Sodium is the main electrolyte in the fluid outside your cells, and a blood test measures how much is circulating in your bloodstream. A normal serum sodium level sits between 135 and 145 mEq/L. The most important idea to grasp is that this number tells you far more about your body's water balance than about how much salt you eat — most abnormal results reflect too much or too little water, not too much or too little salt.
What is the sodium blood test?
Sodium is a positively charged particle that your body uses to hold water in the right compartments, conduct nerve signals, contract muscles and keep blood pressure stable. Because sodium and water move together, your kidneys, brain and several hormones work constantly to keep the concentration in a tight band. When you are dehydrated, hormones tell the kidneys to conserve water; when you are overloaded, they let water go. Sodium is the marker that reflects whether that system is in balance.
A doctor usually measures sodium as part of a serum electrolyte panel, a basic metabolic panel or a kidney function test. It is ordered when you have confusion, unusual drowsiness, seizures, muscle cramps or weakness, persistent vomiting or diarrhoea, swelling, signs of dehydration, or when you take medicines that affect fluid balance such as diuretics ("water tablets"). In Indian clinical practice it is one of the most frequently checked values in hospitalised, elderly and post-operative patients precisely because fluid shifts are so common.
Sodium normal range
For most healthy adults, a normal blood sodium is between 135 and 145 mEq/L. A level below 135 mEq/L is called hyponatraemia (low sodium), and a level above 145 mEq/L is called hypernatraemia (high sodium). The table below shows the general severity bands ExaHealth uses to interpret a result. The unit is milliequivalents per litre (mEq/L), which is standard across Indian pathology labs and is numerically the same as mmol/L for sodium — always confirm the unit on your own report before comparing.
| Sodium level (mEq/L) | Interpretation |
|---|---|
| 135 – 145 | Normal |
| 130 – 134.9 | Borderline low (mild hyponatraemia) |
| 125 – 129.9 | Moderately low |
| 120 – 124.9 | Severely low |
| Below 120 | Critically low |
| 145.1 – 149.9 | Borderline high (mild hypernatraemia) |
| 150 – 154.9 | Moderately high |
| 155 – 159.9 | Severely high |
| 160 and above | Critically high |
These bands are drawn from standard laboratory reference ranges. What matters clinically is not only how far the number sits from normal, but how quickly it changed — a sodium that falls or rises rapidly is far more dangerous than the same value reached slowly, because the brain has had no time to adapt.
Normal range by age, sex and condition
Unlike some markers, blood sodium does not have widely differing reference numbers for men, women or age groups — the 135–145 mEq/L band applies broadly. What changes between groups is the risk of drifting out of range and the usual direction of that drift. The table below explains these patterns qualitatively; where your report gives no group-specific figure, the general band still applies.
| Group or condition | Typical pattern | Why it differs |
|---|---|---|
| Healthy adults | 135 – 145 mEq/L | Reference range for a well-hydrated person with normal kidney and hormone function. |
| Older adults | More prone to low sodium | A blunted thirst response, reduced kidney concentrating ability and a higher burden of medicines all make hyponatraemia much more common with age. |
| People on diuretics | Often runs low | Thiazide diuretics in particular promote sodium and water loss and are a leading cause of low sodium, especially in older women. |
| Heart, liver or kidney failure | Tends to run low | The body retains excess water that dilutes sodium, even though total body sodium may be normal or high — this is "dilutional" hyponatraemia. |
| Infants and young children | Vulnerable to rapid swings | Their smaller fluid reserves mean vomiting, diarrhoea or incorrect feed dilution can shift sodium quickly in either direction. |
| Endurance athletes | Can fall after long events | Drinking large volumes of plain water during prolonged exertion can dilute sodium, a recognised cause of exercise-associated hyponatraemia. |
Because sodium moves with so many everyday factors — illness, medicines, heat, hydration — a single reading is best understood in context and over time. Tracking it alongside your other electrolytes and kidney markers with a tool like ExaHealth makes it far easier to separate a one-off dip during a stomach bug from a genuine trend that needs review.
What high sodium means
High sodium (hypernatraemia), above 145 mEq/L, almost always signals a water deficit rather than excess salt. The blood becomes too concentrated because the body has lost more water than sodium, or has not taken in enough. The single commonest cause is simple dehydration — not drinking enough, especially in the elderly, in very hot weather, or in anyone who cannot access or ask for water. Other causes include prolonged vomiting or diarrhoea, high fever with heavy sweating, uncontrolled diabetes driving large urine losses, and a rarer hormone problem called diabetes insipidus in which the kidneys cannot conserve water.
Because water leaves brain cells when the blood outside them is too salty, symptoms are largely neurological: intense thirst, dry mouth, restlessness, irritability, muscle twitching, lethargy and, in severe cases, confusion or seizures. In infants and frail older adults, hypernatraemia can develop quietly and is a medical emergency at the higher bands. As with low sodium, correction must be careful and gradual — bringing the level down too fast can cause the brain to swell.
What low sodium means
Low sodium (hyponatraemia), below 135 mEq/L, is one of the most common electrolyte abnormalities seen in clinical practice, and it is usually a problem of too much water relative to sodium rather than true salt depletion. It is especially frequent in older adults and in people taking certain medicines. The main mechanisms are:
- Medicines and diuretics. Thiazide diuretics are a leading cause, and several other drugs — including some antidepressants, anti-epileptics and painkillers — can lower sodium. This is why a fresh medication review is part of working up any low result.
- SIADH (syndrome of inappropriate antidiuretic hormone). Here the body releases too much of the hormone that tells the kidneys to hold on to water, so water is retained and sodium becomes diluted. SIADH can be triggered by lung and brain conditions, certain cancers, infections and some drugs.
- Fluid-retaining illnesses. Heart failure, cirrhosis of the liver and advanced kidney disease all cause the body to hold excess water, diluting the sodium even when the total amount of salt in the body is not low.
- Genuine losses replaced with water. Prolonged vomiting, diarrhoea or heavy sweating lose both salt and water; if the fluid is then replaced with plain water alone, sodium falls further. Drinking very large volumes during endurance events works the same way.
Mild low sodium may cause only vague symptoms — headache, nausea, tiredness or difficulty concentrating. As it falls further, confusion, unsteadiness, muscle cramps and, at severe or critical levels, seizures and reduced consciousness can follow. Because the causes are so varied, a low reading is a starting point: your doctor may check your fluid status, urine sodium, kidney and thyroid function, and review every medicine you take.
Why sodium must be corrected slowly
Whether sodium is too low or too high, one rule stands out: it must be corrected gradually, under medical supervision. The reason is that brain cells adapt to an abnormal sodium level over hours to days by adjusting their own internal contents to match. If the blood sodium is then moved back to normal too quickly, the balance between the brain cells and the fluid around them swings the other way and can cause serious, sometimes permanent injury — brain swelling when high sodium is dropped too fast, and a condition called osmotic demyelination when low sodium is raised too fast. This is why you should never try to "fix" an abnormal sodium yourself with salt, salt restriction or large amounts of water. Doctors correct sodium in small, monitored steps for exactly this reason.
How to manage and support healthy sodium
For most people, sodium looks after itself through normal eating, drinking to thirst and healthy kidneys. When it drifts, the fix is to address the cause — not to chase the number. Sensible, everyday steps include:
- Drink to thirst, not by the clock. During ordinary days and prolonged heat or exercise, let thirst guide you rather than forcing very large volumes of plain water, which can dilute sodium.
- Replace losses properly during illness. With vomiting, diarrhoea or heavy sweating, use oral rehydration solution (ORS) rather than plain water alone, so you replace both salt and fluid. This is standard advice across Indian households for good reason.
- Review your medicines. If you take diuretics or other drugs that affect sodium, keep your check-ups and ask your doctor before making changes — do not stop them on your own.
- Do not self-treat with salt or salt restriction. Adding extra salt or cutting it out sharply can do harm; any correction of an abnormal sodium belongs with your doctor.
- Retest to see the trend. One value can mislead, especially after an acute illness. A repeat test shows whether things are settling back into range.
When to see a doctor: seek urgent care for confusion, drowsiness, severe headache, repeated vomiting, seizures or fainting, or any very low or very high sodium result on a report — these need prompt evaluation rather than home management. You can compare sodium with related electrolytes such as potassium and chloride, and explore the full lab tests library to make sense of your panel. To follow your electrolytes and kidney markers over time, see how ExaHealth brings your reports together.
Guidelines and references
The interpretation bands above are based on standard laboratory reference ranges. Sodium is universally reported on serum electrolyte, basic metabolic and kidney function panels; always interpret your result against the reference range printed on your own lab report and in discussion with your doctor, who will weigh how quickly the value changed and your overall fluid status.
- Standard laboratory reference ranges as reported by accredited pathology laboratories.
Frequently asked questions
What is a normal sodium level in a blood test?
For most healthy adults, a normal blood sodium is between 135 and 145 mEq/L (numerically the same as mmol/L). Always compare against the reference range printed on your own report.
Does a low sodium level mean I don't eat enough salt?
Usually not. Blood sodium reflects your body's water balance more than your salt intake, so most low results come from holding on to too much water — for example from medicines, SIADH, or heart, liver or kidney conditions — rather than eating too little salt.
What causes high sodium?
High sodium almost always means a water deficit rather than excess salt. Common causes are dehydration, prolonged vomiting or diarrhoea, high fever with heavy sweating, and uncontrolled diabetes; it is especially common in the elderly.
Why must low sodium be corrected slowly?
The brain adapts to an abnormal sodium level over time. Raising a low sodium too quickly can trigger a serious condition called osmotic demyelination, so doctors correct it in small, monitored steps rather than all at once.
Is low sodium serious?
It can be. Mild hyponatraemia may cause only headache or tiredness, but a rapidly falling or very low level can lead to confusion, seizures and reduced consciousness and needs prompt medical care.
Can drinking too much water lower my sodium?
Yes. Drinking very large volumes of plain water, particularly during prolonged exercise, can dilute blood sodium and cause exercise-associated hyponatraemia. Replacing losses with an oral rehydration solution rather than plain water helps prevent this.