Sodium (Na)
A blood test that measures the concentration of sodium in the bloodstream, one of the most important electrolytes for maintaining fluid balance, nerve function, and muscle contractions.
Last updated: April 10, 2026
Sodium is the most abundant electrolyte in the blood and plays a vital role in nearly every body function. It helps regulate fluid balance, blood pressure, nerve signals, and muscle contractions. The body keeps blood sodium levels within a very tight range — normally 136–145 mEq/L (also expressed as mmol/L). Even small changes outside this range can cause noticeable symptoms and may indicate an underlying problem with fluid balance, kidney function, or hormonal regulation.
Why Is Sodium Tested?
Sodium testing is fundamental to evaluating electrolyte balance and fluid status. The kidneys, along with hormones like aldosterone and antidiuretic hormone (ADH), work constantly to keep sodium within the normal range by adjusting how much water and sodium are retained or excreted.
Sodium is a standard component of the Comprehensive Metabolic Panel (CMP) and the Basic Metabolic Panel (BMP). It is one of the most frequently ordered lab tests in both outpatient and hospital settings. Sodium imbalances can develop quickly during illness, medication changes, or surgery, making it an essential monitoring tool.
A healthcare provider may order a sodium test as part of routine blood work, to evaluate symptoms like confusion, nausea, headache, or fatigue, to monitor the effects of medications that affect fluid balance (especially diuretics), to assess patients with heart failure, kidney disease, or liver disease, or to investigate changes in blood pressure or swelling.
Normal Reference Ranges
Reference ranges may vary slightly by laboratory. Always use the ranges on your lab report. The mEq/L and mmol/L units are numerically identical for sodium.
| Category | Range | Unit |
|---|---|---|
| Adults | 136 – 145 | mEq/L (mmol/L) |
| Children | 136 – 145 | mEq/L (mmol/L) |
| Newborns | 133 – 146 | mEq/L (mmol/L) |
| Status | Sodium Level (mEq/L) |
|---|---|
| Normal | 136 – 145 |
| Borderline Low (Mild Hyponatremia) | 130 – 135 |
| Low (Hyponatremia) | Less than 130 |
| High (Hypernatremia) | Greater than 145 |
What Does a HIGH Sodium Level Mean?
A sodium level above 145 mEq/L is called hypernatremia and most often results from the body losing too much water relative to sodium, rather than consuming too much salt.
Possible causes of high sodium include:
- Dehydration — the most common cause; inadequate water intake, excessive sweating, fever, or prolonged vomiting/diarrhea can concentrate sodium in the blood
- Diabetes insipidus — a condition where the body produces large volumes of dilute urine, leading to water loss (not related to diabetes mellitus)
- Excessive sodium intake — rare from diet alone, but can occur from IV saline solutions or sodium bicarbonate administration in hospital settings
- Cushing syndrome — excess cortisol can cause the kidneys to retain sodium
- Primary hyperaldosteronism — overproduction of aldosterone, a hormone that tells the kidneys to hold onto sodium
- Inability to access water — infants, elderly individuals, and hospitalized patients who cannot drink water freely are at higher risk
- Osmotic diuresis — high blood sugar (as in uncontrolled diabetes) can cause excessive urination and water loss
Severity matters. Mild hypernatremia (146–150 mEq/L) may cause thirst and mild confusion. Severe hypernatremia (above 160 mEq/L) is a medical emergency that can cause seizures, coma, and brain damage. The rate at which sodium rises is also important — gradual changes are better tolerated than rapid ones.
What Does a LOW Sodium Level Mean?
A sodium level below 136 mEq/L is called hyponatremia and is the most common electrolyte abnormality seen in clinical practice. It usually means the body has too much water relative to sodium, rather than not enough sodium.
Possible causes of low sodium include:
- Medications — thiazide diuretics (water pills) are the most common drug cause; SSRIs, carbamazepine, and some pain medications can also lower sodium
- SIADH — syndrome of inappropriate antidiuretic hormone causes the body to retain too much water, diluting sodium (can be caused by lung disease, brain injury, certain cancers, or medications)
- Heart failure — fluid retention dilutes sodium in the blood
- Liver cirrhosis — fluid accumulation in advanced liver disease
- Kidney disease — impaired ability to excrete excess water
- Excessive water intake — sometimes called water intoxication; seen in endurance athletes, psychiatric conditions (psychogenic polydipsia), and certain social contexts
- Severe vomiting or diarrhea — loss of sodium-containing fluids
- Adrenal insufficiency (Addison disease) — reduced aldosterone causes the kidneys to lose sodium
- Hypothyroidism — severe thyroid underactivity can impair water excretion
Symptoms of low sodium range from mild (nausea, headache, fatigue) to severe (confusion, seizures, coma). Hyponatremia is the most common electrolyte disorder in hospitalized patients, occurring in up to 30% of inpatients. Mild chronic hyponatremia (130–135 mEq/L) may cause subtle symptoms like poor balance and increased fall risk, especially in older adults.
Factors That Can Affect Your Results
Because the body regulates sodium so tightly, most healthy people will have stable sodium levels regardless of dietary salt intake. However, several factors can shift the result.
- Hydration status: Both dehydration and overhydration are the most common non-disease causes of abnormal sodium. Dehydration concentrates sodium (making it appear high), while excess water dilutes it (making it appear low).
- Medications: Diuretics are the single most common medication cause of sodium abnormalities. Thiazide diuretics tend to lower sodium, while loop diuretics can have variable effects. SSRIs, ACE inhibitors, and certain anticonvulsants can also affect levels.
- Age: Older adults are more susceptible to sodium imbalances due to decreased kidney function, reduced thirst sensation, medication use, and concurrent illnesses.
- Exercise: Prolonged endurance exercise (marathon running, triathlons) combined with excessive water intake and sodium loss through sweat can cause exercise-associated hyponatremia, which can be life-threatening.
- Diet: Contrary to popular belief, eating salty food does not typically raise blood sodium in a meaningful way. The kidneys quickly adjust to maintain balance. However, very low sodium diets combined with high water intake can contribute to low sodium.
- Pregnancy: Sodium levels naturally decrease slightly during pregnancy due to increased blood volume. This mild decrease is expected and normal.
- Gender: Women, particularly postmenopausal women, are at higher risk for hyponatremia, partly due to hormonal effects on water balance and higher prevalence of diuretic use.
- Lab technique: Pseudohyponatremia (a falsely low sodium reading) can occur when blood has very high levels of lipids or proteins, which artificially dilute the measurement. Modern lab methods largely avoid this issue, but it can still occur.
Should I Be Concerned About My Sodium Level?
Sodium is one of the most tightly regulated substances in the body. In a healthy person eating a normal diet and drinking adequate fluids, sodium levels almost always fall within the normal range. A result of 135 or 146 mEq/L — just barely outside the reference range — is very common and often not clinically significant.
Context is everything with sodium. A sodium of 133 mEq/L in an older adult taking a thiazide diuretic has a very different meaning than the same level in a young marathon runner. A healthcare provider will consider the result alongside symptoms, medication list, fluid intake history, kidney function, and other electrolytes (especially potassium and chloride).
If sodium is mildly abnormal without symptoms, the next step is often simply repeating the test after adjusting fluid intake or reviewing medications. Significantly abnormal levels (below 125 or above 155 mEq/L) require prompt medical attention, as severe sodium imbalances can affect brain function.
Common Next Steps
Depending on the result, a healthcare provider may recommend:
- Urine sodium and osmolality — measures how much sodium the kidneys are excreting, which helps determine the cause of abnormal blood sodium
- Serum osmolality — measures the concentration of dissolved particles in the blood; helps distinguish true sodium abnormalities from pseudohyponatremia
- Potassium and chloride levels — other electrolytes that are closely related to sodium and are often affected together
- Kidney function tests — creatinine and BUN to assess whether kidney disease is contributing
- Thyroid and adrenal hormone tests — if SIADH, hypothyroidism, or adrenal insufficiency is suspected
- Medication review — adjusting or changing medications that may be affecting sodium balance, particularly diuretics
- Fluid restriction or adjustment — for low sodium caused by excess water intake; or increased water intake for high sodium from dehydration
- Repeat testing — to confirm the result and monitor trends
Frequently Asked Questions
What is a normal sodium level?
A normal blood sodium level is 136–145 mEq/L (which is the same as mmol/L for sodium). This is the standard reference range for adults and children. The body maintains sodium within this narrow range through a complex system involving the kidneys, hormones, and thirst mechanisms.
What is hyponatremia?
Hyponatremia means low blood sodium (below 136 mEq/L). It is the most common electrolyte disorder, especially in hospitalized patients and older adults. In most cases, hyponatremia is caused by excess water in the body rather than a lack of sodium. Mild hyponatremia may cause no symptoms, while severe cases (below 120 mEq/L) can cause confusion, seizures, and even be life-threatening.
What is hypernatremia?
Hypernatremia means high blood sodium (above 145 mEq/L). It is almost always caused by water deficit — the body losing or not taking in enough water. The most common cause is simple dehydration. It is particularly common in elderly individuals who may have a diminished thirst sensation, infants, and hospitalized patients. Symptoms include intense thirst, confusion, muscle twitching, and in severe cases, seizures.
Can drinking too much water lower sodium?
Yes. Drinking excessive amounts of water can dilute sodium in the bloodstream, a condition sometimes called water intoxication. Healthy kidneys can excrete about 800 mL to 1 liter of water per hour, so it takes a large amount of water consumed rapidly to overwhelm this capacity. This is most commonly seen in endurance athletes (marathon runners) who drink large volumes of water during races without replacing electrolytes, and in certain psychiatric conditions.
Does eating salty food raise blood sodium?
Not significantly, in most cases. The body has powerful mechanisms to keep blood sodium stable. When dietary sodium intake increases, the kidneys respond by retaining more water (which may raise blood pressure) and excreting the extra sodium in urine. This is why a high-salt diet is more strongly associated with high blood pressure than with high blood sodium. However, in people with impaired kidney function, the ability to regulate sodium may be reduced.
Which medications can affect sodium levels?
Several medication classes can lower sodium: thiazide diuretics (hydrochlorothiazide, chlorthalidone) are the most common cause; SSRIs (sertraline, fluoxetine); anticonvulsants (carbamazepine, oxcarbazepine); NSAIDs; and some diabetes medications. Medications that can raise sodium include corticosteroids and sodium-containing IV fluids. Diuretic-induced hyponatremia is one of the most common reasons for sodium abnormalities in outpatient settings.
What is SIADH?
SIADH stands for syndrome of inappropriate antidiuretic hormone. In this condition, the body produces too much antidiuretic hormone (ADH), which causes the kidneys to retain excess water. This dilutes sodium in the blood, causing hyponatremia. SIADH can be triggered by lung diseases (pneumonia, COPD), brain injuries or tumors, certain cancers (especially small cell lung cancer), pain, nausea, and some medications. It is one of the most common causes of hyponatremia in hospitalized patients.
Is sodium included in routine blood work?
Yes. Sodium is included in the Comprehensive Metabolic Panel (CMP), the Basic Metabolic Panel (BMP), and dedicated electrolyte panels. It is one of the most frequently ordered laboratory tests, tested during annual physicals, emergency room visits, hospital admissions, and whenever fluid or electrolyte status needs to be assessed.
What is the difference between mEq/L and mmol/L for sodium?
For sodium, mEq/L (milliequivalents per liter) and mmol/L (millimoles per liter) are numerically identical because sodium has a valence of 1. A sodium level of 140 mEq/L is the same as 140 mmol/L. The United States commonly uses mEq/L, while many other countries use mmol/L. The number is the same regardless of which unit is listed on the lab report.
How does sodium relate to potassium?
Sodium and potassium work together as opposing forces in the body. Sodium is the primary electrolyte outside cells, while potassium is the primary electrolyte inside cells. The sodium-potassium pump in cell membranes constantly moves sodium out and potassium in, which is essential for nerve signals and muscle contractions. Conditions that affect one electrolyte often affect the other, which is why they are almost always tested together.
Sources
- Adrogué HJ, Madias NE. Hyponatremia. N Engl J Med. 2000;342(21):1581-1589.
- Sterns RH. Disorders of Plasma Sodium. N Engl J Med. 2015;372(1):55-65.
- MedlinePlus. Sodium Blood Test. U.S. National Library of Medicine.
- Verbalis JG, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-42.
- National Kidney Foundation. Electrolytes. 2024.
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Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice. Reference ranges vary by laboratory, testing method, and individual factors. Always discuss your results with a qualified healthcare provider.