Sodium

What is sodium?

Sodium is an electrolyte which is found in all body fluids and is crucial for normal body function, including nerve and muscle function.

Sodium, together with other electrolytes such as potassium and calcium, helps the cells to work normally and helps to regulate the amount of fluid in the body. Although sodium is present in all body fluids, the highest sodium concentration is found in the blood. All sodium is regulated by the kidneys.

Most people get sufficient sodium through their diets, e.g. table salt (sodium chloride) and, to an extent, the foods we eat. Hence most people have a sufficient sodium intake. The body uses what it needs and the kidneys eliminate the rest of the sodium in the urine.

The body attempts to maintain the sodium concentration in the blood within a very narrow range and regulates this by:

  • Producing hormones which can increase or reduce the amount of sodium excreted in the urine
  • Producing a hormone that prevents dehydration
  • Regulating thirst: even a 1 per cent increase in the sodium level can trigger thirst, causing us to drink in order to achieve a normal sodium level.

Sodium levels outside the reference values are caused by problems with these systems. When the sodium level changes, it also causes a change in the body water level. These changes are often associated with too little or too much fluid, which may result in swelling of the legs.

Why should you analyse sodium?

A sodium blood test is carried out in order to check sodium levels. The sodium test is often carried out as part of an electrolyte panel or a basic kidney function examination.

Sodium is an electrolyte which is present in all body fluids and is crucial for normal body function, including nerve and muscle function. It helps the cells to work normally and helps to regulate the amount of fluid in the body.

Sodium tests are also carried out in order to locate problems and/or help to monitor treatments in individuals suffering from dehydration or excess fluid or who are experiencing various symptoms (such as weakness, confusion, thirst and/or dry mucous membranes). The sodium level in the blood may turn out to be abnormal in many diseases. The sodium test, together with other electrolytes, identifies an electrolyte imbalance or whether there are symptoms involving the brain, lungs, liver, heart, kidneys, thyroid gland or adrenal glands.

In people with a known electrolyte imbalance, the sodium test can be used to monitor the efficacy of a treatment or monitor medication that can affect sodium levels.

Sodium levels in the urine can be tested in people with abnormal sodium levels in order to determine the cause of the imbalance, e.g. consuming too much sodium or losing too much sodium. Urine sodium tests are also carried out on people with test results outside the reference values in order to establish the cause of kidney disease and/or provide guidance on potential treatment.

High sodium levels

A high level of sodium is almost always linked with dehydration. In rare cases, it may be due to an increase in salt intake without having compensated with sufficient water to dilute the excess sodium.

It may be wise to combine the evaluation of sodium levels in the urine with the sodium level in the blood. The body normally eliminates excess sodium in the urine, and so the sodium concentration in the urine may increase if there is excess sodium in the blood. It is also possible to have an elevated sodium level in the urine when the body has lost too much sodium – and in this case the sodium level in the blood would appear to be low to normal. If the sodium levels in the blood are low due to insufficient intake, the sodium level in the urine will also be low.

Some medications such as anabolic steroids, antibiotics, laxatives, cough medicines and the contraceptive pill may increase sodium levels.

Low sodium levels

Low sodium levels are rarely due to a reduction in sodium intake. A low level of sodium may be due to:

  • Losing too much sodium, usually because of problems such as diarrhoea, vomiting, excessive sweating, kidney disease or low levels of cortisol and sex hormones
  • Drinking too much water, e.g. when exercising too hard
  • Excess fluid in the body caused by heart failure and kidney disease, for example, causing protein loss or malnutrition. A number of diseases – particularly those involving the brain and lungs and many types of cancer – and some medications cause the body to produce too much antidiuretic hormone (ADH), which causes excess water to be retained in the body.

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