Blood sodium: what it is, what it tells you and what a low level means

Blood sodium measures the balance of water and salt in your body, not how much salt you eat. Low sodium is the most common imbalance of water and salts. Usually there is too much water for the salt, often because of a medicine, drinking a lot of water, or vomiting and diarrhoea. High sodium, on the other hand, usually means too little water.

Why we test it

Sodium keeps water where it belongs, in your blood and around your cells. Your kidneys keep it in balance, guided by a hormone that decides how much water to hold on to.

When it drops fast, cells take on water and swell a little, in the brain too. That is why it can cause headache and confusion.

Common medicines often shift it, such as diuretics and some antidepressants. A blood test can find it before you notice anything.

What it tells you, and what it does not

It tells you whether water and salt are in the right proportion. If it is low, there is usually too much water. If it is high, there is too little.

It does not tell you how much salt you eat. Your body holds it in a very narrow range, and people who eat a lot of salt usually have a normal level.

Nor does it tell you why. To find out, your doctor looks at your medicines, how much you drink and other tests.

That is why it is read together with other tests:

Normal blood sodium levels

The lab decides what is normal. It is usually 136 to 145 mmol/L, the same for women and men. Your own report may print different limits.

Some reports print mEq/L instead of mmol/L: for sodium the number is the same. You may also see it written as Na.

Your body keeps it very steady: in the same person it barely changes from one day to the next. So when it moves out of range there is usually a reason, often how much you drank or a medicine.

In the Rosa report you also see your optimal range, next to potassium, creatinine and eGFR.

What your result means

Find the row for your result, in mmol/L. Which values are dangerous? Below 130 should always be seen by a doctor, and below 125 or above 150 today.

If your sodium is low or high

Low sodium: the causes

Usually salt is not missing: there is too much water. Often more than one cause is at work.

Low sodium: the symptoms

If the drop is mild or slow, often none. When it is more marked, you may notice:

Even a drop with no obvious symptoms can make you unsteady on your feet if it lasts. Confusion, severe drowsiness, seizures or fainting are an emergency: call 112 or go to the emergency department.

High sodium: what it means

It usually means there is too little water in your body: you drank little, sweated a lot, or had a fever, vomiting or diarrhoea. With enough water it usually settles.

The salt you eat moves it very little. A very high intake can show up a little in the number, but people who eat a lot of salt usually have a normal sodium. Extra salt weighs mostly on your blood pressure, far more than on this number.

With age you feel thirst less, and drinking enough gets harder. More rarely, the body loses too much water through the urine. If you pass a lot of urine and are always thirsty, talk to your doctor.

What to do if your sodium is out of range

It depends on the cause, and your doctor usually finds it. In the meantime:

What can skew the result

Sodium moves mostly with water. It helps to know this before you read it.

When to worry

Talk to a doctor if:

With Rosa, a sodium below 130 mmol/L is always flagged to a doctor, and below 125 or above 150 urgently.

Frequently asked questions

Hyponatraemia, hypernatraemia: what do they mean? Hyponatraemia means low sodium in the blood, hypernatraemia means high sodium. On your report sodium may also be written as Na.

Does high sodium mean I eat too much salt? Usually not. Your body holds blood sodium in a very narrow range, and people who eat a lot of salt usually have a normal level. Extra salt matters for your blood pressure.

Low sodium in older people: why is it more common? With age the body controls water less precisely, and people more often take diuretics and antidepressants. Even a drop with no obvious symptoms can make you unsteady and cause falls if it lasts: talk to your doctor.

Low sodium: what should I eat? Food is usually not the cure: the cause matters. Do not add a lot of salt or take salt tablets on your own: if they are needed, your doctor decides.

How much water should I drink if my sodium is out of range? Usually it is enough to drink when you are thirsty, and more in hot weather or when you exercise. If your sodium is low, your doctor sometimes asks you to drink less for a while. It depends on the cause, so do not do it on your own.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. If it is out of range, your doctor decides when to check it again.

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