⚠️ Educational only. LabPlain does not provide medical advice or diagnosis. Always discuss your specific results with your healthcare provider.
What this test measures
Sodium is the most abundant electrolyte in your blood and the fluid surrounding your cells. It plays a central role in controlling the amount of water in your body, maintaining blood pressure, and enabling your nerves and muscles — including your heart — to fire properly.
Your kidneys are the primary regulators of sodium, constantly filtering and reabsorbing it in response to hormones like aldosterone and ADH (antidiuretic hormone). When this balance is disrupted — by illness, dehydration, medications, or kidney disease — sodium levels fall outside the normal range.
A sodium test is almost always included in a Basic Metabolic Panel (BMP) or Comprehensive Metabolic Panel (CMP), which means most people get their sodium checked routinely at annual physicals or during hospital stays.
Normal reference range
135–145 mEq/L (or mmol/L)
This range applies to adults. Most labs use the same reference range, though the units may appear as mEq/L or mmol/L — these are numerically identical for sodium.
Sodium outside this range — especially below 125 or above 155 mEq/L — is considered a medical emergency. Even modest deviations (below 130 or above 150) can cause serious symptoms and usually require prompt medical attention.
💧 Hydration affects your result
Sodium is measured relative to the water in your blood. Drinking an unusually large amount of water before your draw can dilute sodium and push it slightly low. Dehydration from not drinking enough — or from sweating, vomiting, or diarrhea — can push it slightly high. If your result is borderline, your doctor may repeat the test under more controlled conditions.
What your result might indicate
↑ If High (Hypernatremia)
High sodium (above 145 mEq/L) usually means the body has too little water relative to sodium — from dehydration, excessive sweating, diabetes insipidus, or not drinking enough fluids. It can also result from consuming too much sodium, though this is less common.
↓ If Low (Hyponatremia)
Low sodium (below 135 mEq/L) is the most common electrolyte disorder. It can result from drinking too much water, heart failure, cirrhosis, kidney disease, hypothyroidism, Addison's disease, certain medications (especially diuretics and antidepressants), or SIADH — a condition where the body retains too much water.
Symptoms associated with abnormal sodium
↑ High Sodium Symptoms
Intense thirst
Dry mouth and sticky saliva
Dark-colored urine
Confusion or restlessness
Muscle twitching or spasms
Rapid heartbeat
Seizures (severe cases)
Coma (extreme cases)
↓ Low Sodium Symptoms
Headache
Nausea and vomiting
Fatigue and low energy
Confusion or difficulty concentrating
Muscle cramps or weakness
Irritability or mood changes
Seizures (severe cases)
Loss of consciousness (extreme cases)
Common causes of abnormal sodium
What causes high sodium?
The most common cause of elevated sodium is dehydration — when you lose more water than you take in through sweating, fever, vomiting, diarrhea, or simply not drinking enough. Diabetes insipidus (a condition affecting ADH that is separate from diabetes mellitus) causes the kidneys to excrete too much water, concentrating the sodium that remains. In hospitalized patients, receiving excessive sodium through IV fluids is another cause.
Rarely, high sodium results from a brain or kidney problem that disrupts the hormonal signals controlling fluid balance.
What causes low sodium?
Low sodium is more common and has more varied causes. Drinking excessive amounts of water (overhydration) dilutes sodium in the blood. Heart failure, cirrhosis, and kidney disease cause the body to retain water, diluting sodium. Diuretic medications (water pills) cause the kidneys to excrete sodium along with water. SIADH (syndrome of inappropriate antidiuretic hormone secretion) causes the kidneys to hold onto too much water. Hypothyroidism and adrenal insufficiency (Addison's disease) can also cause low sodium by disrupting the hormones that regulate fluid balance.
Tests often ordered alongside sodium
Sodium is almost always interpreted alongside other electrolytes and kidney markers. Your doctor may also review:
Potassium — the other major intracellular electrolyte; often imbalanced alongside sodium
Chloride and bicarbonate (CO2) — complete the electrolyte picture and help evaluate acid-base balance
BUN and creatinine — assess kidney function, which directly controls sodium regulation
Glucose — very high blood sugar can cause a falsely low sodium reading (pseudohyponatremia)
Urine sodium and osmolality — help determine whether low sodium is due to excess water or true sodium loss
TSH — hypothyroidism is a reversible cause of low sodium
What to do next
A mildly abnormal sodium result is common and doesn't automatically signal a serious problem — but it always deserves a conversation with your doctor. The cause matters as much as the number: low sodium from drinking too much water before your draw is very different from low sodium caused by heart failure. Your doctor will interpret your result alongside your symptoms, medications, kidney function tests, and hydration status before deciding on next steps.
Questions to ask your doctor
01Is my sodium result clinically significant, or is it a mild variation that just needs monitoring?
02Could any of my current medications — especially diuretics or antidepressants — be affecting my sodium?
03Do I need to check my urine sodium or osmolality to figure out the underlying cause?
04Should I change how much water I'm drinking, or adjust my salt intake?
05Could a thyroid or adrenal problem be contributing to this result?
06How quickly does this need to be corrected, and what's the plan?
Frequently asked questions
Can eating too much salt cause high sodium on a blood test?
In healthy people with normal kidneys, no — the kidneys are very efficient at excreting excess sodium, so a salty meal alone rarely raises blood sodium. High dietary sodium is more likely to raise blood pressure than blood sodium levels. Elevated blood sodium usually points to a water deficit (dehydration) rather than excess salt intake.
Is low sodium dangerous?
It depends on the level and how quickly it developed. Mild, chronic low sodium (130–134 mEq/L) may cause subtle symptoms like fatigue and brain fog. Severe low sodium (below 125 mEq/L) or a rapid drop can cause brain swelling, seizures, and is a medical emergency. Correction must be done carefully — correcting sodium too quickly can itself cause serious neurological damage.
Why do endurance athletes sometimes have low sodium?
This is called exercise-associated hyponatremia, and it happens when athletes — especially marathon runners — drink large volumes of plain water without replacing the sodium lost in sweat. The excess water dilutes blood sodium. Sports drinks containing electrolytes help prevent this, which is why they're recommended for prolonged exercise rather than plain water alone.
What is SIADH and why does it cause low sodium?
SIADH stands for syndrome of inappropriate antidiuretic hormone secretion. ADH is the hormone that tells your kidneys to hold onto water. In SIADH, ADH is released when it shouldn't be — often due to lung disease, brain injury, certain cancers, or medications — causing the kidneys to retain too much water, which dilutes sodium in the blood. Treatment focuses on fluid restriction and addressing the underlying cause.
Can low sodium cause falls in older adults?
Yes — this is a well-documented and often underappreciated risk. Even mildly low sodium (130–134 mEq/L) can cause subtle impairments in balance, attention, and reaction time that significantly increase fall risk in older adults, even without obvious symptoms. This is one reason doctors take even borderline low sodium more seriously in elderly patients.