Quick answer: Low sodium on a blood test means the sodium concentration in your blood is below the laboratory’s range. It often reflects the balance between water and sodium in your body, not simply how much salt you ate. The right response depends on your symptoms, medicines, other results and how quickly the level changed. Do not try to correct it by taking salt tablets or drastically changing how much you drink without medical advice.
You open a routine blood report and see “Na” marked low. Should you add salt to dinner, drink an electrolyte beverage or worry about your kidneys? None of those is a reliable answer from one number. Here is how to make sense of the finding and decide what to ask next.
Anthony Ofori | Medical Graduate in General Medicine | Founder, WellNest Central
Last updated: 30 September 2026
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What does the sodium number actually measure?
A sodium blood test measures the concentration of sodium in the blood. Sodium helps regulate fluid balance and supports nerve and muscle function. It is commonly included in an electrolyte panel or metabolic panel. A typical reference range is about 135–145 mmol/L (numerically equivalent to mEq/L for sodium), but check the range printed by your laboratory. A value below about 135 is called hyponatremia. The MedlinePlus sodium blood test guide explains why clinicians interpret it alongside the other results and your history.
The number is a concentration, like the strength of a mixture. It can fall when the body holds too much water relative to sodium, when both water and sodium are lost in different amounts, or for other medical reasons. A low result does not by itself tell you how much sodium is in your entire body.
Why can sodium be low even when you eat salt?
Excess water relative to sodium is a frequent explanation. This can happen with some kidney, heart or liver conditions; with hormone-related problems; or after drinking a very large amount of water. Vomiting or diarrhoea can also disturb the balance. Some medicines, including certain diuretics and antidepressants, are relevant. The National Kidney Foundation’s hyponatremia guide notes that low blood sodium is often more about body water than too little dietary salt.
Very high blood glucose can also affect the measured sodium concentration. That is one reason a clinician may check glucose and the rest of the panel before deciding what the low value means. A review of medicines, recent illness and changes in drinking or urination helps narrow the possibilities.
Should you eat more salt or drink less water?
Do not choose a treatment from the label “low sodium” alone. Eating more salt may be inappropriate if the problem involves excess fluid, heart failure, kidney disease or another condition. Restricting fluid can be useful in some circumstances, but the amount and duration must match the cause. In other situations, a person needs a different approach, sometimes in hospital.
The speed of correction matters too. A level that fell suddenly or is very low requires careful medical assessment; raising sodium too quickly can cause harm. The National Kidney Foundation advises against trying to treat hyponatremia on your own. Do not stop a prescribed water pill or other medicine without discussing it with the prescriber.
Which symptoms make this urgent?
Mild hyponatremia may cause no obvious symptoms. Headache, nausea, unusual fatigue, weakness, muscle cramps or unsteadiness can occur, but these symptoms have many possible causes. New severe confusion, a seizure, marked drowsiness or reduced consciousness needs emergency care now. Tell the emergency team about the low sodium result and any recent medicines, vomiting, diarrhoea or unusually high fluid intake. MedlinePlus on low blood sodium describes the serious neurological complications that can occur when the level drops too far or too fast.
If a laboratory or clinician has flagged the result as critically low, follow its urgent instructions even if you feel well. If the result is only slightly below range and you have no symptoms, contact the ordering clinician for an individual interpretation and follow-up plan rather than assuming it can wait indefinitely.
What might your clinician check next?
The first step is to confirm the result, compare it with earlier sodium values and consider how quickly it changed. Your clinician may review glucose, kidney function, potassium, medicines and fluid status. Depending on the case, blood and urine osmolality, urine sodium or hormone testing can help identify the cause. These tests are selected for the situation; one abnormal result does not mean everybody needs the entire list.
If kidney results also look unusual, our guide to three kidney tests that add context beyond creatinine explains what different tests answer. If potassium is flagged too, see our guide on high potassium on a blood test. Neither article replaces prompt assessment of a concerning electrolyte result.
Five questions to bring to your appointment
- How low is my sodium, and how does it compare with my previous results?
- Do my symptoms or the speed of change make this urgent?
- Could my medicines, recent illness, glucose or fluid intake be involved?
- Do I need a repeat sample, urine tests or other checks to find the cause?
- What exact instructions should I follow for fluids, medicines and follow-up?
The takeaway
Low sodium on a blood test is a clue about fluid and sodium balance, not an automatic instruction to eat more salt. The number, symptoms, trend and underlying cause guide the next step. Seek emergency care for serious neurological symptoms, and get a clinician’s plan for an abnormal result. If you would like help organising your daily habits and questions before a healthcare appointment, connect with WellNest Central.
This article provides general health education and does not replace individual medical advice, diagnosis, treatment or emergency care.
