High Sodium on Your Blood Test
What a raised sodium usually means, why it is almost always about water rather than salt, and what your Australian GP checks next — in plain English.
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The Quick Answer
Sodium is the main electrolyte in your blood, and its level is controlled mostly by water balance and thirst rather than by how much salt you eat. The Australian reference range is roughly 135-145 mmol/L. A result above 145 mmol/L is called hypernatraemia.
The most important concept to take from this page is that a high sodium is almost always a water problem — too little water — rather than a salt problem. Your blood has simply become more concentrated. A mildly high sodium is frequently just mild dehydration that settles quickly once you rehydrate.
What Is Sodium and Why Is It Really About Water?
Sodium sits mostly outside your cells, in the fluid that bathes them and in your bloodstream. Its concentration is tightly controlled by a partnership between your brain, your pituitary gland, and your kidneys. When your blood becomes even slightly too concentrated, specialised sensors in the brain trigger both thirst and the release of antidiuretic hormone (ADH), which tells the kidneys to hold onto water and produce more concentrated urine.
This system is built around adjusting water, not sodium. Your kidneys are very efficient at excreting excess dietary salt, so eating a salty meal rarely moves your blood sodium level much at all. What does move it is not drinking enough, or losing too much water through sweat, fever, vomiting, diarrhoea, or urine.
Less commonly, the system itself can be faulty — the kidneys may be unable to concentrate urine properly (diabetes insipidus), or a hormone imbalance may drive excess sodium and water retention (excess mineralocorticoid activity). These causes are rarer, but important to consider when the more common explanations do not fit.
Causes of High Sodium
Causes are grouped as inadequate intake (not enough water going in), excess water loss (too much going out), or a rarer kidney or hormone problem affecting how water is regulated.
Inadequate water intake
The single most common cause, especially in older or frail people whose thirst sensation is blunted. Common in dementia, after a stroke, or during sedation, when the person simply does not feel or cannot act on thirst.
Reduced access to fluids
Immobility, hospital admission, or cognitive impairment can mean someone wants to drink but cannot easily reach a cup or ask for one. A frequent, preventable finding in aged care and post-surgical settings.
Fever and heavy sweating
Insensible losses through the skin and lungs rise sharply with fever, hot weather, or vigorous exercise. Usually mild and corrects quickly once fluids are replaced.
Vomiting and diarrhoea
Gastrointestinal illness causes water loss that outpaces sodium loss, especially in children and older adults who cannot keep up with oral replacement.
Poorly controlled diabetes
High blood glucose spills into the urine and drags water out with it (osmotic diuresis). This can be the first clue to undiagnosed or poorly managed diabetes, and in severe cases points to a hyperosmolar emergency.
Diuretic medications
Loop and thiazide diuretics increase free water loss through the kidneys. A medication review, especially in older patients on multiple tablets, often finds the cause.
Diabetes insipidus
The kidneys cannot concentrate urine properly, producing large volumes of dilute urine. Can follow head injury or pituitary surgery (central), or relate to lithium or kidney disease (nephrogenic).
Excess mineralocorticoid activity
Conditions such as primary hyperaldosteronism cause mild sodium and water retention alongside low potassium and high blood pressure. Usually only a small rise in sodium.
Symptoms That Can Accompany a High Sodium
Symptoms depend on how high the sodium is and how quickly it rose. Many people with a mildly raised sodium feel completely well, while a sharp or severe rise can affect the brain and needs prompt attention.
Increased thirst
Often the earliest and most sensitive symptom, as the body tries to correct the water deficit. May be blunted or absent in older or frail people.
Dry mouth and dry mucous membranes
A simple bedside sign doctors look for, alongside reduced skin elasticity and a dry tongue, when assessing dehydration.
Lethargy and tiredness
A general sense of low energy and sluggishness that can be mistaken for simply being unwell or run down.
Irritability or restlessness
Mood and behaviour changes are common, particularly in young children and older adults with dementia.
Muscle weakness or cramps
Reflects the effect of altered fluid balance on muscle and nerve function.
Reduced, dark, concentrated urine
The kidneys try to conserve water by producing less, more concentrated urine — a useful sign to watch for at home.
Confusion or altered mental state
A red-flag symptom, especially if it develops over hours rather than days. Reflects the effect of fluid shifts on brain cells and needs prompt assessment.
Seizures or reduced consciousness
Seen with severe or rapidly rising sodium. This is a medical emergency requiring immediate hospital care.
Red Flags — When to Seek Prompt Care
Most mildly high sodium results can be followed up with your GP in the ordinary way. But some combinations of findings should prompt same-day medical attention:
Sodium above 160 mmol/L
Considered severe hypernatraemia. Needs prompt medical assessment, usually in an emergency department or under specialist guidance.
Confusion, drowsiness or reduced consciousness
Suggests the brain is being affected by the fluid shift. This needs same-day medical review — do not wait for a routine appointment.
Seizures
A medical emergency. Call an ambulance (000) if a high sodium result is accompanied by a seizure.
Rapid onset over hours rather than days
Acute hypernatraemia carries a higher risk of harm from fluid shifts in the brain than a slowly developing, chronic rise, and typically needs urgent assessment.
Frail or elderly person with poor intake plus fever, vomiting or diarrhoea
A high-risk combination, particularly in aged care or when living alone. Needs prompt medical review rather than advice to simply drink more at home.
High sodium plus high blood glucose
Can indicate a diabetic emergency (hyperosmolar hyperglycaemic state) and needs urgent same-day assessment.
What Your GP Will Do Next — The Workup
Australian GPs follow a fairly standard pathway when investigating a high sodium result, usually starting with the simplest and most common explanations before moving to rarer ones. A urea and electrolytes (U&E) blood test is Medicare-rebated when ordered by your GP.
Confirm the result and assess hydration
A repeat urea and electrolytes (U&E) test confirms the finding is real, alongside a clinical check of hydration — blood pressure, pulse, skin turgor and mucous membranes.
Review fluid balance and recent losses
Your GP will ask about how much you have been drinking, and whether you have had fever, sweating, vomiting or diarrhoea in the days beforehand.
Check blood glucose
Ruling out hyperglycaemia-driven osmotic diuresis is a key early step, particularly if diabetes is known or suspected.
Review medications
Diuretics, lithium, and other medications that affect how the kidneys handle water are checked against your current prescription list.
Assess kidney function and urine concentration
Urea, creatinine, and sometimes urine osmolality help show whether the kidneys are concentrating urine appropriately, or whether diabetes insipidus needs to be considered.
Correct the fluid deficit gradually
Mild cases are treated with oral fluids at home. More significant or symptomatic cases are managed with supervised intravenous fluids in hospital, with sodium lowered slowly and rechecked regularly.
Specialist referral if unexplained
If the cause is not clear from history and simple tests, particularly with unexplained large urine volumes, referral to an endocrinologist may be arranged to investigate diabetes insipidus or another hormonal cause.
Treatment — What Happens Once You Know the Cause
Mild dehydration-related hypernatraemia
The most common scenario is treated with straightforward oral rehydration and addressing whatever triggered the fluid loss — a fever, a stomach bug, or simply not drinking enough. Your GP will usually recheck your sodium level once you have rehydrated to confirm it has settled back into the normal range.
Osmotic diuresis from high blood glucose
Where uncontrolled diabetes is the cause, treatment focuses on bringing blood glucose back under control alongside careful fluid replacement. Severe presentations, such as a hyperosmolar hyperglycaemic state, are managed in hospital with cautious, closely monitored intravenous fluids.
Diabetes insipidus
Central diabetes insipidus is usually treated with desmopressin (DDAVP), which replaces the missing antidiuretic hormone. Nephrogenic diabetes insipidus focuses on ensuring free access to water, treating any underlying cause, and sometimes using thiazide diuretics, which paradoxically reduce urine volume in this condition.
Excess mineralocorticoid activity
If a hormonal cause such as primary hyperaldosteronism is found, treatment is guided by an endocrinologist and may include a specific medication or, if an adrenal adenoma is identified, surgery.
The golden rule — correction must be gradual
Whatever the cause, bringing sodium down too fast is dangerous. In chronic hypernatraemia, brain cells adapt to the higher sodium level, and correcting it too quickly can cause water to shift rapidly into those cells, risking serious neurological harm. Doctors typically aim to lower sodium by no more than about 8 to 10 mmol/L over 24 hours, with frequent blood tests to track progress — which is why more significant cases are managed in hospital rather than at home.
Staying Well Hydrated
Sip water regularly through the day
EveryoneDo not wait until you feel thirsty, especially during hot weather, illness, or exercise, when thirst can lag behind your actual fluid needs.
Keep a drink within easy reach
Reduced mobilityFor anyone with limited mobility, a drink bottle beside the bed or chair removes the barrier of having to get up for water.
Check your urine colour
EveryonePale straw-coloured urine generally means you are well hydrated; dark, concentrated urine is a simple prompt to drink more.
Extra care in heatwaves and during exercise
EveryoneIncrease fluid intake proactively on hot days or before, during and after vigorous activity, rather than waiting for symptoms.
Increase fluids during fever, vomiting or diarrhoea
UnwellSmall, frequent sips are often better tolerated than large volumes. Oral rehydration solutions can help replace both water and electrolytes.
Review medications with your GP or pharmacist
On regular medicationDiuretics and some other medications affect fluid balance — a periodic review is worthwhile, particularly for older adults on several tablets.
Check in on older relatives
Carers and familyReduced thirst sensation with age means dehydration can develop quietly. Regular check-ins matter most for those with dementia or who live alone.
Track your results over time
EveryoneKnowing your usual sodium level makes it easier for you and your GP to spot a meaningful change early.
High Sodium — Frequently Asked Questions
What does a high sodium level mean?
A high sodium (hypernatraemia) on your blood test means the concentration of sodium in your blood is above the normal Australian reference range of roughly 135-145 mmol/L. Importantly, this is almost always caused by having too little water in your body relative to the amount of sodium, rather than eating too much salt. Your GP will look at your hydration, recent fluid losses, and medications to find the cause.
What is the normal range for sodium in Australia?
Australian pathology labs typically report a normal blood sodium range of 135-145 mmol/L, though the exact figures vary slightly between labs. A result above 145 mmol/L is called hypernatraemia. Mild elevations of 146-150 mmol/L are common and often resolve with simple rehydration, while levels above 160 mmol/L are considered severe and need prompt medical assessment.
Is a high sodium level caused by eating too much salt?
Usually not. While it seems logical that salt intake would raise blood sodium, your body is very good at excreting excess dietary sodium through the kidneys. A high sodium level on a blood test is almost always a sign that you have too little water in your body - through reduced drinking, fever, vomiting, diarrhoea, or a medical condition affecting water balance - rather than a sign you have eaten too much salt.
What symptoms does high sodium cause?
Mild elevations often cause no symptoms at all, or just increased thirst. As sodium rises further, or if it rises quickly, symptoms can include lethargy, irritability, muscle weakness, and dry mucous membranes. Severe or rapidly developing hypernatraemia can cause confusion, drowsiness, and in serious cases seizures, because the shift in fluid affects brain cells.
Why are older people more at risk of high sodium?
The sensation of thirst becomes less reliable with age, and older or frail people - particularly those with dementia, reduced mobility, or who are unwell - may not recognise or be able to act on thirst in the same way. This means dehydration, and the resulting high sodium, can develop gradually without the person realising they need to drink more. Aged care settings and hospitals watch for this closely.
How is high sodium treated?
Treatment focuses on replacing the water deficit and addressing the underlying cause - whether that is inadequate fluid intake, fever, vomiting and diarrhoea, poorly controlled diabetes, or a rarer hormonal cause such as diabetes insipidus. Mild cases are usually managed with oral fluids at home. More significant or symptomatic cases may need supervised intravenous fluids in hospital, with sodium levels checked regularly.
Why does high sodium need to be corrected slowly?
If chronic hypernatraemia is corrected too quickly, brain cells that have adapted to the higher sodium level can swell rapidly as water shifts back in, which can cause serious neurological injury. Doctors aim to lower sodium gradually - typically no more than about 8 to 10 mmol/L over 24 hours in chronic cases - with frequent blood tests to track the pace of correction safely.
Related Reading
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This page provides general educational information about elevated sodium and hypernatraemia. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your GP about abnormal blood test results — they have access to your full medical history and can interpret your results in context. SmarterBlood does not provide medical care.