Hyponatraemia means abnormally low levels of sodium (salt) in the bloodstream or a deficiency of sodium in the body.

Alternative names for hyponatraemia

Low blood sodium; sodium deficiency; low sodium levels; hyponatremia

What is hyponatraemia?

Hyponatraemia means abnormally low levels of sodium in the bloodstream or a deficiency of sodium in the body.

What causes hyponatraemia?

Hyponatraemia is a deficiency of sodium in the blood. The type of hyponatraemia that an individual has can be categorised according to the water volume levels ('hydration status') in particular parts of the body:

  1. Dehydration (hypovolaemic hyponatraemia) – this means that a person will be in a dehydrated condition, with low water levels in the body. It is caused by two mechanisms:
    • Loss of sodium and water through the kidneys as a result of:
      • Addison’s disease
      • kidney problems
      • the use of diuretics (particularly thiazide diuretics such as bendroflumethiazide).
    • Loss of sodium and water through other sources such as:
      • diarrhoea and vomiting
      • pancreatitis
      • trauma
      • burns
      • fistulas.
  2. Fluid overload (hypervolaemic hyponatraemia) – in this case, the patient will have swelling of the arms and legs due to too much water in the body. The causes can include:
    • cardiac failure
    • cirrhosis of the liver
    • nephrotic syndrome.
  3. Normal hydration status ('euvolaemic hyponatraemia) – this means water levels in the body are normal so the condition can be caused by:
    • syndrome of inappropriate secretion of anti-diuretic hormone. This is due to the secretion of too much anti-diuretic hormone, the hormone which is responsible for managing the excretion of fluid and salt from the body. Syndrome of inappropriate secretion of anti-diuretic hormone can be caused by a number of medications taken to treat other conditions. It can also be the result of chest infection (pneumonia) or cerebral disorders such as meningitis
    • water overload, which can be caused by drinking excessive amounts of water
    • a severely underactive thyroid gland (hypothyroidism). The reason for this is unknown.

What are the signs and symptoms of hyponatraemia?

The symptoms of hyponatraemia depend on the severity of the condition. In mild cases, there are usually no symptoms but, as the severity increases, patients may experience headache, nausea, vomiting, lack of energy and anorexia. More serious hyponatraemia can lead to confusion, muscle cramps and weakness, problems with gait and personality changes.

In severe cases, the main symptoms are seizures and/or drowsiness. However, there are other signs that could lead to a diagnosis of hyponatraemia. These are generally:

  • neurological symptoms, including reduced level of consciousness, fits, difficulty in breathing and coma
  • dehydration-related symptoms, including dry mucous membranes and reduced elasticity of the skin
  • over-hydration-related symptoms, such as swelling of the arms, legs, swelling of the abdomen and breathlessness.

How common is hyponatraemia?

Hyponatraemia is the most common electrolyte condition and is most often seen in elderly, female and hospitalised patients. Estimates of how common it is do vary. It is thought that approximately 3–5% of all hospitalised patients, and 30% of elderly patients have severe hyponatraemia. Hyponatraemia is common in patients taking certain medciations including thiazide diuretics, and drugs that cause syndrome of inappropriate secretion of anti-diuretic hormone, such as anti-epileptics and some antidepressants.

Is hyponatraemia inherited?

Hyponatraemia is not itself inherited. However, some conditions with a genetic component can be associated with hyponatraemia. Because there are so many different causes of hyponatraemia, it is important to consider all causes in patients whatever the underlying chronic illness.

How is hyponatraemia diagnosed?

In the community, hyponatraemia is usually picked up by a routine blood test. This test is usually may be carried out annually by the GP if a patient is on any medication that could affect kidney function. In hospital, almost every patient has a blood test on admission.

Once hyponatraemia is diagnosed, there is a need for other tests to identify causes of hyponatraemia. These would be glucose, a cholesterol profile, cortisol and thyroid function tests. A very high glucose or cholesterol can make sodium levels appear low. Other specialised tests to establish the cause of low sodium may also be carried out, such as a serum and urine test for osmolality and to establish the amount of sodium being passed in the urine. Chest x-rays should be taken for patients showing symptoms of a cough and loss of weight to exclude lung cancer, as this is a common cause of syndrome of inappropriate secretion of anti-diuretic hormone.

How is hyponatraemia treated?

The treatment given will depend on the underlying cause. In cases of mild hyponatraemia, where the patient has no symptoms, or very minimal symptoms, treatment can be managed as an outpatient. This can include:

  1. Stopping any medication that may be responsible for the condition such as diuretics, antidepressants or anti-epileptics. Sodium levels are then rechecked regularly and, if levels do not rise, further investigations need to be started.
  2. Starting specific treatment if blood glucose or cholesterol levels are high.
  3. Restricting daily fluid intake to 1.0–1.5 litres if necessary, if the diagnosis is syndrome of inappropriate secretion of anti-diuretic hormone. Usually the sodium will correct when the underlying cause of syndrome of inappropriate secretion of anti-diuretic hormone is treated.
  4. Administering a medication called demeclocycline if sodium levels do not improve after the above action has been taken, although this drug is not commonly used and is usually supervised by a specialist.  

Where hyponatraemia is more serious, patients will usually be admitted to hospital. Treatment will include:

  1. Initially, controlled fluid replacement if dehydration is diagnosed.
  2. Stopping any medication that may be responsible for the condition.
  3. Assessing the amount of fluid a patient has. If fluid overloaded is diagnosed, the fluid and salt intake may be restricted and the relevant specialist may be consulted depending on whether overload is due to heart, kidney or liver problems.
  4. Patients are often given diuretics to offload the excess fluid, and this requires close monitoring of sodium levels.
  5. A new class of drugs called ‘vaptans’ may rarely be given in syndrome of inappropriate secretion of anti-diuretic hormone patients with lung cancer, although this requires a specialist to prescribe them.

In the most severe cases of hyponatraemia, the majority of patients will show neurological symptoms and will require admission to a hospital high dependency unit. Correction of sodium levels will be carefully controlled. 

Are there any side-effects to the treatment?

The majority of patients with hyponatraemia do not experience side-effects to treatment. However, a small minority may experience:

  1. A rare adverse reaction in the brain causing reduced conscious level and a ‘locked-in’ syndrome. This is called osmotic demyelination syndrome (ODS), previously called central pontine myelinosis. It refers to loss of the lining of the nerve fibres (‘demyelination’) in the brain stem, if hyponatraemia is corrected too rapidly.
  2. Drug-specific side effects – demecolycline can cause nausea, vomiting, gastritis, skin rashes and blood-related disorders such as anaemia and low platelet count. The vaptans may cause a dry mouth and thirst and/or increased urinary frequency, and may also give rise to over-rapid correction of sodium, so require close monitoring of blood sodium levels.

If the patient has any concerns about these side-effects, they should speak to their doctor.

What are the longer-term implications of hyponatraemia?

The longer-term implications of hyponatraemia depend on the underlying cause of the condition and how severe the hyponatraemia is.

In cases of chronic or long-term hyponatraemia, neurological impairment such as confusion or altered attention can result. It can affect the gait, which can lead to trips and falls, and recent studies indicate that chronic hyponatraemia can also cause osteoporosis.

Last reviewed: Feb 2017