A thyroid antibodies test measures antibodies your immune system may be producing against your own thyroid gland - mainly thyroid peroxidase (TPO) antibodies and thyroglobulin (TG) antibodies. Their presence points to an autoimmune process affecting the thyroid, most commonly Hashimoto's thyroiditis (the underactive form) or Graves' disease (the overactive form). A standard thyroid function test (TSH, free T3, free T4) shows how your thyroid is currently performing; a thyroid antibodies test shows whether the immune system is the reason it might not be performing well.
This is the piece a lot of people are missing. It's common to be told a thyroid test is "normal" because TSH sits within range, without antibodies ever being checked - even though antibodies can be elevated well before TSH moves outside its reference range, and TPO-antibody-positive people move from a subclinical to an overt result at roughly double the rate of antibody-negative people.[5][6]
Thyroid antibodies vs a thyroid function test - what's the difference
These two tests answer different questions, and the confusion between them is one of the most common reasons people feel like they've been given an incomplete picture.
| | Thyroid function test | Thyroid antibodies test | | --- | --- | --- | | What it measures | TSH, free T3, free T4 | TPO antibodies, thyroglobulin antibodies | | Question it answers | Is my thyroid over- or under-active right now? | Is an autoimmune process involved? | | When it's usually done | First, as a general screen | Alongside or after a function test, if symptoms persist or a cause is being investigated | | Can results be normal on one and not the other? | Yes - antibodies can be raised while TSH is still in range | Yes - this pattern is common in early autoimmune thyroid disease |
Because the two tests are answering different questions, one being "normal" doesn't rule out the other. If you've already had a thyroid function test come back within range but you still don't feel like yourself, thyroid antibodies are usually the next logical step, not a repeat of the same test.
What is Hashimoto's, and how do antibodies show it?
Hashimoto's thyroiditis is the most common cause of an underactive thyroid (hypothyroidism), which NICE guidance puts at around 2% of the UK population, rising to more than 5% of people over 60, with a further 4-20% having a subclinical form depending on the population studied.[1][10] It's an autoimmune condition - the immune system mistakenly produces antibodies that gradually target and damage the thyroid gland, most often TPO antibodies, sometimes alongside thyroglobulin antibodies. Over time, this can reduce the thyroid's ability to produce hormone, which is what eventually shows up as a change in TSH.
Graves' disease is the overactive counterpart, driven by a different antibody (TSH receptor antibodies), and is far less common.[7]
The presence of thyroid antibodies is a marker relevant to an autoimmune thyroid process - it isn't, on its own, a diagnosis. It's a result to take to your GP alongside your thyroid function results, symptoms and history, so the two pictures can be read together.
Signs that might point to an autoimmune thyroid process
Autoimmune thyroid disease shares symptoms with a simple underactive or overactive thyroid, so the antibody test itself is what distinguishes the cause, not the symptoms alone. Things people commonly report before testing include:
- Persistent fatigue despite adequate sleep
- Unexplained weight change
- Feeling cold when others don't (or the reverse - feeling overheated)
- Dry skin, brittle hair, or hair thinning
- Low mood, anxiety, or brain fog
- Irregular periods or fertility concerns
- A family history of thyroid or other autoimmune conditions
None of these confirm an autoimmune thyroid process on their own - they're reasons to test, not results in themselves.
TPO vs thyroglobulin antibodies
Panels that test "thyroid antibodies" are usually testing for two distinct markers:
| Antibody | What it targets | Most relevant to | | --- | --- | --- | | TPO (thyroid peroxidase) antibodies | An enzyme involved in thyroid hormone production | The most common marker checked in suspected Hashimoto's | | Thyroglobulin (TG) antibodies | The protein thyroid hormone is built from | Often checked alongside TPO for a fuller autoimmune picture |
TPO antibodies tend to be the more consistently elevated of the two, though how often each is raised varies between studies, and testing both still gives a fuller picture than either alone.[2][3][4] This is why i-screen's Advanced Thyroid Check includes both antibodies alongside thyroid function, rather than a single marker on its own.
Who should consider testing thyroid antibodies
Thyroid antibody testing is commonly considered by people who:
- Have thyroid symptoms but a thyroid function test that came back within range
- Have a family history of Hashimoto's, Graves', or other autoimmune conditions[8][9]
- Have another autoimmune condition already (autoimmune conditions often cluster)[6]
- Are investigating unexplained fatigue, weight change, or mood symptoms alongside a thyroid function test
- Want a fuller picture before or during pregnancy planning, where thyroid antibodies are a relevant marker to discuss with a GP[11]
If you're still weighing up which panel fits, i-screen's thyroid testing hub compares the full range side by side.
How i-screen tests for thyroid antibodies
You can test thyroid antibodies without a GP referral. Order online, then book a venous blood draw through a phlebotomy service (a separate collection fee applies, £40 at a clinic or £60 for a mobile appointment), and view results in your dashboard.
- Thyroid Check - £88, finger-prick sample. TSH, free T3 and free T4 only - the right starting point if you haven't tested your thyroid at all yet, but it doesn't include antibodies.
- Advanced Thyroid Check - £132 plus the phlebotomy fee. TSH, free T3 and free T4 plus TPO and thyroglobulin antibodies in one panel - function and autoimmune status together, and the test to reach for if antibodies are specifically what you want to check.
- Autoimmune Check - £140 plus the phlebotomy fee. A broader six-marker autoimmune panel that includes TPO antibodies and ANA (antinuclear antibodies) alongside mitochondrial, smooth muscle, gastric parietal cell and liver-kidney microsome antibodies, for anyone whose question extends beyond the thyroid to autoimmunity more generally.
There isn't currently a UK panel that adds reverse T3 to a thyroid antibody result the way some other i-screen markets offer; a reverse T3 question is better raised separately with your GP.
Whichever panel you choose, take your results to your GP to discuss what they mean for you and any next steps.
Related reading
Because Hashimoto's and Graves' are autoimmune conditions, they connect to the wider question of immune regulation - i-screen's immunity and autoimmunity testing hub covers the Autoimmune Check (ANA and other autoimmune markers) alongside thyroid.
For the broader symptom picture that often prompts thyroid testing in the first place, read 8 reasons to get your thyroid checked.
This information is not intended to be a substitute for professional medical advice. Always seek the guidance of your doctor or other qualified health professional with any questions about your health or a medical condition. i-screen provides wellness and educational services only and does not diagnose, treat, cure or prevent any disease or medical condition.
