High Potassium: Causes, Symptoms and What to Do Next
Someone rings from the surgery to say a blood test showed high potassium and would you come in. It sounds minor — potassium is a mineral, it is in bananas — and it is the one abnormal result on a routine panel that genuinely can stop a heart.
It is also, quite often, not real. Both of those things need saying in the same breath, because the first without the second causes unnecessary terror and the second without the first causes people to ignore something that matters.
🧪 Start here: a substantial proportion of raised potassium results are artefacts of how the blood was taken, not a reflection of what is in the body.
Potassium sits mainly inside cells, at a concentration far higher than in the fluid around them. So if any red cells rupture between the vein and the machine, potassium leaks out and the measured value rises — even though nothing was wrong with the person.
What causes it: the tourniquet left on too long, clenching or pumping the fist during the draw, a difficult collection through a small needle, the sample being shaken, or a delay before it reaches the laboratory. Very high platelet or white cell counts can do it too. Laboratories often flag this on the report with a note about the sample being haemolysed.
What to do: a repeat test, taken carefully, before drawing any conclusions. What not to do is dismiss a repeat result that comes back high again, or ignore any of the symptoms below while waiting for one.
⚡ Why this particular mineral
Potassium controls how nerve and muscle cells reset electrically between one signal and the next. Heart muscle depends on that reset happening precisely, every beat, and it is unforgiving of drift in either direction.
Too little potassium destabilises the rhythm, which is the mechanism behind the danger described in the article on purging behaviour. Too much does the same thing by the opposite route, slowing conduction until the heart stops responding. The safe range is narrower than most people would guess, and the body normally holds it there with considerable precision, largely by adjusting how much the kidneys excrete.
Which means a persistently raised level is almost always telling you something about the kidneys, or about a drug acting on them.
🚨 When it cannot wait
🔴 The most dangerous feature of high potassium is that it usually produces no symptoms at all until it is severe. People are not gradually warned. The first sign can be a collapse.
Seek emergency care for:
- Palpitations, a slow or irregular pulse
- Muscle weakness, particularly starting in the legs, or difficulty standing
- Numbness or tingling in hands, feet or around the mouth
- Chest pain, breathlessness, or feeling faint
- Nausea and vomiting alongside any of the above
Also treat as urgent any high result in someone with known kidney disease, or anyone who has recently been vomiting or unable to drink, since dehydration compounds it quickly.
If the laboratory or the surgery describes the level as severe, that is a same-day matter regardless of how well you feel. An ECG shows the effect on the heart before any symptom does, which is why it is the first thing done.
💊 The commonest real cause
When a raised level is genuine and the person is not acutely unwell, medication is the usual explanation — and frequently several medications acting together, each of which alone would have been fine.
| Medicine | How it raises potassium |
|---|---|
| Names ending in pril | Reduce aldosterone, the hormone instructing the kidney to excrete potassium. Very widely prescribed for blood pressure, heart failure and kidney protection |
| Names ending in sartan | The same effect by a slightly different route |
| Spironolactone, eplerenone, amiloride | Block the potassium-excreting mechanism directly. The strongest single contributors |
| Anti-inflammatory painkillers | Reduce blood flow through the kidney and lower aldosterone. Bought without prescription and rarely mentioned at appointments |
| Trimethoprim | Blocks potassium excretion directly. A short course for a urine infection is enough to matter in someone already at risk |
| Heparin | Suppresses aldosterone production |
| Certain transplant medicines | Reduce potassium excretion |
| Potassium supplements and potassium-based salt substitutes | Add potassium directly, and are often not thought of as medicines at all |
The pattern that causes most trouble is accumulation. Someone on a blood pressure tablet ending in pril is stable for years. They add a potassium-based salt substitute on sound dietary advice, take an anti-inflammatory for a sore knee, and are given trimethoprim for a urine infection. Each step is reasonable. Together they can be dangerous, and no single prescriber sees the whole picture.
The article on salt substitutes covers that specific trap in more detail, since low-sodium salt is the contributor people are least likely to report.
🫀 The kidneys, and the hormone that runs them
Reduced kidney function is the other major cause, and it is why potassium is watched closely in kidney disease. Less filtering capacity means less potassium removed, and the margin shrinks as function declines.
But there is a second, more specific mechanism, and it is what the older name for this topic refers to.
Aldosterone is the hormone that instructs the kidney to hold sodium and get rid of potassium. Where it is deficient or the kidney stops responding to it, potassium accumulates and sodium is lost, even when overall kidney function looks only mildly impaired.
The commonest version of this occurs in people with long-standing diabetes and mild to moderate kidney impairment, where the system producing aldosterone becomes underactive. It is considerably more frequent than its obscurity suggests, and it explains persistently raised potassium in someone whose kidney numbers do not look bad enough to account for it.
It also occurs in adrenal insufficiency, where aldosterone and cortisol are both lacking — a situation with its own distinctive picture of fatigue, weight loss, low blood pressure, salt craving and darkening skin, described in the article on cortisol.
Where the deficiency is confirmed, it is treated by replacing the missing hormone with a synthetic equivalent, which restores the kidney signal and brings potassium down. Blood pressure, ankle swelling and potassium all need monitoring on treatment, since the same effect that corrects the problem can cause fluid retention if the dose is too high.
🍌 Where diet fits, and where it does not
Diet is usually not the cause on its own. Healthy kidneys handle an enormous range of potassium intake without difficulty, and the mineral is present in most of the food anyone would call good for them.
Diet becomes important when something else has already reduced the ability to excrete it — kidney impairment, or one of the medicines above. At that point intake matters, and the surprise is usually which foods are involved.
| Higher than expected | Notes |
|---|---|
| Potatoes | Among the largest contributors in most diets, well above bananas. Boiling in plenty of water and discarding it removes a useful proportion |
| Tomato paste, purée and sauces | Concentrated, so quantities add up fast |
| Dried fruit | Water removed, potassium concentrated |
| Avocado, spinach, beans and lentils | All substantial |
| Orange juice and coconut water | Easy to drink in quantity without registering it |
| Nuts and chocolate | Modest per portion, significant as a habit |
| Low-sodium salt substitutes | The single most concentrated source in a kitchen, and not thought of as food |
Nobody should restrict potassium without being told to. It lowers blood pressure, and the foods containing it are the foods most worth eating. Restriction belongs to people with a specific reason, guided by their actual results.
📋 What should happen next
A sensible sequence after a raised result, in order.
- A repeat sample, taken without a prolonged tourniquet and without clenching the fist, and processed promptly
- An ECG if the level is more than mildly raised, since it shows the effect on the heart directly
- Kidney function and sodium checked alongside
- A full medication review including anything bought without prescription, any supplement, and any salt substitute
- Consideration of aldosterone deficiency where potassium is persistently raised out of proportion to kidney function, particularly in diabetes
- A plan — which drug is being reduced or stopped, when the level will be rechecked, and what to do if symptoms appear before then
One thing to be clear about: the answer is frequently not to stop the medication permanently. Drugs ending in pril and sartan protect the kidneys and the heart, and a modest rise in potassium is often accepted deliberately because the benefit outweighs it. What changes is monitoring and dose, and sometimes removing a different contributor instead. This is a decision for the prescriber rather than for the patient reading a result.
Where illness intervenes, the sick-day rules matter here as much as anywhere — several of these medicines are paused during vomiting or diarrhoea, as set out in the article on stomach infections.
🩺 When to seek help
- Palpitations, a slow or irregular pulse, or feeling faint — emergency
- Muscle weakness starting in the legs, or difficulty rising — emergency
- Numbness or tingling with any raised result
- A severe result reported by the laboratory — same day, however well you feel
- Any raised result alongside vomiting, diarrhoea or reduced fluid intake — urgent
- Before starting a potassium-based salt substitute, if you take anything for blood pressure or the heart
- Before taking anti-inflammatory painkillers regularly on those medicines
- If prescribed trimethoprim while on a drug ending in pril or sartan — mention it
- Persistent raised potassium with mild kidney impairment and diabetes — ask about aldosterone deficiency specifically
- Fatigue, weight loss, low blood pressure, salt craving or darkening skin — assessment for adrenal insufficiency
Where aldosterone deficiency is confirmed, replacement is stocked as Florinef (fludrocortisone), which restores the kidney signal to excrete potassium and retain sodium, with blood pressure, swelling and blood tests monitored on treatment. Among the medicines that raise potassium, Aldactone (spironolactone) has the strongest effect, and drugs ending in pril or sartan in the blood pressure category contribute in the same direction. None of these should be started, changed or stopped without a doctor, and all of them require periodic blood tests.
❓ Frequently asked questions
Could my high potassium result be wrong?
Quite possibly, and a substantial proportion of raised results are artefacts. Potassium sits mainly inside cells at high concentration, so if any red cells rupture during the blood draw it leaks out and the measured value rises. Causes include a prolonged tourniquet, clenching the fist, a difficult collection, shaking the sample, or a delay reaching the laboratory. A carefully taken repeat is the first step.
What does high potassium feel like?
Usually nothing until it is severe, which is exactly what makes it dangerous — there is often no gradual warning. When symptoms appear they are palpitations, a slow or irregular pulse, muscle weakness starting in the legs, tingling in hands, feet or around the mouth, and feeling faint. Any of those alongside a raised result is an emergency rather than something to raise at the next appointment.
Which medicines raise potassium?
Anything ending in pril or sartan, and spironolactone, eplerenone or amiloride most strongly of all. Also anti-inflammatory painkillers, the antibiotic trimethoprim, heparin, certain transplant medicines, potassium supplements, and potassium-based salt substitutes. The usual problem is accumulation — several individually reasonable additions where no single prescriber sees the whole list.
Should I stop my blood pressure tablet?
Not on your own, and frequently not at all. Drugs ending in pril and sartan protect the kidneys and the heart, and a modest rise in potassium is sometimes accepted deliberately because the benefit outweighs it. What usually changes is the dose and the monitoring, or the removal of a different contributor such as a painkiller or a salt substitute. This is a prescriber decision, not a reaction to a number.
Is it caused by eating bananas?
Almost never on its own. Healthy kidneys handle a wide range of intake without difficulty. Diet matters once something else has reduced excretion — kidney impairment or one of the medicines above. And bananas are not the main culprit: potatoes, tomato paste, dried fruit, avocado, beans, orange juice and coconut water all contribute more than their reputation suggests, and salt substitutes most of all.
Should I cut potassium out of my diet?
Not unless you have been told to. Potassium lowers blood pressure and the foods containing it are among the most worth eating, so restricting it without a specific reason does harm rather than good. Restriction belongs to people with reduced kidney function or a confirmed problem, guided by their actual results rather than by a general worry.
What is aldosterone deficiency?
Aldosterone is the hormone telling the kidney to hold sodium and get rid of potassium. Where it is lacking, or the kidney stops responding to it, potassium accumulates and sodium is lost even when kidney function looks only mildly impaired. The commonest version occurs in long-standing diabetes with mild to moderate kidney impairment, and it explains persistently raised potassium that the kidney numbers alone do not account for.
How is that treated?
By replacing the missing hormone with a synthetic equivalent, which restores the kidney signal and brings potassium down while helping the body retain sodium. Blood pressure, ankle swelling and blood tests are all monitored on treatment, because the same effect that corrects the problem causes fluid retention if the dose is too high. Alongside it, contributing medicines are reviewed and dietary potassium may be adjusted.
Does a stomach bug make this worse?
It can, considerably, because dehydration reduces blood flow through the kidneys and several of the medicines involved become risky in that state. That is why drugs ending in pril or sartan, water tablets and anti-inflammatory painkillers are usually paused during vomiting or diarrhoea and restarted once eating and drinking normally. Steroid tablets, by contrast, go up rather than down.
What should I ask for after a high result?
A carefully taken repeat sample, an ECG if the level is more than mildly raised, kidney function and sodium checked alongside, and a full medication review including anything bought over the counter, any supplement and any salt substitute. Then a plan: which drug is changing, when the level will be rechecked, and what symptoms should prompt contact before then.
📑 Sources and editorial
- Literature on pseudohyperkalaemia, preanalytical haemolysis and sample handling effects on measured potassium
- Physiology of potassium distribution, membrane repolarisation and cardiac conduction
- Guidance on electrocardiographic changes and severity stratification in hyperkalaemia
- Pharmacological literature on renin-angiotensin inhibitors, aldosterone antagonists and potassium retention
- Evidence on trimethoprim-associated and NSAID-associated hyperkalaemia
- Literature on hyporeninaemic hypoaldosteronism and its association with diabetes and chronic kidney disease
- Clinical features and biochemical findings of primary adrenal insufficiency
- Guidance on mineralocorticoid replacement and monitoring requirements
- Dietary potassium content data and the effect of boiling on potassium in vegetables
- Guidance on sick day rules for medicines during acute dehydrating illness
- Related reading: salt substitutes and potassium, cortisol and adrenal insufficiency, medicines to pause when ill
- Related products: endocrine category, blood pressure category
- RXshop Editorial Team — reviewed by Robert Hayes, MD, Cardiology
Medical Disclaimer: The information in this article is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek guidance from a qualified healthcare provider with any questions you may have regarding a medical condition, and before starting, stopping or changing any medication.