Sodiummmol/L
Fluid & electrolyte status
Typical optimal range
133–146 mmol/L
A typical reference range for blood sodium is 133–146 mmol/L, though exact ranges vary slightly between laboratories and should always be interpreted using your own lab's reference interval. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
Interpretation of sodium and electrolyte results follows NHS laboratory medicine practice and NICE clinical knowledge summaries on hyponatraemia and electrolyte disorders.
What is Sodium?
Sodium is the body's principal extracellular electrolyte, meaning it is the dominant electrically charged mineral found in the fluid surrounding your cells. It plays a central role in regulating fluid balance, blood pressure, and the electrical signals that drive nerve and muscle function. A blood sodium measurement (taken as part of a urea and electrolytes panel, or U&Es) reflects how well your body is managing the balance between sodium and water, rather than simply how much salt you eat. Because even modest shifts outside the normal range can affect the brain and other organs, sodium is one of the most tightly regulated substances in the body.
What a high Sodium means
A sodium level above the upper end of the reference range — a state called hypernatraemia — is relatively uncommon in otherwise healthy people and usually signals that the body has too little water relative to its sodium content, rather than too much salt in the diet. It warrants a closer look with your GP, particularly if it persists on repeat testing.
- Dehydration. Not drinking enough fluid, sweating heavily, or losing water through vomiting or diarrhoea without adequately replacing it can raise blood sodium. This is the most common cause of a mildly elevated reading in an otherwise well person.
- Reduced fluid intake. Older adults in particular may have a diminished sense of thirst, making it easier to become relatively dehydrated without realising it, which can push sodium higher.
- Underlying medical causes. Less commonly, conditions such as diabetes insipidus (where the kidneys cannot retain water properly) or certain hormonal disorders can raise sodium. These are usually accompanied by other symptoms and investigated by a GP or specialist.
A single mildly raised result in someone who has been unwell, sweating, or not drinking well is often transient; repeat testing after rehydration typically resolves it. Specimen handling issues can occasionally affect results.
What a low Sodium means
A sodium level below the lower end of the reference range — hyponatraemia — is actually the more clinically significant direction and one of the most common electrolyte abnormalities seen in clinical practice. It usually signals that there is too much water relative to sodium in the body, and the cause needs to be identified.
- Overhydration during endurance exercise. Drinking large volumes of plain water during prolonged exercise such as marathons or triathlons without replacing electrolytes can dilute blood sodium, sometimes to levels that cause symptoms including headache, confusion, or nausea.
- SIADH (syndrome of inappropriate antidiuretic hormone). In SIADH, the body retains too much water because antidiuretic hormone is released inappropriately; this dilutes sodium. It can be triggered by certain medications, lung conditions, or brain disorders, and requires medical investigation.
- Diuretics and medications. Some blood pressure medications (particularly thiazide diuretics) promote water and sodium loss from the kidneys and are a common cause of low sodium, especially in older adults. Other drugs including some antidepressants (SSRIs) are also associated with hyponatraemia.
Mild, isolated hyponatraemia can sometimes be a laboratory artefact or reflect technique; your GP will usually repeat the test and check it in clinical context before acting on it.
Should I be worried?
A single sodium result slightly outside the reference range — in either direction — is not a diagnosis, and many mild deviations resolve on their own or reflect something straightforward such as hydration status at the time of the blood draw. That said, sodium sits at the heart of how your body manages fluid, so a result that is clearly abnormal, that persists on repeat testing, or that comes with symptoms (such as unusual thirst, headaches, confusion, swelling, or feeling very unwell) is worth discussing with your GP promptly. Your GP will typically interpret sodium alongside other results from the same U&E panel — particularly potassium, urea, and creatinine — as well as your current medications, kidney function, and any relevant symptoms, to build a full picture. Severe or rapidly developing abnormalities in either direction are treated as medical urgencies, though these are usually picked up in clinical settings rather than routine health checks.
What actually helps
For most people with a sodium level within or close to the normal range, the goal is maintaining good hydration habits and a sensible approach to dietary salt rather than making dramatic changes.
- Stay well hydrated. Drinking adequate fluid throughout the day — generally around 6–8 cups for most adults in a UK climate, more in heat or during exercise — supports normal sodium regulation. Water, lower-fat milk, and unsweetened drinks all count.
- Replace electrolytes during prolonged exercise. During exercise lasting more than about 60–90 minutes, particularly in warm conditions, using a sports drink or electrolyte supplement alongside water can prevent the dilutional hyponatraemia seen in endurance athletes who drink plain water to excess.
- Sensible salt intake. UK dietary guidelines recommend no more than 6 g of salt per day for adults. Keeping within this target supports healthy blood pressure without driving sodium to extremes; most dietary salt in the UK comes from processed and restaurant foods rather than added table salt.
- Review medications with your GP. If you take diuretics, SSRIs, or other medications associated with sodium changes, and your result is outside range, speak to your GP before making any changes — never stop prescribed medicines without advice.
There is good supporting evidence that certain dietary patterns help maintain electrolyte balance and healthy blood pressure, which are closely linked to sodium regulation.
Vegetables and fruit
High in potassium, which works in tandem with sodium to regulate fluid balance and blood pressure; a diet rich in produce is consistently associated with better cardiovascular and kidney outcomes in UK and international evidence.
Wholegrains
Naturally low in sodium and contribute to overall dietary patterns (such as the DASH diet) that support healthy blood pressure and electrolyte balance.
Low-fat dairy (milk, yoghurt)
Provides a useful combination of potassium, calcium, and some sodium in proportions that support electrolyte balance, and is counted towards fluid intake.
Unsalted nuts and seeds
Good sources of magnesium and potassium; replacing salted snacks with unsalted varieties meaningfully reduces dietary sodium without sacrificing nutritional value.
Oily fish
Part of a heart-healthy dietary pattern; not directly linked to sodium but contributes to overall cardiovascular health which shares risk factors with sodium-related blood pressure conditions.
Water and low-sugar electrolyte drinks (during exercise)
Adequate fluid intake maintains plasma volume; for prolonged exercise, electrolyte drinks help prevent sodium dilution, directly relevant to exercise-associated hyponatraemia.
Track your own Sodium
Upload a lab PDF and Better Days trends Sodium over time, flags it when it drifts, and suggests foods that help.
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Sources & further reading
These are reputable UK health resources for further reading. This page is written to be consistent with their guidance but is not a substitute for them.
For information only — not medical advice, and not individually reviewed by a clinician. Reference ranges vary by lab and individual. Discuss any results with a qualified clinician before making changes.