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Kidney Health

Sodium

Sodium is a salt that helps keep your body's water and nerve signals in balance, and your kidneys hold it within a tight range. The result on your report shows whether that balance is being kept. A result inside the range is reassuring; a result outside usually points to something simple, like how much fluid you have had, that your GP can look into.

Standard Care
CategoryKidney Health
Reference range135–145 mmol/L mmol/L
Evidence✓Standard Care
Last updated3 July 2026
On this page

What is sodium?

Sodium is an electrolyte — a mineral that carries an electric charge when dissolved in your blood. Electrolytes like sodium are how your nerves fire and your muscles contract: as the RCPA explains, "cells use electrolytes to transmit electrical impulses internally and externally, to other cells. In this way, the nerves, heart and muscles can create nerve impulses and muscle contractions." Sodium's most important job is fluid balance. Your kidneys use it as a way of controlling how much water stays in the blood and how much is removed through the urine. Between sodium and its partner electrolyte potassium, the body keeps its fluid levels precisely regulated. Because the brain, heart, and muscles all depend on that balance, even a moderate shift in either direction matters.

Why is it tested?

Sodium is nearly always measured alongside potassium, chloride, and bicarbonate as part of an electrolytes panel. Your GP may order it as routine screening, alongside kidney markers such as creatinine and eGFR (an estimate of how well your kidneys are filtering), or to investigate symptoms such as unusual thirst, fatigue, or confusion. It is also checked when starting or monitoring medicines that affect how the kidneys handle water, and in people who have kidney, liver, or heart conditions.

Reference range (Australia)

135–145 mmol/L mmol/L

The AHRIA harmonised reference interval (AACB/RCPA-endorsed) applies to adults of all sexes. The range printed on your report may vary slightly between laboratories — use that range when reading your result.

Reference intervals vary between laboratories. Always use the range printed on your own report.

What your result means

If your level is low

A sodium below the reference range is called hyponatraemia — the most common electrolyte abnormality picked up on a blood test. It can happen when there is too much water relative to sodium in the blood: excessive fluid intake, some medicines (including certain antidepressants and diuretics), kidney or liver disease, and other conditions are all possible causes. Mild hyponatraemia often causes no symptoms, but a larger or rapid drop can cause nausea, headaches, confusion and, when severe, more serious effects such as seizures. Your GP will work through the likely cause based on your medicines, fluid history, and other results.

If your level is in range

A sodium within the reference range shows your kidneys are balancing fluid and sodium as expected. The best support for that balance is staying well hydrated day to day and letting your GP know if any new medicines are started, since some can gradually shift sodium over time.

If your level is high

A sodium above the reference range is called hypernatraemia — meaning the blood has more sodium than water relative to normal. The most common explanation is dehydration or fluid loss (from sweating, fever, or illness), where the water content of the blood drops and sodium becomes relatively concentrated. Certain kidney conditions and some medicines can also contribute. Your GP will ask about your fluids, recent illness, and medicines, and usually a repeat test after rehydration helps clarify the picture.

What can affect your result

Sodium is primarily regulated by the kidneys and is not a simple reflection of salt in your diet — in a healthy person the kidneys adjust excretion so that blood sodium stays stable regardless of how much salt you eat. A few things can push it outside the usual range. On the high side: dehydration from any cause (illness, heat, not drinking enough), and certain conditions that reduce the kidneys' ability to retain water. On the low side: drinking very large amounts of plain water in a short time, some medicines (certain antidepressants and fluid tablets are common ones), and conditions affecting the kidneys, liver, or adrenal glands. Some medicines alter sodium by changing how the kidneys handle water rather than sodium directly. Your GP will factor in your full medicine list when interpreting the result.

When to act

A sodium outside the usual range is worth a conversation with your GP at your next appointment. If you also have symptoms — confusion, a severe headache, or significant dizziness — it is worth seeking attention sooner rather than later. Your GP will usually look at the result alongside your other electrolytes, creatinine and eGFR, and your fluid and medicine history, and may recommend a repeat test to see whether the level is stable or changing. This page is general information and is not a substitute for advice from your GP.

Frequently asked questions

The harmonised reference interval endorsed by the Australasian Association of Clinical Biochemists (AACB) and the RCPA is 135–145 mmol/L for adults. Because different laboratories can set their own intervals, the range printed on your own report is the one your GP will use for your result.

In most healthy people, no. Your kidneys adjust how much sodium is passed into the urine so that blood sodium stays within a narrow range regardless of how much salt you eat. This is why blood sodium reflects hydration and kidney regulation rather than yesterday's diet. Sodium's job, as the RCPA puts it, is to help transmit nerve signals telling your muscles to contract and relax — not simply to track salt intake.

Hyponatraemia means a sodium level below the reference range — the most common electrolyte abnormality found in routine blood tests. It develops when there is too much water relative to sodium in the blood, which can happen with excessive fluid intake, certain medicines (such as some antidepressants and diuretics), kidney or liver disease, and other conditions. Mild cases often have no symptoms; a marked drop can cause nausea, headache, or confusion. Your GP investigates the cause before deciding whether any change is needed.

Hypernatraemia means sodium above the reference range — usually a sign that the blood has become relatively concentrated, most often through dehydration. It is less common than low sodium. The usual first step is to assess fluid intake and any recent illness or fluid loss. Your GP will interpret the level in the context of your other results and how you are feeling.

These four electrolytes work together to maintain fluid balance and the body's acid-base (pH) balance. Sodium and potassium together help maintain the body's fluid balance, and changes in one can shift the others. Reading them as a group gives your GP a much more complete picture than any single marker alone — and helps identify whether an abnormal result is a true finding or a temporary fluctuation.

Test your sodium

NATA-accredited testing. Results in 2–5 days. No GP referral required.

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