Symptom guide
Constipation That Won’t Shift: When to Check Your Thyroid
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed August 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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Constipation is one of the classic signs of an underactive thyroid. A slow thyroid slows the muscles that push food through your gut. If fibre, fluid and movement have not helped after a few weeks, a blood test reading TSH and free T4 is the sensible next check.
Find the test that fits your symptoms →As of August 2026. Figures here come from the NHS and NHS inform pages cited in the text, each checked on 26 August 2026.
You have upped the fibre. You drink more water. You bought the husk powder, then the tablets. Nothing much changed. At some point the useful question stops being which fibre, and starts being why.
1. When is constipation more than a fibre problem?
First, the definition. The NHS says you are constipated if you “have not had a poo at least 3 times during the last week or you’re pooing less often than usual” (NHS, Constipation).
Most of the time the cause is ordinary. The NHS lists too little fibre, too little fluid, not moving enough, ignoring the urge to go, a change of routine, stress, and side effects of medicine. Iron tablets are a well known culprit here, which our guide on iron tablets that are not working covers.
So fibre first. It is the cheapest thing to try and it works for plenty of people. Our guide on how much fibre per day sets out the UK target of 30g and where to find it.
The moment worth paying attention to is the one after that. You have eaten well for a month. You are drinking enough. Nothing has moved. Fibre cannot fix a gut that is running slow for a hormonal reason, and a slow thyroid is the most common hormonal reason there is.
2. Why does a slow thyroid slow your gut?
Your thyroid is a small gland in your neck. It sets the pace of your metabolism, which is the speed your body runs at. When it makes too little hormone, the medical name is hypothyroidism, and everything drops a gear.
Your gut is muscle. Food is pushed along by a wave of squeezing called peristalsis. That wave is driven by the same metabolic pace as the rest of you. Slow the pace, and the wave slows too. Food sits longer, more water is drawn out of it, and stools get harder.
This is why NHS inform lists constipation right alongside tiredness and feeling the cold as a common symptom of an underactive thyroid. It is not a separate gut problem. It is the same slowdown, showing up somewhere you notice it.
“A blood test measuring your hormone levels is the only accurate way to find out whether there’s a problem.”
— NHS inform, Underactive thyroid
3. The pattern that points at your thyroid
Constipation on its own is weak evidence. Half the country has been constipated this month. What raises the odds is the company it keeps. NHS inform lists these alongside it:
- Tiredness that a full night of sleep does not shift. Our always tired guide runs the full checklist.
- Feeling the cold when nobody else does.
- Weight gain with no change to how you eat, covered in our unexplained weight gain guide.
- Dry, scaly skin and brittle hair or nails.
- Low mood, slow thinking, muscle aches and cramps.
- Heavy or irregular periods in women.
One of these means little. Three or four, sitting on top of constipation that will not shift, is worth a test. The full list is in our guide to underactive thyroid symptoms.
It is also more common than people expect, and much more common in women. NHS inform puts it at 15 in every 1,000 women and 1 in every 1,000 men in the UK.
4. Which blood markers actually answer this?
You cannot feel a hormone level. Two markers do the work:
- TSH, or thyroid-stimulating hormone. This is the signal your brain sends to the gland. If the gland is underperforming, the brain shouts louder, so TSH goes up. Our TSH guide explains the range.
- Free T4, or thyroxine. The main hormone the thyroid makes. A low free T4 with a high TSH is the classic underactive pattern.
Read them together, not one alone. Our normal thyroid levels guide shows the UK reference ranges. Those ranges vary a little between laboratories.
TSH and free T4 both sit in Helvy’s General Energy & Wellness panel (£149), alongside B12, vitamin D, magnesium and cortisol. The Thyroid & Vital Organs panel (£159) adds free T3, the active form, plus kidney and liver markers. If iron tablets are in the picture, the Vitamins & Minerals panel (£159) reads ferritin and the rest of the iron set. The build-my-test tool matches you to the right mix. All are home finger-prick tests, with results in about five working days from UKAS-accredited UK laboratories.
5. When to see your GP instead
A home test is the wrong first move for some symptoms. Book a GP appointment rather than testing at home if you have any of these:
- Blood in your poo, or black stools.
- Weight loss you cannot explain.
- A change in your usual bowel habit lasting three weeks or more.
- Tummy pain that keeps coming back.
- Constipation that is not getting better with treatment.
Coeliac disease is another cause worth ruling out, and its antibody test is not one Helvy sells. Ask your GP, and read our coeliac blood test guide first so you know what to ask for. The same goes for IBS, which is diagnosed by a clinician rather than by any single marker.
A blood test measures biomarkers and gives you wellness insight. It does not diagnose anything on its own. If a result comes back outside its range, take it to a qualified clinician.
READY TO TEST?
Put a number behind a stubborn gut
Helvy’s home blood tests read TSH and free T4 alongside the other markers that shape your energy, with clear context on each one. Results in about 5 working days from UKAS-accredited UK laboratories.
Frequently asked questions
Can an underactive thyroid cause constipation?
Yes. NHS inform lists constipation as a common symptom of an underactive thyroid. Too little thyroid hormone slows your metabolism, and that includes the muscle wave that moves food through your gut. This is general information, not medical advice.
I have tried fibre and it did not help. What now?
Look at what else has changed. If you are also tired, cold, gaining weight or feeling flat, a thyroid test is a reasonable next step. If you have blood in your poo, weight loss you cannot explain, or a bowel habit that has changed for three weeks or more, see a GP first.
Which blood test checks the thyroid?
One that reads TSH and free T4 together. NHS inform calls this a thyroid function test. Free T3 adds the active form of the hormone. Helvy reads TSH and free T4 in the General Energy & Wellness panel, and all three in Thyroid & Vital Organs.
Can iron tablets make constipation worse?
The NHS lists side effects of medicine among the common causes of constipation, and iron supplements are a familiar example. If you take iron, it is worth reading ferritin and the rest of your iron markers rather than guessing at the dose. Speak to a qualified clinician before changing anything.
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