Sodium blood test: normal range and results (Na)

Sodium is the most important ion of blood plasma and extracellular fluid: it determines how much water is held in the blood vessels and around the cells, so it directly affects blood volume and blood pressure, and together with potassium it affects nerve impulses and muscle contraction. The sodium concentration is regulated by the kidneys, the thirst mechanism and hormones — antidiuretic hormone (vasopressin) and aldosterone. Because of this, a change in sodium most often means a water problem rather than a salt problem: hyponatraemia (low sodium) arises when the body retains too much water or loses fluids together with salts through vomiting and diarrhoea. Hypernatraemia arises when water is lost or too little is drunk. Sodium is tested as part of an electrolyte test together with potassium, chloride and creatinine. Severe deviations affect the brain — they cause confusion and seizures — and are therefore assessed urgently.

• As part of an electrolyte test — as a check-up, before an operation, or with a chronic illness
• With dehydration, or prolonged vomiting or diarrhoea
• With confusion, weakness or seizures — especially in older age
• While taking diuretics, certain antidepressants or blood pressure medicines — for monitoring
• With kidney, heart, liver, adrenal or thyroid disease

Sodium counts as raised when the result is above the upper limit of the adult range — 145 mmol/l according to the MedlinePlus medical encyclopedia, and laboratory intervals differ slightly. Raised sodium (hypernatraemia) most often means a lack of water: too little is drunk, fluids are lost with fever, diarrhoea or sweating, or thirst is impaired in older age. Other causes are diabetes insipidus, adrenal disorders, too much sodium, and certain medicines. The causes listed are:

• Too little fluid — impaired thirst, being unable to drink
• Large fluid loss — fever, diarrhoea, sweating
• Diabetes insipidus
• Adrenal disorders
• Too much sodium from food or from infusion fluids
• Certain medicines

Symptoms that can occur:

• Thirst
• Weakness, irritability
• Confusion, drowsiness
• In severe cases — seizures

What to discuss with a doctor:

• How much and what you drink, and whether there has been any fluid loss
• Creatinine and GFR — kidney function
• Urine tests, if diabetes insipidus is suspected
• The medicines you take

Sodium counts as low when the result is below the lower limit of the adult range — 135 mmol/l according to the MedlinePlus medical encyclopedia, and laboratory intervals differ slightly. Low sodium (hyponatraemia) is the most common electrolyte disorder. The causes are too much water in the body (heart, liver and kidney disease, inappropriate antidiuretic hormone secretion), fluid loss through vomiting or diarrhoea, adrenal insufficiency, hypothyroidism, diuretics and certain antidepressants. Very high glucose or triglycerides can make sodium appear falsely low. The causes listed are:

• Too much water in the body — heart failure, liver cirrhosis, kidney disease
• Inappropriate secretion of antidiuretic hormone (vasopressin)
• Fluid loss — vomiting, diarrhoea
• Adrenal insufficiency
• Hypothyroidism
• Medicines — diuretics, certain antidepressants
• Very high glucose or triglycerides — falsely low sodium
• Ketonuria

Symptoms that can occur:

• Nausea, headache
• Weakness, tiredness
• Confusion, irritability
• In severe cases — seizures, disturbed consciousness

What to discuss with a doctor:

• Whether the sodium fell quickly or gradually — the urgency depends on that
• Fluid balance — how much is being drunk, whether there is any swelling
• Medicines — especially diuretics and antidepressants in older age
• Glucose, thyroid (TSH) and adrenal (cortisol) markers, and urine sodium — to tell the causes apart

Related tests

Sodium is tested as part of an electrolyte test together with potassium, chloride and creatinine. Depending on the cause suspected, a doctor may add glucose, thyroid and adrenal markers. Related pages: potassium, chloride, creatinine, glucose, TSH, cortisol.