The Sodium (Serum) test is a common blood test that measures the amount of sodium (chemical symbol Na) in the liquid portion of your blood. Sodium is one of the body’s main electrolytes—minerals that carry an electrical charge and help regulate many essential functions.

Sodium plays a key role in:

• Maintaining the right fluid (water) balance inside and outside your cells
• Supporting nerve signaling and muscle contraction (including the heart)
• Helping control blood pressure and overall circulation in combination with other electrolytes and hormones

Your serum sodium level is tightly regulated by your kidneys and hormones (such as aldosterone and antidiuretic hormone/ADH). Results are usually interpreted as:

Low sodium (hyponatremia): This often means there is too much water relative to sodium in the bloodstream (dilution) or that the body is losing sodium. Depending on how low it is and how quickly it changed, it may cause symptoms such as headache, nausea, confusion, unsteadiness, or—in severe cases—seizures.

High sodium (hypernatremia): This usually means there is too little water relative to sodium (dehydration) or too much sodium intake/retention. It can cause thirst, weakness, irritability, confusion, and—in severe cases—serious neurologic symptoms.

Sodium is often checked as part of a Basic Metabolic Panel (BMP) or Comprehensive Metabolic Panel (CMP) and interpreted along with other electrolytes such as potassium, chloride, and bicarbonate (CO2), as well as kidney function tests.

Doctors commonly order a serum sodium test to evaluate electrolyte balance, hydration status, and kidney or hormone-related causes of symptoms. It may be ordered for general screening, to investigate symptoms, or to monitor treatment.

It’s often done if you have symptoms that could be related to an electrolyte imbalance, such as:

• Ongoing nausea, vomiting, or diarrhea
• Excessive thirst or signs of dehydration (dry mouth, low urine output, dizziness)
• Swelling/edema (puffy legs, abdominal swelling) or sudden weight changes from fluid shifts
• Muscle cramps, weakness, twitching, or unusual fatigue
• Headache, confusion, trouble concentrating, drowsiness, unsteadiness, or fainting
• Severe symptoms such as seizures or decreased responsiveness (urgent evaluation)

It may also be ordered if you have, or are at risk for, conditions that affect fluid and sodium balance, including:

• Kidney disease or changes in urination
• Heart failure or fluid overload
• Liver disease (such as cirrhosis with fluid retention/ascites)
• Endocrine (hormone) disorders affecting ADH, aldosterone, cortisol, or thyroid function

Monitoring serum sodium is especially important if you:

• Take medications that can shift sodium/water balance (for example, diuretics/water pills, certain antidepressants like SSRIs, anti-seizure medicines, or desmopressin)
• Are receiving IV fluids or are hospitalized/critically ill
• Have recently had surgery, significant infection, or major illness
• Are older, have limited access to fluids, or have conditions that reduce the ability to sense thirst

• Hyponatremia (low blood sodium)
• Hypernatremia (high blood sodium)
• Dehydration from poor intake, vomiting/diarrhea, fever, or excessive sweating
• Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
• Heart failure (often associated with dilutional hyponatremia)
• Chronic kidney disease and other kidney disorders affecting fluid/electrolyte handling
• Cirrhosis and advanced liver disease with fluid retention (ascites)
• Adrenal insufficiency (Addison’s disease) and other disorders of aldosterone/cortisol
• Diabetes insipidus (water balance disorder that can lead to high sodium)
• Thyroid disease (hypothyroidism can contribute to low sodium in some cases)

• Checking hydration status and preventing complications from dehydration or overhydration
• Electrolyte monitoring as part of routine labs (BMP/CMP) for general wellness or annual physicals
• Kidney health monitoring, especially when kidney function is reduced or changing
• Medication safety monitoring (e.g., diuretics or other drugs that can affect sodium and water balance)
• Managing heart failure and fluid balance to support cardiovascular health
• Monitoring liver disease and fluid retention/ascites management
• Evaluating hormone-related causes of fatigue, confusion, or abnormal fluid balance (adrenal, thyroid, ADH-related)
• Supporting safe endurance training and recovery planning when there is risk of exercise-associated hyponatremia (in appropriate clinical contexts)