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Swollen ankles and water retention: the blood tests to run before you buy water tablets
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed September 20266 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
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Most swollen ankles come from standing, sitting still, salt, weight or a medicine such as amlodipine. Water tablets are prescription-only in the UK. Before you buy anything, four organs are worth checking with one blood test: thyroid, kidneys, liver and the blood protein albumin. Sudden, one-sided or painful swelling needs NHS 111, not a home test.
Find the panel that reads thyroid, kidneys, liver and albumin →As of September 2026. Long days on your feet make puffy ankles a late-summer complaint, and the question underneath is the one a GP hears every week: is it serious, and can I have some water tablets? The British Heart Foundation had its cardiac nurse, Regina Giblin, answer the first half in November 2025. Her list of causes runs from salty food to heart failure.
This guide is the step in between: which of those causes a blood test can check, which need a GP today, and why the real water tablets are not on the shelf.
1. Why are my ankles swollen?
Usually because fluid has pooled where gravity put it. Nothing is wrong with your organs.
Oedema is the medical word for fluid building up in the tissues. The NHS page on swollen ankles, feet and legs, reviewed in February 2026, puts the everyday causes first: “standing or sitting in the same position for too long”, “eating too much salty food”, “being overweight” and “being pregnant”. Then the medicines: “some blood pressure medicines, contraceptive pills, hormone therapy, antidepressants or steroids”.
One tablet deserves its own line. Amlodipine, a widely used blood pressure medicine, lists “swelling in your ankles, feet and legs (oedema)” among its common side effects on the NHS medicines page. If your ankles swelled after a new prescription, ask your pharmacist first. No blood test answers that.
Further down the NHS list come the causes worth a test: “thyroid problems” and “problems with your kidneys, liver or heart”. The rest of this guide is about those four.
2. When is swelling not a home-test question?
When it is sudden, one-sided, painful or comes with breathlessness. Then it is a phone call, today.
The NHS says to ask for an urgent GP appointment or call 111 if:
- “you have swelling in 1 ankle, foot or leg and you don't know what's causing it”
- “the swelling is severe, painful or starts very suddenly”
- “the swollen area is red or feels hot”
- “you have diabetes”
And to call 999 if “you feel short of breath” or “your chest feels tight, heavy or painful”.
Swelling in one leg can be a clot in a leg vein. The BHF's Regina Giblin lists “sudden or severe swelling in one leg, especially if painful, red, or warm” as a possible sign of one. Neither that nor breathlessness waits for a kit in the post.
3. Which blood tests find the cause of water retention?
Four organ checks, ten markers. All ten sit on one home panel.
- Thyroid.TSH is thyroid stimulating hormone, the signal your brain sends to ask the thyroid for more. Free T4 and free T3 are the hormones themselves. An underactive thyroid slows the whole body, and the NHS names “thyroid problems” as a cause of oedema. Our TSH guide explains how the three are read together.
- Kidneys.Creatinine is a waste product from muscle that healthy kidneys clear. eGFR is the filtering rate the lab works out from it, and urea is a second waste product. When the kidneys under-filter, salt and water stay in the body. The NHS lists “swollen ankles, feet or hands” from water retention as a symptom of chronic kidney disease. See the kidney function guide.
- Liver.ALT is an enzyme that leaks from stressed liver cells. ALP is a second enzyme from the liver and bile ducts. The NHS lists “swollen tummy, legs, ankles or feet” among the symptoms of cirrhosis, which is advanced liver scarring. The liver function guide covers both enzymes.
- Albumin and total protein. Albumin is the main protein in your blood, made by the liver, and it holds water inside your blood vessels. The NHS explains the link plainly: “the low level of protein in the blood reduces the flow of water from body tissues back into the blood vessels, leading to swelling”. Total protein is albumin plus everything else. Our albumin guide reads the pair.
Helvy's Thyroid & Vital Organs panel (£159) carries all ten from a home finger-prick sample, with cholesterol alongside. The thyroid hub has more on reading the thyroid trio.
Three checks are not on any home panel we sell. The heart test is a blood marker called NT-proBNP, which NICE guideline NG106 says to measure “in people with suspected heart failure”. A full blood count and a urine dipstick for protein are GP tests too. If your GP suspects the heart, they will run them.
A blood test measures biomarkers and offers wellness insight. It does not diagnose a condition. Kidney and liver markers also move with hydration and hard training, so a qualified clinician reads the whole set rather than one number.
4. Should I just buy water tablets?
You cannot buy the real ones, and that is the point.
Furosemide is the water tablet doctors mean. The NHS page on it, reviewed in July 2026, says: “It's only available on prescription.” It is “mainly used to treat a build-up of fluid in the body (oedema)” caused by heart failure, chronic kidney disease or cirrhosis. Every condition on that list is one a blood test or a GP visit finds first.
Water tablets also need watching. Cambridge University Hospitals tells people taking them: “Your doctors will monitor your kidneys and the salts such as potassium and sodium in your blood”. The same leaflet warns that they “can make you lose too much water from your body”.
Anything sold as a water retention tablet without a prescription is not furosemide, and it will not tell you why your ankles are swollen. Our dehydration guide shows what losing fluid does to the same kidney markers.
5. What can I do while I wait for results?
The NHS's own three, and a look at your medicine cabinet.
- “Raise your legs or the swollen area on a chair or pillows.”
- “Get some gentle exercise, like walking, to improve your blood flow.”
- “Wear wide, comfortable shoes with a low heel and soft sole.”
Check whether the swelling began with a new tablet, and take the box to your pharmacist. If it “has not improved after treating it at home for a few days”, the NHS says to see a GP.
READY TO TEST?
Start with the numbers a GP would check first
Thyroid, kidney, liver and albumin markers sit on one home finger-prick panel. Results in around 5 working days from UKAS-accredited UK laboratories.
Frequently asked questions
Can a blood test tell me why my ankles are swollen?
It can check four of the causes the NHS lists: thyroid, kidneys, liver and low albumin. It cannot check the heart, which needs a GP test called NT-proBNP, and it cannot see a clot. Sudden or one-sided swelling needs 111 first.
Can I buy water tablets over the counter in the UK?
Not furosemide. The NHS says it is only available on prescription, and people taking it have their kidneys, potassium and sodium monitored. Products sold as water retention tablets without a prescription are something else, and none of them finds the cause.
Does an underactive thyroid cause swollen ankles?
It can. The NHS lists thyroid problems among the causes of oedema. TSH, free T4 and free T3 show whether the gland is keeping up, and all three sit on a home finger-prick panel.
Which blood test shows fluid retention from the kidneys or liver?
Creatinine, eGFR and urea for the kidneys. ALT and ALP for the liver. Albumin, the blood protein that holds water in the vessels, for both. All of them are on the Thyroid & Vital Organs panel.
Related guides
Kidney Function Test UK
Creatinine, eGFR and urea, and what each one tells you.
Liver Function Test UK
ALT, ALP and albumin, and how a liver panel is read.
Albumin Blood Test UK
The protein that holds water in your vessels, and why it falls.
TSH Blood Test UK
The first thyroid number to move, and how it is read.