Low sodium (hyponatraemia) in a blood test: what it means and what happens next

Lab Tests Online UK describes sodium as an essential electrolyte present in all body fluids and vital to normal body function, including nerve and muscle function. The body keeps blood sodium within a very small range by adjusting hormones that change sodium loss in urine, a hormone (antidiuretic hormone, also called vasopressin) that prevents water loss, and by thirst. An abnormal blood sodium is usually due to some problem with one of these systems.

A low result is hyponatraemia. Importantly, the number mainly reflects the balance of sodium and water. Lab Tests Online UK says that when the concentration of sodium in the blood changes the water content of the body changes, and these changes can be associated with dehydration, with oedema (too much fluid, often leading to swollen legs) and with changes in blood pressure.

Lab Tests Online UK says hyponatraemia is rarely due to decreased sodium intake. Most commonly it is due to sodium loss, for example through diarrhoea, vomiting, excessive sweating, diuretic medicines, kidney disease or Addison's disease. In some cases it is due to excess fluid in the body, such as drinking too much water, heart failure, cirrhosis, kidney diseases that cause protein loss (nephrotic syndrome) and malnutrition.

It also notes that in a number of diseases, particularly those involving the brain and the lungs, many kinds of cancer and some drugs, the body makes too much antidiuretic hormone, causing it to keep too much water. Doctors may describe this pattern as SIADH. The NHS describes Addison's disease as a condition in which the adrenal glands do not produce enough hormones; its symptoms include extreme tiredness, dizziness on standing, loss of appetite, weight loss, tummy pain and cravings for salty foods.

• Medicines: diuretics, sulphonylureas, ACE inhibitors, NSAIDs, carbamazepine, tricyclic antidepressants, heparin and others (Lab Tests Online UK).

• Fluid and salt losses: vomiting, diarrhoea, heavy sweating.

• Fluid retention: heart failure, cirrhosis, nephrotic syndrome.

• Hormonal: Addison's disease, and conditions that make the body hold on to water through antidiuretic hormone.

• Drinking very large volumes of water.

• Kidney disease.

Lab Tests Online UK says that if sodium falls quickly you may feel weak and tired, and in severe cases may experience confusion or even fall into a coma, whereas a slow fall may cause no symptoms. In practice, people with a mild low sodium may notice nothing, or may feel off colour, nauseated, headachy or unsteady. Confusion, drowsiness, seizures or collapse are emergency symptoms.

Because symptoms are non-specific and can reflect the underlying illness as well as the sodium itself, a low sodium that comes with vomiting, diarrhoea or being generally unwell should be assessed rather than waited out.

Lab Tests Online UK says people who have diarrhoea, vomiting, kidney disease, Addison's disease, adrenocortical disorder or congestive heart failure may have low sodium. Older people are more likely to be taking several of the medicines linked to low sodium, which is one reason the result is so often found on routine blood tests. The NHS lists diuretics (water tablets) among medicines that make you pee more and can contribute to dehydration.

The sodium blood test is a blood sample from a vein in the arm and needs no preparation. Lab Tests Online UK says a urine sodium test is typically used in patients with abnormal blood sodium to help determine whether an imbalance is due to taking in too much sodium or losing too much. The osmolality of blood or urine may also be checked. The clinician will look at your medicines, fluid status, other electrolytes such as potassium, kidney function and sometimes hormone tests.

Lab Tests Online UK also advises checking with your doctor if you have concerns about drugs or supplements you take and their effect on blood results. Do not stop prescribed medicines on your own; a prescriber can decide whether a change is needed.

Treatment depends entirely on the cause and the speed and severity of the fall, so it is not a matter of simply adding salt. If the cause is a medicine, the prescriber may change the dose or choose another medicine. If the cause is fluid overload, fluid intake may be limited. If it is dehydration from vomiting or diarrhoea, the NHS says to drink fluids and that a pharmacist can recommend oral rehydration solutions. Severe or symptomatic hyponatraemia is treated in hospital with carefully monitored fluids, because correcting sodium too quickly can itself be harmful.

Call 999 or go to A&E for confusion, severe drowsiness, a fit or collapse. Ask for an urgent GP appointment or call NHS 111 if you have a low sodium result and are vomiting repeatedly, feel increasingly weak or unsteady, or are unusually tired and dizzy. The NHS dehydration advice also lists being confused or sleepier than normal or difficult to wake as reasons to call 999 or go to A&E.

Sodium is measured within routine NHS blood panels such as urea and electrolytes. If a private health check shows a low sodium, share the result with your GP, because low sodium is more often a sign of an underlying cause or medicine effect than a problem in its own right, and the follow-up usually needs your medical record.