Is my potassium low?
Yes, if it’s below 3.5. That’s the bottom of normal at NHS labs like South Tees. But the number is only half the story. What pulled it down decides what happens next.
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Is my potassium level low?
Yes, if it’s below 3.5 mmol/L. South Tees NHS lab gives a normal adult range of 3.5 to 5.3, and North Bristol uses the same. Your lab prints its own range next to your result.
How far below matters. A guide for GPs from RUH Bath calls 2.5 to 3.4 mild to moderate. Below 2.5 is severe.
- Below 2.5 mmol/LSevere. The lab phones these to your GP.
- 2.5 up to 3 mmol/LModerate. A heart trace if below 3.
- Above 3 to just under 3.5 mmol/LMild. Your GP looks for the cause.
- 3.5 up to 5.3 mmol/LNormal for adults at South Tees and North Bristol.
- Above 5.3 mmol/LAbove normal. This is high, not low.
Sources: South Tees NHS; North Bristol NHS; RUH Bath
Think of it like a fuel gauge. A 3.4 is just under the line. It doesn’t mean you’ll break down, but your GP will want to know why it dropped.
A 2.4 is close to empty. Severe low potassium can cause life threatening heart rhythm problems, so it needs urgent care.
Find your band, then look at what might have pulled it down. Over the top of the range? See high potassium instead.
What causes low potassium?
Usually losing it, not eating too little. In a GP guide from RUH Bath, stomach losses like diarrhoea and vomiting are listed as 51% of common causes, and water tablets 47%.
| Cause | Examples |
|---|---|
| Losing it through your gut | Diarrhoea, vomiting, or laxatives used too often. |
| Water tablets (diuretics) | Ones like furosemide or indapamide. |
| Low magnesium | It often comes with low potassium and makes it harder to fix. |
| Other medicines | Steroids, asthma relievers like salbutamol, insulin, and theophylline (a breathing medicine). |
| Hormone problems | Cushing’s syndrome (too much of the stress hormone cortisol) or too much aldosterone, a salt-balancing hormone. Liquorice can act the same way. |
| Eating again after a long gap | After starving, heavy drinking or an eating disorder. |
| The sample, not your blood | A slow trip to the lab in warm weather, or very high white cells. |
Source: RUH Bath
That cause is what pulled it down, and it decides your next step. A tummy bug that’s passed is a different job from a daily water tablet.
On one of these medicines? Never stop or change a prescribed medicine without your prescriber. Bring a full list to your GP.
What are the symptoms of low potassium?
Often none. RUH Bath says people with low potassium are often free of symptoms. In a mild low, symptoms often come from the cause, such as a stomach bug.
When low potassium does cause symptoms, the list includes:
- muscle cramps, pain and weakness
- tiredness
- constipation or feeling short of breath, from weak muscles
- palpitations (a racing, fluttering or pounding heartbeat) or fainting
- heart rhythm problems
- confusion or seeing things that aren’t there
The NHS adds needing to pee a lot. If you take digoxin, a heart medicine, or have a heart condition, even a mild low raises the risk of rhythm problems.
Cramps have many causes. See blood tests for muscle cramps and blood tests for palpitations.
What will my GP do after a low result?
Look for the cause, then treat it. RUH Bath says the cause should be found and managed.
What usually happens after a low potassium result
- 1
Ask about the cause
Recent sickness, diarrhoea, and every medicine you take.
- 2
Run more blood tests
Kidneys, magnesium, bicarbonate and a full blood count.
- 3
Check your heart if below 3
A heart trace (ECG). Below 2.5 or with symptoms, it’s urgent.
- 4
Treat, then retest
For a mild or moderate low: tablets for 3 to 4 days, then a repeat test.
Source: RUH Bath
Magnesium matters here. Low potassium is often hard to fix until low magnesium is corrected. See the magnesium blood test.
If a water tablet is the cause, your prescriber may change your medicines. Ask when your repeat test is due, and book it.
Before your GP appointment about low potassium
- Write down your result and the range printed next to it.
- Note any recent diarrhoea, vomiting or laxative use.
- List every medicine and supplement you take, including water tablets and inhalers.
- Ask whether your magnesium has been checked.
- Ask when your repeat test is due, and book it.
- Never stop or change a prescribed medicine without your prescriber.
Can food raise low potassium?
Food can help, and RUH Bath lists foods GPs can suggest. But the cause is usually a loss, so that gets treated first. Adults aged 19 to 64 need 3,500mg a day.
Good sources include bananas, broccoli, parsnips, beans, nuts, fish and chicken. RUH Bath adds tomato juice, coffee and chocolate.
Take potassium tablets only as your GP prescribes. With kidney problems, doses must be smaller. The NHS says older people should not take potassium supplements unless a doctor advises it.
More in potassium supplements.
When to speak to a GP
Get help if:
- You have heart palpitations that don’t go away, or with chest pain, shortness of breath, feeling faint or fainting. Call 999 or go to A&E. If they’ve stopped, ask your GP for an urgent appointment, or call 111.
- You still have signs of dehydration after using rehydration sachets, or keep being sick and can’t keep fluid down. Or you have bloody diarrhoea or bleeding from your bottom, or diarrhoea for more than 7 days or vomiting for more than 2 days. Call 111 now.
- Your potassium is below 2.5. The lab phones these to your GP or out-of-hours service.
- You have a low result with cramps, weakness, palpitations or fainting. RUH Bath says this needs urgent checks with a heart trace. Ask your GP for an urgent appointment, or call 111.
- You take digoxin or have a heart condition, and your result is low at all.
Potassium usually comes with your kidney numbers. See U&E results explained.