Sodium Blood Test: Hyponatraemia, Hypernatraemia and SIADH Explained
Hyponatraemia is the commonest electrolyte disorder in hospital. Learn the volume-status workup, SIADH, and safe correction rates.
Normal sodium is 135–145 mmol/L. Hyponatraemia (< 135) is classified by volume status: hypovolaemic (diarrhoea, diuretics), euvolaemic (SIADH, hypothyroidism, adrenal insufficiency), or hypervolaemic (heart failure, cirrhosis, nephrotic). Correct no faster than 10 mmol/L in 24h to avoid osmotic demyelination.
Sodium disorders reflect water balance, not sodium balance. Hyponatraemia is the commonest electrolyte disturbance in hospital (up to 30% of admissions) and independently predicts mortality. The European Renal Best Practice (ERBP) 2014 pathway remains the standard: first assess acute vs chronic and symptom severity; symptomatic acute hyponatraemia (< 48 h) needs 3% saline bolus (150 mL over 20 min) regardless of level; chronic hyponatraemia (> 48 h or unknown) must be corrected slowly — no more than 10 mmol/L in 24 h — to prevent osmotic demyelination syndrome. Diagnostic workup: plasma osmolality, urine osmolality, urine sodium, TFTs, cortisol. SIADH criteria: euvolaemia, urine osm > 100 mOsm/kg, urine Na > 30 mmol/L, plasma osm < 275, normal thyroid + adrenal function, no diuretics. Common SIADH causes: SSRIs, thiazides, carbamazepine, pneumonia, small-cell lung cancer, CNS pathology. Hypernatraemia (> 145) is usually free-water deficit (elderly, cognitive impairment, poor access to water).
- Normal sodium
- 135 – 145 mmol/L
- Max correction
- ≤ 10 mmol/L per 24 h
- Symptomatic acute
- 3% saline 150 mL over 20 min
- SIADH urine osm
- > 100 mOsm/kg
- Osmotic demyelination
- Risk if correction too fast
Hyponatraemia by volume status
| Volume status | Causes | Urine Na |
|---|---|---|
| Hypovolaemic | Diarrhoea, vomiting, diuretics, addison | Low (< 20) if extrarenal; high if diuretic/Addison |
| Euvolaemic | SIADH, hypothyroidism, adrenal insufficiency, primary polydipsia | High (> 30) SIADH; low primary polydipsia |
| Hypervolaemic | Heart failure, cirrhosis, nephrotic syndrome | Low (< 20) |
SIADH criteria (ERBP 2014)
Serum osmolality < 275 mOsm/kg.
Urine osmolality > 100 mOsm/kg while hyponatraemic.
Clinical euvolaemia.
Urine sodium > 30 mmol/L on a normal salt/water intake.
Normal thyroid and adrenal function; no recent diuretic.
- Sodium < 125 mmol/L — same-day hospital assessment.
- Any hyponatraemia + seizure, coma, or focal neurology — emergency 3% saline.
- Correction rate exceeded (> 10 mmol/24h) — risk of osmotic demyelination.
Hyponatraemia workup
- 1Symptomatic (seizure/coma)?3% saline 150 mL bolus over 20 min. Repeat if needed. Max rise 5 mmol/L in first hour.
- 2Assess volume status.Skin turgor, JVP, blood pressure, urine output.
- 3Send urine osm, urine Na, TFT, cortisol.Interpret with volume status.
- 4Meets SIADH criteria?Fluid restrict 800–1000 mL/day. Identify and treat cause.
Related questions people ask
Frequently asked questions
- Sodium disorders are water balance disorders.
- Classify by volume status.
- Max correction 10 mmol/L per 24 h — avoid demyelination.
- SIADH is a diagnosis of exclusion.
- Symptomatic acute hyponatraemia = 3% saline.
References
2 sources- ERBPHyponatraemia Guideline
European Renal Best Practice.
academic.oup.com
- NICE CKSHyponatraemia
UK primary care.
cks.nice.org.uk
Evidence Snapshot
This snapshot shows verified evidence records matched to this article. It is not a diagnosis or personal medical advice.
The serum anion gap is a calculated value derived from measured electrolytes, most commonly as sodium minus the sum of chloride and bicarbonate, and reflects the balance between measured and unmeasured ions in blood.
Limitation. The anion gap is calculated and therefore inherits any measurement error in sodium, chloride and bicarbonate; laboratory reference ranges also vary.
Serum osmolality is influenced most strongly by serum sodium and its accompanying anions, with smaller contributions from glucose and urea in most adults.
Limitation. Additional osmoles from ingested substances such as alcohols are not captured by the standard calculation and can produce an osmolal gap.
Serum osmolality is a measure of the total number of dissolved particles per kilogram of serum water and is used to evaluate hydration and sodium-water balance.
Limitation. A single osmolality value does not identify the underlying cause of an abnormal result.
Related articles
Laboratory TestsTSH Test: Normal Range, High, Low & What Your Result Actually Means
TSH is the single most important thyroid number. Understand normal ranges by age and pregnancy, what a high or low value really tells you, and the two follow-up tests that clinch the diagnosis.
Laboratory TestsHbA1c Test: Normal Range, Prediabetic, Diabetic Cut-offs & How to Lower It
HbA1c reflects your average blood sugar over ~3 months. Understand every threshold — normal, prediabetes, diabetes — and the realistic actions that move the number.
Laboratory TestsLDL Cholesterol: Optimal Numbers, High Numbers, and How to Read Yours
LDL is the "bad" cholesterol only if you leave the story there. Understand optimal targets by risk group, the ApoB perspective, and the interventions that actually shift the number.