Blood sodium is used to detect the cause and help monitor treatment in persons with dehydration, oedema, or with a variety of symptoms. Blood sodium is abnormal in many diseases; your doctor may request this test if you have symptoms of illness involving the brain, lungs, liver, heart, kidney, thyroid, or adrenal glands.

Urine sodium levels are typically tested in patients who have abnormal blood sodium levels, to help determine whether an imbalance is due to taking in too much sodium or losing too much sodium. Urine sodium is also used to see if a person with high blood pressure is eating too much salt. It is often used in persons with abnormal kidney tests to help the doctor determine the cause of kidney disease, which can help guide treatment.

This test is a part of the routine laboratory evaluation of most patients. It is one of the blood electrolytes, which are often requested as a group, when someone has non-specific health complaints. It is also tested when monitoring treatment involving intravenous (IV) fluids or when there is a possibility of developing dehydration. Electrolytes are also commonly used to monitor treatment of certain problems, including high blood pressure, heart failure, and liver and kidney disease.

A low level of blood sodium is called hyponatraemia, and is usually due to either too much sodium loss, too much water intake or retention, or fluid accumulation in the body (oedema). If sodium falls quickly, you may feel weak and tired; in severe cases, you may experience confusion or even fall into a coma. When sodium falls slowly, however, there may be no symptoms. That is why sodium levels are often checked even if you don't have any symptoms.

Hyponatraemia is rarely due to decreased sodium intake (deficient dietary intake or deficient sodium in IV fluids). Most commonly, it is due to sodium loss (diarrhoea, vomiting, excessive sweating, diuretic administration, kidney disease or Addison's disease). In some cases, it is due to excess fluids in the body (drinking too much water, heart failure, cirrhosis, kidney diseases that cause protein loss [nephrotic syndrome]) and malnutrition. In a number of diseases (particularly those involving the brain and the lungs, many kinds of cancer, and with some drugs), your body makes too much anti-diuretic hormone, causing you to keep too much water in your body.

A high blood sodium level is referred to as hypernatraemia, and is almost always due to excessive loss of water (dehydration) without enough water intake. Symptoms include dry mucous membranes (mouth, eyes etc.), thirst, agitation, restlessness, acting irrationally, and coma or convulsions if levels rise extremely high. In rare cases, hypernatraemia may be due to increased salt intake without enough water, Cushing's syndrome, or too little anti-diuretic hormone (called diabetes insipidus).

Recent trauma, surgery, or shock may increase sodium levels because blood flow to the kidneys is decreased.

Drugs such as lithium and anabolic steroids may increase sodium levels; this is uncommon with most other drugs.

Drugs such as diuretics, sulphonylureas (used to treat diabetes), ACE inhibitors (such as captopril), heparin, NSAIDs (such as ibuprofen), tricyclic antidepressants, and vasopressin, among others, can decrease sodium levels.

Check with your doctor if you have any concerns about drugs you are taking and their effect on your body.