist input and monitoring.
Do not start this management independently — discuss with a senior and obtain expert help.
It is used only in severe symptomatic hyponatraemia, irrespective of fluid status.
Avoid rapid correction: For acute/chronic hyponatraemia, do not correct sodium by ≥10 mmol/L per 24 hours because of the risk of central pontine myelinolysis (osmotic demyelination).
The Foundation doctor hyponatraemia framework
LOW SODIUM → PLASMA OSMOLALITY → VOLUME STATUS → URINARY Na / URINARY OSMOLALITY → CAUSE → BROAD MANAGEMENT → AVOID RAPID CORRECTION
The key message
The algorithm is built around classification. Work out whether the hyponatraemia is isotonic, hypotonic or hypertonic, then use volume status and the relevant urine test to narrow the cause before choosing the broad management approach.
Educational note: This article is for Foundation doctor education.
For an individual patient, use NICE CKS hyponatraemia, local NHS hyponatraemia guidance and senior/endocrine advice.
Content for this version was updated from the clinical information visible in the supplied hyponatraemia management images.
