Low Sodium (Hyponatraemia)
Sodium is the main salt in your blood, and its level mostly reflects the balance of water in your body. A low result usually means too much water for the amount of sodium, not too little salt in your diet.
Quick answer: Sodium below 135 mmol/L is low. Mild results of 130–134 are common, often from water tablets, antidepressants or vomiting, and many people feel fine. Under 125 is profound. Confusion, drowsiness, vomiting or a seizure with low sodium is an emergency, whatever the number.
What the test measures
The sodium level is part of the electrolytes (often reported as “UEC” or “EUC” in Australia, or “U&E” in the UK). Your kidneys and a hormone called antidiuretic hormone (ADH, or vasopressin) control how much water you keep. When the body holds on to too much water, sodium is diluted and the level falls. Speed matters: a fall over less than 48 hours is more dangerous than one that has crept down over weeks.
Typical adult ranges
- Normal: 135–145 mmol/L. US labs report mEq/L, which is the same number.
- Mild hyponatraemia: 130–134 mmol/L
- Moderate: 125–129 mmol/L
- Profound: under 125 mmol/L
- These bands come from the European hyponatraemia guideline. Symptoms don’t always match the number, so how you are matters as much as the result.
- False lows can happen with very high blood sugar, very high fats or proteins in the blood, or a sample taken from an arm with a drip running.
What a low result usually means
Common causes:
- Medicines: thiazide-type water tablets (hydrochlorothiazide, indapamide, chlortalidone), SSRI and SNRI antidepressants such as sertraline, carbamazepine and oxcarbazepine. The risk is higher in older people and in the first weeks.
- Vomiting or diarrhoea, especially when lost fluid is replaced with plain water.
- Fluid build-up conditions: heart failure, liver cirrhosis and some kidney diseases.
- Drinking a lot of water: during long endurance events, or with compulsive drinking.
- Very low food intake with lots of fluid, such as a “tea and toast” diet in older people, or heavy beer drinking with little food.
- SIAD (syndrome of inappropriate antidiuresis): the body releases too much ADH, from chest infections, lung disease, brain conditions, pain, nausea or after surgery.
Less common but important:
- Adrenal insufficiency (Addison’s disease or pituitary problems), often with high potassium. See Addison’s disease.
- A severely underactive thyroid. See underactive thyroid.
- Cancers that make ADH, especially small-cell lung cancer.
- MDMA (ecstasy), particularly with large amounts of water.
Symptoms
- Often none when mild or slow.
- Nausea, headache, tiredness, muscle cramps and poor concentration.
- In older people: unsteadiness, falls and new confusion.
- Severe: vomiting, drowsiness, seizures or reduced consciousness.
What happens next
- Repeat the test with glucose, kidney function and other checks, often thyroid function and a morning cortisol.
- Paired blood and urine tests (osmolality and urine sodium) help show the cause.
- A medicine review. A doctor may stop or swap a water tablet or antidepressant. Don’t stop prescribed medicines without advice.
- Treat the cause: for SIAD, this often means drinking less fluid for a while, as advised by your doctor.
- In hospital, profound or symptomatic hyponatraemia is corrected carefully: guidelines limit the rise to 10 mmol/L in the first 24 hours, because correcting too fast can damage the brain (osmotic demyelination).
- Don’t take salt tablets or drink salty drinks to fix it yourself unless a doctor advises it.
Get urgent help (000 in Australia, or your local emergency number) for low sodium with vomiting, confusion, drowsiness or a seizure, or for headache, vomiting or confusion after an endurance event or a party where you drank a lot of water. See a doctor the same day if your sodium is under 125 mmol/L, or under 130 with nausea, headache, unsteadiness or new confusion. See your GP within a week or so for a mild result, especially after starting a new medicine.
Practise reading results. Play Panelysis, the daily blood-test puzzle: a fictional patient, a real-looking panel, three guesses. More in medical games.
Common questions
What sodium level is dangerous?
Under 125 mmol/L is called profound hyponatraemia and usually needs urgent medical review. But symptoms matter more than the number: vomiting, confusion, drowsiness or a seizure with any low sodium is an emergency, and a fast fall is more dangerous than a slow one.
Is a sodium of 132 bad?
It’s mildly low. Many people feel fine, and it’s often due to a water tablet, an antidepressant or a recent tummy bug. Your GP will usually repeat it and review your medicines. In older people even mild low sodium can add to falls and unsteadiness.
Can drinking too much water lower sodium?
Yes. Drinking far more than you need, especially during long endurance events or with MDMA, can dilute sodium dangerously. Drink to thirst during exercise rather than forcing fluids.
Which medicines cause low sodium?
The commonest are thiazide-type water tablets (such as hydrochlorothiazide and indapamide), SSRI and SNRI antidepressants, and the epilepsy medicines carbamazepine and oxcarbazepine. The risk is highest in older people and in the first weeks of treatment.
Should I eat more salt if my sodium is low?
Usually not. Low sodium is mostly about too much water rather than too little salt, and extra salt can make fluid build-up worse in heart or liver disease. Your doctor will treat the cause, which may mean drinking less, not eating more salt.
What are the symptoms of low sodium in older people?
Often subtle: unsteadiness, falls, tiredness, poor concentration and new confusion. Because these are easy to blame on age, doctors often check sodium when an older person becomes more confused or starts falling.
Evidence and more information
- Pathology Tests Explained (RCPA): Sodium
- Spasovski et al, European Journal of Endocrinology 2014: Clinical practice guideline on diagnosis and treatment of hyponatraemia (severity bands, correction limits)
- Society for Endocrinology 2022: Emergency management of severe and moderately severe symptomatic hyponatraemia in adults
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