Sodium

What is sodium?

Sodium is the most abundant electrolyte in the blood and in the fluid outside the cells. It plays a key role in regulating the amount of water in the body, maintaining blood volume and blood pressure, and keeping nerve and muscle cells working. Most of our daily sodium comes from table salt and processed foods, and the kidneys excrete any excess in the urine. In situations such as sweating, diarrhea and vomiting, the body loses sodium along with fluid.

The sodium level in the blood reflects how much that sodium is diluted by water rather than the total amount of sodium in the body; this is why the result often tells us about water balance. This balance is maintained by the sense of thirst, the kidneys, antidiuretic hormone, which regulates urine volume, and aldosterone, which is secreted by the adrenal glands. Sodium is measured in a blood sample taken from the arm using ion-selective electrodes and is usually ordered together with potassium and chloride as an electrolyte panel.

When is a sodium test ordered?

Sodium is a core part of routine biochemistry tests and of the electrolyte panel. It is monitored regularly, especially in people who have conditions that can affect fluid balance or who take medications that can alter this balance. It is also one of the tests most often checked when older adults have unexplained weakness, inattentiveness or falls. It may also be ordered as a check after a hospital stay or shortly after starting a new medication.

• Headache, nausea, weakness, confusion or inattentiveness
• Fluid loss after vomiting, diarrhea or heavy sweating
• Marked thirst or an unexplained change in urine output
• Monitoring of diuretics and other medications that can affect sodium balance
• Heart failure, kidney or liver disease
• Investigation of hormonal disorders and diseases of the adrenal glands

How do you prepare for a sodium test?

No special fasting is needed before a sodium measurement; your blood can be drawn in the morning or afternoon, at a time that suits you. Simply keep to your usual eating and drinking habits before the test; drinking a lot of water in a short time or deliberately restricting fluids can change the result. If other fasting tests have also been ordered, you need to follow your doctor's fasting instructions.

Diuretics, some antidepressants, epilepsy medications and blood pressure medications can affect sodium levels. If you take any of these medications, tell your doctor; only your doctor should decide on changing the dose or stopping a medication. Only a small amount of blood is needed for the sodium measurement, and if you wish, a nurse can take the sample at your home or workplace.

What does a sodium result mean?

Low sodium, or hyponatremia, usually indicates that there is too much water in the body relative to sodium. The main causes are diuretics, heart failure, liver cirrhosis, kidney disease, prolonged vomiting and diarrhea, some hormonal disorders and drinking excessive amounts of water in a short time. A mild decrease may cause no symptoms, whereas a marked decrease can lead to headache, nausea, confusion and, in severe cases, seizures. In older adults, even mild decreases may be associated with poor concentration and an increased risk of falls.

High sodium, or hypernatremia, mostly develops when the body loses water or cannot take in enough fluid, and it can be seen particularly in older adults with a reduced sense of thirst. How quickly the change in sodium level develops also matters, and correction must always be done slowly, under a doctor's supervision. Although the range considered normal for sodium is narrow, it may vary slightly by laboratory, age and sex; the range of the laboratory that runs your test is shown on this page. Your doctor will interpret the result together with your symptoms, fluid status, medications and other test results.