Sodium
Serum Sodium
What this test measures
Sodium is a mineral that helps control fluid balance in your body and is essential for nerve and muscle function. Your kidneys carefully control sodium levels.
Understanding the numbers
Typical range: 136-145 mmol/L
Sodium levels are tightly controlled by the body. Even small changes can be significant.
- Severe low: 115–125 mmol/L
- Moderate low: 125–130 mmol/L
- Mild low: 130–135 mmol/L
- Normal: 136–145 mmol/L
- High: 146–160 mmol/L
Key numbers
- 130 mmol/L — Below = moderate hyponatraemia — investigate
- 125 mmol/L — Below = severe hyponatraemia — admit
- 146 mmol/L — Above = hypernatraemia — usually dehydration
- 120 mmol/L — Critical — seizure and cerebral oedema risk
UK reference range is typical. NICE CG174 / hyponatraemia guidance: sodium below 125 mmol/L or symptomatic hyponatraemia needs admission. Rapid correction of chronic hyponatraemia can cause osmotic demyelination — always correct slowly, usually in hospital.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean (Hypernatraemia)
- Often caused by dehydration
- Can happen if you are not drinking enough fluids
- Some medicines can raise sodium
- Your GP may want to find the underlying cause
When not to wait
Rapid or severe sodium changes affect the brain and can cause fits or collapse.
- Confusion or drowsiness that is new or worsening
- Seizures (fits) or severe headache
- Collapse or losing consciousness
- Repeated vomiting that prevents fluid intake
- Known sodium below 125 mmol/L with any symptoms
- Very high sodium with extreme thirst and low urine output
Call NHS 111 for urgent advice, or 999 if someone is seriously unwell.
Questions you could ask your GP
- What might be causing my abnormal sodium level?
- Could any of my medicines be affecting this?
- Do I need to change how much I drink?
- When should this be rechecked?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
SIADH from a new medication
Mavis had been started on sertraline two weeks ago. She was confused and unsteady. Sodium 122, urine sodium and osmolality showed SIADH.
What the GP said: “Your sertraline is causing your body to hold on to too much water, diluting the sodium. This is a medical emergency at 122 — I'm admitting you for gradual correction in hospital.”
What happened next: Sertraline was stopped; fluid restriction was started. Sodium rose slowly over three days. She was switched to mirtazapine with no recurrence.
SSRIs, diuretics, carbamazepine and tramadol can cause SIADH, especially in older adults. Confusion, unsteadiness and a sodium below 125 is a hospital problem, not a community one.
Mild drop on bendroflumethiazide
John's sodium was 132 mmol/L on a routine check. He'd been on bendroflumethiazide for hypertension. He felt well.
What the GP said: “Bendroflumethiazide can lower sodium. A mild drop is often tolerated, but we should switch you to a different blood pressure tablet and recheck in a month.”
What happened next: Switched to amlodipine; sodium at one month was 138.
Thiazide diuretics are a common cause of mild chronic hyponatraemia in older adults. It's usually reversible by switching class.
Hypernatraemia — dehydrated in a care home
Ravinder had not been drinking well. Sodium 154, urea 14, creatinine up from baseline. He was confused and thirsty.
What the GP said: “This is dehydration with high sodium. You need admission for IV fluids — we must replace the water slowly to avoid shifting fluid too fast.”
What happened next: Admitted; sodium corrected gradually over 48 hours; returned to care with a drinking plan in place.
Hypernatraemia is almost always water deficit, not salt excess. It happens most often to older adults who can't access fluids or don't feel thirst. Correction must be gradual.
Marathon — exercise-induced hyponatraemia
Trisha collapsed at the end of a marathon. She had drunk a lot of water without electrolytes during the race. Sodium 128.
What the GP said: “Drinking too much plain water during prolonged exercise dilutes the sodium. For your next race, drink to thirst and use an electrolyte drink if running for more than 90 minutes.”
What happened next: She recovered overnight in hospital with fluid restriction; sodium normalised within 36 hours.
Over-drinking plain water during endurance events can cause exercise-associated hyponatraemia. Drinking to thirst, with electrolyte drinks on longer efforts, prevents it.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.