Low sodium (hyponatraemia)
How fast it fell matters as much as how low it is. Severe symptoms need 3% saline now. Chronic low sodium must never be corrected too fast.
A 58-year-old woman, two days after a hysterectomy, has been on 5% dextrose drips "to keep the line open". She has vomited and now had a fit. Sodium 118 mmol/L (mEq/L), was 138 before surgery.
Is this acute or chronic, and how do you treat it?
- Severe symptoms?Fits, drowsiness, falling GCS, vomiting, confusion. These need treatment now.
- Severe: 3% saline bolus150 mL over 20 minutes, recheck sodium, repeat up to twice. Senior and HDU.
- Check the sugar and repeat the sodiumHigh sugar lowers sodium falsely.
- Send tests before giving saline if possibleSerum osmolality, urine osmolality, urine sodium. Do not delay treatment of severe symptoms.
- Assess fluid statusDry, normal or overloaded. This points to the cause.
- Stop the culpritsDextrose drips, thiazides, SSRIs, carbamazepine, PPIs, and others.
- Plan the target and recheck timesSodium every 4–6 hours while correcting. Write the limit on the chart.
- Think of the adrenal glandLow BP, high potassium, low sugar: give hydrocortisone.
- Fits, falling GCS or persistent vomiting
- Sodium below 120
- Sodium rising more than 10 in 24 hours
- Low BP with low sodium and high potassium
- Alcohol use, malnutrition or low potassium (high risk)
- Brain swelling in acute hyponatraemia
- Osmotic demyelination from overcorrection
- Adrenal crisis
- Severe hypothyroidism
- High sugar causing a falsely low sodium
| Severe symptoms | 150 mL 3% saline IV over 20 minutes, repeat up to twice, aiming for a rise of 5 mmol/L |
| Daily limit | Rise no more than 10 mmol/L in the first 24 hours (8 if high risk), and 8 in each 24 hours after |
| High risk of overcorrection | Alcohol, malnutrition, low potassium, liver disease, sodium below 120 |
| Suspected adrenal crisis | Hydrocortisone 100 mg IV immediately |
| Mild and chronic | Treat the cause, slowly |
Check every dose against a current drug reference and your hospital protocol.
On this page
Why speed matters
When sodium falls, water moves into brain cells and they swell. If it falls fast (within 48 hours), the brain has no time to adapt: swelling causes vomiting, fits, coma and death. If it falls slowly, the brain adapts, and the danger switches: correcting it too fast shrinks the brain cells and destroys the nerve coverings in the brainstem (osmotic demyelination), causing paralysis, swallowing problems and "locked-in" states days later.
- After surgery with hypotonic drips
- Marathon or heavy water drinking
- Ecstasy (MDMA)
- Brain swelling is the danger
- Treat severe symptoms quickly
- Most ward patients
- Thiazides, SSRIs, SIADH, heart or liver failure
- Overcorrection is the danger
- Correct slowly with strict limits
Symptoms: how bad is it?
| Severity | Symptoms |
|---|---|
| Severe | Vomiting, fits, drowsiness, coma, breathing problems |
| Moderate | Nausea without vomiting, confusion, headache |
| Mild or none | Tiredness, unsteadiness, falls. Many have no symptoms |
Severe symptoms: treat now
- Recheck sodium after each 150 mLStop boluses once sodium has risen by 5 mmol/L or symptoms improve.
- Then stop hypertonic salineTreat the cause. Recheck sodium at 6 and 12 hours, then daily.
- Watch the urineA sudden large output of dilute urine means the sodium is about to rise fast.
- Rising too fast?Senior now. Options include 5% dextrose and desmopressin to bring it back.
Finding the cause
- Vomiting, diarrhoea, diuretics (thiazides), adrenal insufficiency
- Urine sodium low (below 30) unless diuretics or adrenal
- Treatment: 0.9% sodium chloride carefully
- SIADH: chest disease, brain disease, drugs, pain, nausea, after surgery
- Hypothyroidism, adrenal insufficiency
- Too much water, too little salt (tea and toast diet)
- Treatment: fluid restriction, treat the cause
- Heart failure, liver cirrhosis, kidney failure, nephrotic syndrome
- Treatment: fluid restriction, treat the condition
- 5% dextrose drips and other hypotonic fluids, especially after surgery and in children.
- Thiazides (hydrochlorothiazide, indapamide).
- SSRIs (sertraline, fluoxetine), carbamazepine, antipsychotics.
- PPIs, opioids, some chemotherapy.
Investigations
| Test | Why |
|---|---|
| Repeat sodium, sugar | Confirm; high sugar causes a falsely low sodium |
| Serum osmolality | Low in true hyponatraemia |
| Urine osmolality | Below 100 means too much water intake |
| Urine sodium | Low (below 30) suggests dry or overloaded; high suggests SIADH, diuretics or adrenal |
| Potassium, creatinine | Adrenal insufficiency (high potassium), kidney disease |
| TSH, morning cortisol | Thyroid and adrenal causes |
| Chest X-ray, CT head as needed | Causes of SIADH |
Send the serum and urine samples before any saline if you can. Do not delay treating severe symptoms to get them.
Treatment by cause
- SIADH: fluid restriction (often 1 litre a day or less), treat the cause, stop culprit drugs. Further options on senior advice.
- Dry: 0.9% sodium chloride, with frequent sodium checks: once the patient is filled, the kidneys can release water fast and the sodium can shoot up.
- Heart or liver failure: fluid restriction, treat the condition.
- Adrenal insufficiency: hydrocortisone.
Common mistakes
- Leaving post-operative patients on 5% dextrose for days.
- Correcting chronic low sodium by 15 mmol/L in a day.
- Not rechecking the sodium often enough while correcting.
- Missing adrenal crisis in a patient with low BP, low sodium and high potassium.
- Giving 0.9% saline to SIADH: it can make the sodium fall further.
- Forgetting that high sugar causes a falsely low sodium.
Frequently asked questions
What is the dose of 3% saline in severe hyponatraemia?
150 mL IV over 20 minutes, rechecking sodium after each dose, repeated up to twice, aiming for a rise of 5 mmol/L.
How fast can sodium be corrected?
No more than 10 mmol/L in the first 24 hours (8 mmol/L in high-risk patients) and no more than 8 mmol/L in each 24 hours after that.
What is osmotic demyelination syndrome?
Damage to the nerve coverings in the brainstem caused by correcting chronic low sodium too quickly. It causes paralysis, swallowing and speech problems days later and may be permanent.
Which drugs cause hyponatraemia?
Thiazide diuretics, SSRIs, carbamazepine, antipsychotics, PPIs, opioids, and hypotonic IV fluids such as 5% dextrose.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. After surgery, a woman on 5% dextrose has sodium 118 mmol/L and is now fitting. What is the treatment?
Answer: 150 mL of 3% saline. Severe symptomatic hyponatraemia needs hypertonic saline aiming for a 5 mmol/L rise; stop hypotonic fluids and manage in HDU.
- 0.9% saline 1 litre over 1 hour: Not concentrated enough for this emergency.
- Phenytoin only: Treat the sodium.
- Fluid restriction: Too slow for severe symptoms.
- Tolvaptan: Not for severe symptomatic hyponatraemia.
2. A man with alcohol dependence has sodium 116 mmol/L and no symptoms. What is the maximum safe rise in the first 24 hours?
Answer: 8 mmol/L. Alcohol use, malnutrition and low potassium increase the risk of osmotic demyelination. Limit the rise to 8 mmol/L in 24 hours.
- 15 mmol/L: Risks osmotic demyelination.
- 2 mmol/L: Unnecessarily slow.
- 12 mmol/L: Too fast for a high-risk patient.
- 20 mmol/L: Too fast.
3. A patient has sodium 122 mmol/L, potassium 5.9 mmol/L, BP 82/50 and glucose 3.2 mmol/L (58 mg/dL). What is the most likely diagnosis?
Answer: Adrenal crisis. Low sodium, high potassium, low BP and low glucose suggest adrenal crisis. Give hydrocortisone 100 mg IV immediately with fluids.
- Hypothyroidism: Less likely with this pattern.
- Heart failure: Doesn't fit.
- SIADH: SIADH doesn't cause hypotension and high potassium.
- Diuretic use: Would not explain the hypoglycaemia.
4. A 68-year-old smoker has Na 121 mmol/L, serum osmolality 255, urine osmolality 480 and urine Na 45 mmol/L. He is euvolaemic, with normal thyroid and cortisol. What is the first treatment?
Answer: Fluid restriction. SIADH: hypotonic hyponatraemia, inappropriately concentrated urine (> 100 mOsm/kg), urine Na > 30, euvolaemia, normal thyroid/adrenal function, no diuretics. Causes: small-cell lung cancer, CNS disease, pneumonia, drugs (SSRIs, carbamazepine, PPIs). Treat cause; fluid restriction (< 800–1000 mL/day) first; then tolvaptan or urea. Hypertonic saline only for severe symptoms (seizures, coma). Correct by ≤ 8–10 mmol/L in 24 h to avoid osmotic demyelination.
- 0.9% saline infusion: Saline can worsen hyponatraemia in SIADH.
- 3% saline bolus: Hypertonic saline is only for severe symptoms.
- Furosemide alone: Furosemide alone is not first-line.
5. A week after subarachnoid haemorrhage, a patient has Na 127 mmol/L, high urine sodium, polyuria, postural hypotension, dry mucosae and raised haematocrit. What is the most appropriate treatment?
Answer: IV isotonic saline. After SAH or brain injury, hyponatraemia may be SIADH (euvolaemic — fluid restriction) or cerebral salt wasting (renal sodium loss via natriuretic peptides → hypovolaemia with high urine sodium and output). Both have inappropriately concentrated urine and high urine Na — volume status distinguishes them (hypotension, tachycardia, rising haematocrit/urea, negative fluid balance favour CSW). CSW treatment: replace volume and sodium with isotonic (or hypertonic) saline; fludrocortisone may help. In SAH, avoid fluid restriction (vasospasm/delayed ischaemia risk).
- Fluid restriction to 1 L: Fluid restriction worsens hypovolaemia and vasospasm.
- Oral tolvaptan: Tolvaptan is dangerous in hypovolaemia.
- Hypotonic dextrose infusion: Hypotonic fluid lowers sodium further.
6. A woman has tiredness, weight loss, low blood pressure and dark pigmentation of her palmar creases and gums. Which electrolyte pattern is expected?
Answer: Low sodium and high potassium. Primary adrenal insufficiency (Addison's disease, usually autoimmune; TB in some regions) causes loss of cortisol and aldosterone. Low aldosterone causes salt loss and potassium retention: hyponatraemia, hyperkalaemia and hypotension. High ACTH (and MSH) causes pigmentation. Hypoglycaemia may occur.
- High sodium and low potassium: Conn's syndrome.
- Low sodium and low potassium: Diuretics or vomiting.
- High calcium and low phosphate: Hyperparathyroidism.
- Normal sodium and potassium: Not in Addison's.
7. Which metabolic effect is caused by thiazide diuretics?
Answer: Hypercalcaemia. Thiazides block the Na⁺/Cl⁻ co-transporter in the distal convoluted tubule. They increase calcium reabsorption (useful in calcium stones and osteoporosis), and cause hypokalaemia, hyponatraemia, hyperuricaemia (gout), hyperglycaemia and hyperlipidaemia. Loop diuretics cause calcium loss instead.
- Hypocalcaemia: Loop diuretics.
- Hyperkalaemia: Potassium-sparing diuretics.
- Hypoglycaemia: Thiazides raise glucose.
- Hypouricaemia: Thiazides raise uric acid.
References and dose sources
- European clinical practice guideline on hyponatraemia (2014)
- Society for Endocrinology emergency guidance
- Society for Endocrinology — adrenal crisis
- Society for Endocrinology — emergency management of severe and moderately severe symptomatic hyponatraemia (2022)
- European clinical practice guideline on hyponatraemia (2014)
- Society for Endocrinology — emergency guidance
- NICE CKS — hyponatraemia
- Sodium chloride (normal saline, 3% saline) dosing: Drugs.com: Sodium chloride dosage (see also our sodium chloride (normal saline, 3% saline) dose page)
- Hydrocortisone dosing: Drugs.com: Hydrocortisone dosage (see also our hydrocortisone dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.