Sodium

This is the harmonised UK range; some labs still print 135 to 145, and the range is a little lower in pregnancy.

Sodium is the main salt dissolved in the fluid outside the body's cells. Its level in the blood is controlled mainly by how much water the body holds on to, rather than by how much salt is eaten. The test measures the concentration of sodium in a blood sample.

It is part of the routine 'U&E' test and is checked whenever someone is unwell, on water tablets, or has symptoms such as confusion, tiredness or muscle cramps. It is one of the most commonly measured tests in hospital.

• Dehydration, where water is lost faster than salt
• Not drinking enough, especially in older age or hot weather
• Uncommon hormone conditions that make the kidneys lose water
• Some laxatives and water tablets

• Water tablets (thiazide diuretics)
• Drinking a very large amount of water, including during endurance events
• Some antidepressants, epilepsy medicines and painkillers
• Vomiting, diarrhoea or sweating with water replaced but not salt
• Heart, liver and kidney conditions where fluid builds up
• Hormone conditions affecting the adrenal or pituitary glands

• High blood fats or very high protein levels can make the sodium read falsely low on some analysers
• A sample left standing or drawn near a fluid drip can give an odd reading
• Very high blood sugar draws water into the blood and lowers the sodium reading temporarily

No fasting needed. A value taken while unwell or on a drip reflects that moment, and doctors usually repeat it once settled.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.