Sodium (Na) – Understanding Your Blood Test Result

Section Editor: Christopher McCudden Ph.D., DABCC, FADLM, FCACB
September 15, 2026

Sodium is a mineral that carries a small electrical charge in your body. Minerals that behave this way are called electrolytes. Sodium is the most abundant electrolyte in the fluid outside your cells, and its main job is to control how much water your body holds and where it sits. Because water follows sodium, the sodium level in your blood is really a measure of the balance between salt and water rather than of how much salt you eat.

This article explains what the sodium test measures and what a high or low result can mean. It also explains why a low sodium result is usually about too much water rather than too little salt.

The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.

What does the sodium test measure?

Sodium controls water movement between your blood and your cells. Where sodium goes, water follows. Your body closely regulates blood sodium levels. Thirst adds water, and a hormone made in the brain, called antidiuretic hormone, decides how much water your kidneys keep or release.

Because this system is about the balance of salt and water, the number on your report tells your doctor about water balance first. A low sodium result usually means there is too much water in the blood diluting it, not that you need more salt. This surprises many patients, and it is the single most useful thing to understand about this test.

Your report may show sodium in mmol/L or mEq/L, with a reference range printed next to it. These two units are equivalent for sodium. A typical adult level is about 135 to 145 mmol/L, and exact ranges vary by laboratory. Some reports show sodium simply as Na, its chemical symbol.

Why is the normal range so narrow?

The range for sodium is unusually tight compared with most blood tests. That is because your brain is sensitive to changes in water balance. If blood sodium falls, water moves into brain cells, and they swell slightly. If it rises, water moves out.

This is why doctors pay as much attention to how quickly a sodium level has changed as to the number itself. A sodium of 130 that has been stable for years is treated very differently from a sodium of 130 that was 140 last week. This is also why clinicians correct markedly abnormal sodium slowly and carefully rather than quickly.

How is the sodium test performed?

A small blood sample is measured, usually drawn from a vein in your arm. No fasting is needed. It is included in the basic metabolic panel, the comprehensive metabolic panel, and most kidney function testing, so it is one of the most frequently measured substances in medicine.

Your doctor may also order a urine sodium test, and sometimes a measurement called serum osmolality. These help work out why a blood sodium is abnormal by showing whether the kidneys are holding on to water or letting it go.

What do sodium results mean?

Read sodium alongside your other electrolytes, kidney tests, medicines, and how you feel.

A normal sodium level — a result inside the reference range means your body is balancing salt and water as expected.
A low sodium level — a result below the reference range is called hyponatremia. It is the most common electrolyte abnormality seen in medicine. Mild cases often cause no symptoms at all and are found by chance on a routine test. Lower levels can cause tiredness, headache, nausea, confusion, or unsteadiness.
A high sodium level — a result above the reference range is called hypernatremia. It is much less common and almost always means the body is dehydrated. It is most often seen in people who cannot drink freely, such as older adults who are unwell, or young children with a stomach illness.

How far outside the range the result sits matters a great deal. A sodium level of 133 in someone who feels well is common and usually monitored. A sodium in the low 120s is a different situation and is acted on promptly.

What conditions and factors can affect sodium?

Common reasons sodium can be low

Medicines — this is one of the commonest causes. Thiazide diuretics, some antidepressants, and several other common drugs can lower sodium. Do not stop any medicine because of a blood test. Ask the doctor who prescribed it.
Drinking a very large amount of fluid — more water than the kidneys can clear dilutes the blood.
Heart, liver, or kidney disease — these conditions cause the body to hold on to water, diluting sodium even when total body water is high.
A hormone imbalance — the syndrome of inappropriate antidiuretic hormone, often shortened to SIADH, causes the kidneys to keep too much water. It has many triggers, including some medicines, lung conditions, and illness.
Vomiting or diarrhea — fluid and salt are lost together, and replacing them with water alone lowers sodium further.
An underactive thyroid or adrenal gland — both can lower sodium and are usually checked when no other cause is clear.

Common reasons sodium can be high

Not drinking enough water — particularly in people who cannot easily ask for a drink or reach one.
Losing fluid — through fever, sweating, vomiting, or diarrhea, without replacing it.
Diabetes insipidus — an uncommon condition in which the kidneys cannot hold on to water properly.
Uncontrolled diabetes — very high blood sugar draws water out of the body through the urine.

Sometimes the sodium reading is low, but the body’s salt and water balance is normal. Very high blood fats or certain proteins can interfere with older measurement methods and produce a falsely low result. This is called pseudohyponatremia, and the laboratory can usually identify it.

What other tests are ordered with sodium?

Potassium, chloride, and bicarbonate — the other electrolytes, read as a pattern rather than individually.
Creatinine, urea, and eGFR — to see whether the kidneys are part of the explanation.
Serum and urine osmolality — measurements of how concentrated the blood and urine are, used to work out the cause of low sodium.
Urine sodium — which shows whether the kidneys are holding on to salt or losing it.
Thyroid and adrenal testing — when a hormonal cause is being considered.

What happens after a sodium test?

What happens next depends on the number, on how quickly it changed, and on how you feel. A mildly low sodium level in someone who feels well is often repeated first, and your doctor will review your medicines, since medicines are among the most common causes.

If the result is confirmed, the next step is working out whether your body has too much water, too little salt, or both. That is what the osmolality and urine tests are for. Treatment follows the cause. It may mean adjusting a medicine, limiting fluid intake, or treating an underlying condition. A markedly low sodium, or one that has fallen quickly, is usually managed in hospital, and correction is done gradually because raising sodium too fast can be harmful.

Questions to ask your doctor

What was my sodium level, and what is the reference range at this laboratory?
How does this compare with my previous results, and how quickly has it changed?
Could any of my medicines be causing this?
Should I be drinking more or less fluid?
Do you want to repeat the test, and if so, when?
Do I need urine tests or osmolality testing to find the cause?
Should my thyroid or adrenal function be checked?
What symptoms should prompt me to contact you before my next test?