Sodium Blood Test: Normal Range, Low Sodium & High Sodium

Sodium is the main mineral (electrolyte) in the fluid outside your cells, and its blood level — reported on lab work as serum sodium — is one of the most ordered numbers in medicine. Here is the counterintuitive part: sodium levels are mostly a measure of your water balance, not how much salt you eat. A low sodium result (hyponatremia) is the most common electrolyte disorder of all, and it usually means too much water, not too little salt. This guide explains the normal sodium range in mmol/L, what low sodium (hyponatremia) and high sodium (hypernatremia) mean, and why the whole thing comes down to water. Sodium is measured as part of the electrolyte panel and the comprehensive metabolic panel.

Key takeaways

• Sodium (Na⁺) is the main extracellular ion; it controls how much water your body holds and therefore your blood volume.¹
• Typical range: about 135–145 mmol/L (the same as 135–145 mEq/L) — exact cutoffs vary by lab.²
• An abnormal sodium almost always reflects a water imbalance, not too much or too little salt.³
• Low sodium (hyponatremia, below 135) is the most common electrolyte disorder; causes include SIADH, heart, liver, or kidney disease, and diuretics. It must be corrected slowly.³⁴⁵
• High sodium (hypernatremia, above 145) usually means dehydration — loss of water — most often in older adults.⁶⁷
• You do not "raise" or "lower" your own sodium: the cause and the speed of correction are what matter, and both belong to a clinician.⁴

Normal sodium levels

Below are typical adult reference values. Cutoffs vary slightly by lab, so compare against the range printed on your report.

A note on units. U.S. labs report sodium in mmol/L, which is numerically identical to mEq/L for sodium — a result of "140" is the same value whichever label your lab prints. A small, isolated deviation (say, a sodium of 134) does not carry the same weight as a clearly abnormal number paired with symptoms.

What is serum sodium?

Sodium is the dominant electrolyte in the extracellular compartment — the fluid that surrounds your cells and makes up blood plasma. Where potassium rules the inside of cells, sodium governs the outside: it sets how much water the body retains, and therefore your blood volume and pressure.³

That is why serum sodium is not really a measure of "how much salt you eat." It reflects the balance between water and salt in your body. Because sodium and water travel together, the concentration you see on a lab report moves mainly when water shifts — which is the single most important idea in this guide. Sodium is interpreted alongside your hydration, your medications, and your kidney function (creatinine).

Why sodium is measured

Sodium is almost never ordered by itself. It comes bundled in the basic metabolic panel (BMP) or the comprehensive metabolic panel, routine panels that report it next to its electrolyte partners potassium and chloride, plus glucose and kidney markers.⁸ Clinicians check it to:

• run a routine panel or follow known heart, liver, or kidney disease;
• work up symptoms such as confusion, headache, nausea, weakness, or a fall in alertness;
• monitor medications that shift sodium — diuretics, some antidepressants, and some antiseizure drugs;
• assess dehydration, vomiting or diarrhea, or unusual water intake.¹

You typically do not need to fast for a sodium test on its own, though the CMP it usually rides along with may require fasting for the glucose portion — follow your provider's instructions.⁹

Interpreting your results

Low sodium (hyponatremia)

Hyponatremia — a sodium below 135 mmol/L — is the most common electrolyte disorder seen in clinical practice.³ Most of the time it means there is too much water relative to sodium, not a shortage of salt. Common causes:³⁵¹⁰

• Medications, especially diuretics, and some antidepressants and antiseizure drugs;
• Heart failure, liver disease (cirrhosis), or kidney disease, in which the body holds on to water;
• SIADH (syndrome of inappropriate antidiuresis), a leading cause in older adults, where the body makes too much antidiuretic hormone and retains free water;¹¹
• Excess water intake or fluid loss — including exercise-associated hyponatremia, the "water intoxication" seen in endurance athletes who drink too much during a race.¹²

Symptoms range from nausea and headache to confusion and, if the drop is severe or fast, dangerous changes in alertness. Even mild but chronic hyponatremia is not harmless: it is linked to a higher risk of falls, fractures, and problems with balance and concentration, especially in older adults.¹¹ The crucial point is that correction must be slow and gradual. Raising a chronically low sodium too fast can cause a serious brain injury (osmotic demyelination), which is why current standards favor fluid restriction or, in specific cases, carefully monitored hypertonic saline, and why you should never try to "fix" a low sodium yourself — it is a medical decision.⁴¹³ In heart failure, hyponatremia is also a genuine marker of severity, tied to more hospitalizations and worse outcomes.¹⁴

High sodium (hypernatremia)

Hypernatremia — a sodium above 145 mmol/L — most often means a lack of water (dehydration), rather than eating too much salt. It shows up most in older or dependent adults who do not drink enough, or after major fluid losses from fever, vomiting, or diarrhea.⁶⁷ Less commonly it points to diabetes insipidus, a deficiency of or resistance to antidiuretic hormone that causes heavy urination and intense thirst.¹⁵ Treatment is careful rehydration — again, gradual and medically supervised, because correcting sodium too quickly is itself risky.⁶

Why it's about water, not salt

Here is the teaching point worth keeping. Serum sodium is a concentration — sodium divided by the water it is dissolved in. Your body defends that concentration mainly by moving water in and out (through thirst and antidiuretic hormone), not by hunting for salt.³⁶ So:

• A low sodium usually means too much water diluting a normal amount of salt — not a salt deficiency. Eating more salt is generally not the answer and can even be harmful depending on the cause.³⁵
• A high sodium usually means too little water concentrating the salt — dehydration — not salt overload.⁶⁷

That is why clinicians think in terms of body water (tonicity) rather than salt intake when they read a sodium result.⁶ It also explains why a can of soup or a salty meal barely moves your serum sodium in the short term: your kidneys and thirst adjust the water to match.

What can affect sodium

Serum sodium depends on your hydration (water in versus water out), your medications (diuretics above all), your kidney function, and hormones — chiefly antidiuretic hormone, which is why SIADH and diabetes insipidus both show up here. Heart and liver disease shift it by changing how the body handles water. One lab artifact worth knowing: a very high blood glucose can lower the measured sodium (pseudohyponatremia by dilution), so a diabetic emergency can make sodium look falsely low.³ Always tell your clinician what medications you take.

Recent research

According to recent PubMed publications:

• Diagnose and correct hyponatremia without rushing. Current reviews and treatment standards stress slow correction of chronic low sodium to avoid osmotic demyelination, with clearly bounded per-24-hour targets and fluid restriction as a first step.³⁴¹³
• SIADH: a large, underappreciated cause. Recent syntheses detail SIADH as a major driver of hyponatremia, particularly in older adults and with certain medications, and outline how it is managed.⁵¹¹
• Think in tonicity. The modern framing places both hyponatremia and hypernatremia within the management of body water, not salt.⁶
• A marker of severity in heart failure. Work confirms that hyponatremia in heart failure — driven by excess vasopressin (antidiuretic hormone) — signals a worse prognosis.¹⁴
• Exercise-associated hyponatremia. In endurance athletes, drinking too much (not too little) can cause a sometimes-dangerous low sodium: hydration should be appropriate, not maximal.¹²

These findings concern diagnosis and management; they do not justify self-medication or replace your physician's advice.

Get your sodium interpreted by AI DiagMe

A sodium is never read alone: its meaning depends on your hydration, your medications, your kidney function (creatinine), and the rest of your electrolyte panel (potassium, chloride). That cross-referencing is what gives the result its real value.

👉 AI DiagMe interprets your lab results — blood, urine, or stool — in plain language, taking your whole profile into account. An informational service that does not provide a diagnosis and complements, never replaces, your physician.

Frequently asked questions

Bottom line

Serum sodium is the electrolyte of water balance: its abnormalities almost always reflect too much or too little water, not too much or too little salt. Keep the range in mind (135–145 mmol/L, the same as mEq/L, lab-dependent), remember that hyponatremia is the most common electrolyte disorder and is corrected slowly, and that hypernatremia usually signals dehydration. You never correct a sodium on your own. And no single value is read in isolation — what matters is the full set of your markers and your profile, which is what AI DiagMe provides, alongside your physician.

Sources

Official sources and peer-reviewed publications (PubMed) used for this guide: