Sodium and potassium: why electrolytes matter and what shifts them

Electrolytes are salts dissolved in your blood that carry a tiny electrical charge. Sodium and potassium are the two most important. Together they control how water moves in and out of your cells and how nerves and muscles, including your heart, fire. Your kidneys keep both within a narrow range, so a result outside that range is a sign that something has tipped the balance.

Sodium: the water balance marker

Sodium is the main salt outside your cells. A typical lab range is 136 to 145 mmol/L. The sodium level in the blood is less about how much salt you eat and more about the balance between sodium and water. Too much water dilutes it. Too little water concentrates it.

Common causes of a low sodium

• Water tablets (diuretics), especially the type often used for high blood pressure.
• Some antidepressants and seizure medicines.
• Drinking large amounts of water or sports drinks during long endurance events.
• Vomiting or diarrhoea, particularly when fluids are replaced with plain water.
• Heart, liver or kidney conditions that cause the body to hold on to fluid.
• Less often, hormone problems involving the thyroid, adrenal glands or the hormone that controls water balance.

A mildly low sodium often causes no symptoms. Lower levels can cause headache, nausea, tiredness, muscle cramps, unsteadiness and falls, particularly in older people. A sudden, severe drop can cause confusion and seizures.

Common causes of a high sodium

A high sodium almost always means the body is short of water. This can happen with not drinking enough in hot weather, vomiting, diarrhoea, heavy sweating, or high blood sugar that causes a lot of urine. Older people and those who cannot easily get a drink are most at risk. The main symptom is thirst, followed by tiredness and confusion as it rises.

Potassium: the heart rhythm marker

Potassium is the main salt inside your cells. A typical lab range is 3.5 to 5.1 mmol/L. Only a small fraction of your body's potassium is in the blood, so small changes in the blood level matter a lot. Potassium that is too high or too low can upset the heart's electrical rhythm, which is why very abnormal results are taken seriously.

Common causes of a low potassium

• Water tablets, which make the kidneys lose potassium.
• Vomiting, diarrhoea or regular laxative use.
• Some asthma inhalers and steroid medicines, usually at high doses.
• Low magnesium, which makes potassium hard to correct until it is treated too.
• Less often, hormone conditions that make the kidneys lose potassium, which can also cause high blood pressure.

Low potassium can cause muscle weakness, cramps, constipation and palpitations.

Common causes of a high potassium

Real high potassium is most often linked to reduced kidney function, or to medicines that help the body keep potassium. These include some blood pressure tablets, certain water tablets that spare potassium, potassium supplements and salt substitutes. Uncontrolled diabetes can also push potassium out of cells.

When a result is urgent

LifePrint marks a sodium below 125 or above 155 mmol/L, and a potassium below 2.8 or above 6.0 mmol/L, as urgent. Results in these ranges need a doctor the same day, even if you feel well. Seek emergency care straight away if an abnormal result comes with chest pain, palpitations, fainting, marked weakness, confusion or a seizure.

Results just outside the lab range, without symptoms, are far more common. They usually sit in LifePrint's grey zone for results slightly outside a range and are often repeated at the next routine test, or sooner if your doctor prefers.

Reading electrolytes in context

Sodium and potassium are best read alongside your kidney results and your medicines. That is why the Essentials panel includes creatinine, eGFR and urea, and why it helps to tell us what you take. If you start or change a water tablet or blood pressure medicine, your doctor may ask for a repeat electrolyte check a few weeks later.

Do not change your salt intake, fluid intake or medicines in response to a single result without speaking to a registered healthcare practitioner. Your report is health information, not a diagnosis.