All articles · LAB RESULTS · 28 JUNE 2026

Sodium and potassium (electrolytes): what your blood test means

If your blood test included sodium and potassium, you have looked at two of your body's most important electrolytes. They quietly keep your fluids balanced and your nerves and muscles working. This guide explains what they do, what high or low results can signal, and why these numbers are best read alongside the rest of your health rather than on their own.

What electrolytes do

Electrolytes are minerals that carry a small electrical charge when they dissolve in your body's fluids. Sodium, potassium, chloride, and a few others are the main ones. That tiny charge is what makes them so useful. Your body uses it to run three jobs that never switch off.

• Fluid balance. Electrolytes help control how much water sits inside and outside your cells. This keeps your blood volume steady and stops cells from swelling or shrinking.
• Nerve signals. Nerves send messages by moving sodium and potassium in and out of cells. That movement creates the electrical pulse that lets your brain talk to the rest of your body.
• Muscle contraction. Every muscle, including your heart, relies on these mineral shifts to tighten and relax at the right moment.

Because these jobs matter so much, your body keeps electrolytes within a narrow range. Your kidneys, certain hormones, and your sense of thirst work together around the clock to hold the balance steady, which is why a blood test usually catches small shifts before you ever feel them.

Sodium: what it does and why it is watched

Sodium is the main electrolyte in the fluid outside your cells. Its biggest role is managing water. Where sodium goes, water tends to follow, so your sodium level is really a story about the balance between salt and water in your body, not just how much salt you eat.

Doctors watch sodium because it is sensitive to hydration, kidney function, and certain hormones. A sodium result that drifts too high or too low often points to a change in fluid balance somewhere in the body. Importantly, a normal sodium reading does not mean your salt intake is fine or not fine, and a high reading is rarely caused by salty food alone. The number reflects the salt-to-water ratio, which your body usually defends well.

Potassium: what it does and its heart connection

Potassium is the main electrolyte inside your cells, which makes it the mirror image of sodium. It is central to the electrical signals that fire your nerves and contract your muscles. Your heart is a muscle, so potassium has a close and important link to heart rhythm.

This is why potassium is held in an especially tight range. Both high and low potassium can affect how steadily the heart beats, which is part of why labs and doctors take potassium results seriously even when the shift looks small. A single out-of-range potassium can also sometimes be a lab artefact, for example if the blood sample was handled in a way that released potassium from cells, so an unexpected result is often repeated before any conclusions are drawn.

What high or low sodium can mean

Sodium is usually measured in millimoles per litre (mmol/L), and the typical normal range for adults often sits around 135 to 145 mmol/L, though labs vary. These are general bands, not personal targets.

Low sodium is often about too much water relative to salt rather than too little salt. It can be associated with drinking very large amounts of fluid, certain medications such as some diuretics ("water tablets"), heart, liver, or kidney conditions, and some hormone imbalances. High sodium is more commonly about not enough water, so it can be associated with dehydration from illness, heat, or simply not drinking enough, especially in older adults or anyone who cannot easily reach water. Because the kidneys do most of the work balancing sodium, an unusual result can also prompt a closer look at kidney function.

What high or low potassium can mean

Potassium is also measured in mmol/L, with a typical adult normal range often around 3.5 to 5.0 mmol/L, again varying by lab. Small shifts matter more here than they do for many other test results.

Low potassium can be associated with fluid losses from vomiting or diarrhoea, some diuretic medications, and occasionally a diet very low in potassium. High potassium is closely tied to how well the kidneys clear it, so it can be associated with reduced kidney function and with medicines that affect potassium handling, such as some blood pressure drugs. Diet plays a smaller role for most people with healthy kidneys, because the body adjusts to clear extra potassium. Your doctor reads a potassium result alongside your kidney numbers and your medication list, since those usually explain far more than food does.

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Signs of an electrolyte imbalance

Many electrolyte shifts are mild and cause no symptoms, which is exactly why they show up on blood tests rather than through how you feel. When symptoms do appear, they tend to be general and easy to confuse with everyday tiredness or a passing bug. Commonly described signs include:

• Weakness or cramps, since muscles depend on steady sodium and potassium to work smoothly.
• An irregular or racing heartbeat, which is more often linked to potassium and is a reason to seek medical advice promptly.
• Confusion, headache, or trouble concentrating, which can occur when sodium shifts affect how the brain holds water.
• Tiredness, nausea, or feeling generally unwell, which are vague on their own but can accompany larger imbalances.

None of these symptoms can confirm an electrolyte problem by themselves, and most have far more ordinary explanations. Treat them as a reason to check in with a clinician rather than to self-diagnose. Severe or sudden symptoms, particularly a markedly irregular heartbeat, fainting, or new confusion, deserve urgent medical attention.

What to discuss with your doctor

Sodium and potassium results are most useful as part of a bigger picture. Your clinician will read them alongside your kidney function, your hydration, your blood pressure, and a careful look at your medications, since drugs like diuretics and some blood pressure tablets are among the most common reasons these numbers move. They will also consider any recent illness involving vomiting, diarrhoea, or heavy sweating.

Helpful questions to raise include whether your result is mild or one to watch closely, whether any of your medicines could be involved, whether the test should be repeated to confirm it, and whether you need to change anything about fluids or diet. Because electrolyte handling is tied so closely to the kidneys and to medication, this is a conversation worth having rather than a number to act on alone.

Common questions

Related reading

• Kidney function explained: creatinine and eGFR blood test results.
• High uric acid: gout, kidney health, and what your test means.
• How to read your blood test report: a plain-language guide to lab results.