Sodium (Na, Na+, Serum sodium, Blood sodium, Sodium blood test, Natrium)
Sodium is the dominant electrolyte in the fluid outside your cells, and your body defends its level fiercely. It sets the 'osmotic pressure' that decides where water goes — into cells or out of them — which is why sodium matters so much for the brain: brain cells swell when sodium is too low and shrink when it is too high. The blood test measures the concentration of sodium in your serum or plasma, reported in mEq/L in the US.
Here is the counterintuitive part most people miss: your blood sodium result says more about your body's water balance than about how much salt you eat. A low result usually means extra water is diluting your blood; a high result usually means you are short on water. The kidneys, the hormone vasopressin (ADH), and your sense of thirst form a control loop that normally holds sodium within the narrow 135–145 mEq/L band regardless of whether your diet is salty or bland.
Because that control loop involves the kidneys, the brain, several hormones, and many common medications, an abnormal sodium is a clue rather than a diagnosis. Doctors interpret it alongside your fluid status, urine tests, medication list, and other results on the same panel — which is why the same number can mean different things in different people.
Reference ranges
Adults
Most US labs report a normal adult sodium range of about 135 to 145 mEq/L (the convention used by MedlinePlus and Mayo Clinic); some labs use 136 to 145. Critical (panic) cutoffs vary slightly between labs: sodium at or below 120 mEq/L is widely treated as a critical low, and 160 mEq/L and above as a critical high, though some labs flag highs starting at 155.
How to read your result
Low sodium (hyponatremia, below 135 mEq/L) is the most common electrolyte abnormality and usually means excess water relative to sodium — from water pills, antidepressants, high fluid intake, hormone disorders like SIADH, or heart, liver, and kidney disease. How low it is matters, but how fast it got there matters just as much: a slow drift to 128 may cause no symptoms, while a rapid fall to the same number can be serious. High sodium (hypernatremia, above 145 mEq/L) means a water deficit, and because thirst normally prevents it, values much above 146–149 in someone drinking freely deserve a real explanation — dehydration from illness, medications that increase urination, or rarely diabetes insipidus.
At both extremes, values below about 120 and above about 160 (some labs use 155) are treated by most labs as critical results needing immediate attention. In between, context rules: symptoms (especially confusion), the trend across repeat tests, and your medication list usually tell the story better than any single number.
What can shift this value
- Diuretics (water pills), especially thiazides such as hydrochlorothiazide, are a leading cause of low sodium
- Antidepressants (SSRIs/SNRIs) and antiseizure drugs like carbamazepine can trigger SIADH and lower sodium
- How much water you drink — very high intake lowers sodium; too little raises it
- Vomiting, diarrhea, fever, and heavy sweating shift both water and sodium
- Heart failure, cirrhosis, and kidney disease commonly cause dilutional low sodium
- Very high blood glucose pulls water into the blood and lowers the measured sodium (about 1.6–2.4 mEq/L per 100 mg/dL of glucose above normal)
- Very high triglycerides or blood proteins can cause a falsely low reading (pseudohyponatremia)
- Age — older adults have blunter thirst and more medication exposure, so they sit at higher risk on both ends
- Endurance exercise with aggressive plain-water drinking can cause exercise-associated hyponatremia
- Small lab-to-lab differences mean a value 1–2 mEq/L past the cutoff may simply be variation — trends beat single values
When it's tested
Sodium is part of the basic metabolic panel (BMP), the comprehensive metabolic panel (CMP), the electrolyte panel, and renal function panels — so it is checked at most annual physicals, before surgeries, in emergency rooms, and whenever medications like diuretics are monitored. It is not part of the CBC (complete blood count) or the lipid panel.
Sodium converts 1-to-1 between US and SI units — a result in mEq/L (US) is numerically identical in mmol/L (UK, Singapore, and most of the world), so 140 mEq/L equals 140 mmol/L.
Frequently asked questions
Is your value a normal sodium level?
For adults, most US labs consider roughly 135 to 145 mEq/L normal for blood sodium. Where your value falls relative to that range — and whether it matters — depends on your symptoms, medications, and hydration, so review it with your clinician.
Is a sodium level of your value dangerous?
Danger depends on how far the value is from the 135–145 mEq/L range, how fast it changed, and whether you have symptoms like confusion, severe headache, or repeated vomiting. Fast changes are more dangerous than slow ones at the same number.
What causes low blood sodium (hyponatremia)?
Low sodium almost always means too much water relative to sodium, not too little salt in the diet. Common causes include thiazide diuretics (water pills), antidepressants such as SSRIs, drinking large amounts of water, vomiting or diarrhea, SIADH (excess antidiuretic hormone from lung, brain, or medication causes), and heart, liver, kidney, thyroid, or adrenal conditions. Very high blood glucose can also dilute the measured sodium. Your clinician uses urine tests and your fluid status to sort out which applies to you.
Can drinking too much water cause low sodium?
Yes. Drinking water faster than your kidneys can excrete it dilutes your blood sodium — this is water intoxication. It is a known risk during endurance events when people drink heavily without eating, in people who drink very large volumes habitually, and when illness or medications limit how much water the kidneys can release. For most healthy people, drinking to thirst is the safest guide; there is no need to force extreme volumes of water.
Does eating too much salt raise my blood sodium level?
Usually not. A salty diet makes you thirsty and makes your body hold water, which raises blood pressure and causes puffiness — but the blood sodium concentration typically stays normal because thirst and the kidneys rebalance it. High blood sodium (hypernatremia) is almost always a shortage of water rather than an excess of dietary salt. That said, a lower-salt diet is still worthwhile for blood pressure and heart health, which is a separate issue from this test result.
Should I go to the ER for a sodium of your value?
Go to the emergency department for any sodium result if you have marked confusion, extreme drowsiness, seizures, or repeated vomiting. Otherwise, how urgently you need care depends on how far your value is outside the 135–145 mEq/L range — your clinician or the lab's flag on the report can guide you.
Why is my sodium low if I eat plenty of salt?
Because blood sodium is a concentration, not a measure of salt intake. Low sodium nearly always means your body is holding on to extra water that dilutes the sodium — from medications, hormones like ADH, drinking large volumes, or conditions such as heart, liver, or kidney disease. Adding more salt to your food rarely fixes it and can be harmful in some of these conditions, so the right move is finding the cause with your clinician rather than changing your diet on your own.
Browse Sodium values
All 71 values from 100 to 170 mEq/L
Sources
- Sodium Blood Test — MedlinePlus (National Library of Medicine)
- Electrolyte Panel — MedlinePlus (National Library of Medicine)
- Hyponatremia — Symptoms and Causes — Mayo Clinic
- Hyponatremia (StatPearls) — NCBI Bookshelf / StatPearls
- Hypernatremia (StatPearls) — NCBI Bookshelf / StatPearls
Educational information, not medical advice. Full disclaimer