Sodium (Na) in Blood

A sodium test checks how much sodium is in the blood. Sodium is both an Reference electrolyte Opens New Window and mineral. It helps keep the water (the amount of fluid inside and outside the body's cells) and electrolyte balance of the body. Sodium is also important in how nerves and muscles work.

Most of the sodium in the body (about 85%) is found in blood and Reference lymph fluid Opens New Window. Sodium levels in the body are partly controlled by a Reference hormone Opens New Window called aldosterone, which is made by the Reference adrenal glands Opens New Window. Aldosterone levels tell the kidneys when to hold sodium in the body instead of passing it in the urine. (See a picture of the Reference adrenal glands Opens New Window Reference Opens New Window or the Reference kidneys Opens New Window Reference Opens New Window.) Small amounts of sodium are also lost through the skin when you sweat.

Most foods have sodium naturally in them or as an ingredient in cooking. Sodium is found in table salt as sodium chloride or in baking soda as sodium bicarbonate. Many medicines and other products also have sodium in them, including laxatives, aspirin, mouthwash, and toothpaste.

Low sodium levels are uncommon and are most often caused by heart failure, malnutrition, and diarrhea.

Other electrolytes, such as potassium, calcium, chloride, magnesium, and phosphate, may be checked in a blood sample at the same time as a blood test for sodium.

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