TPO Antibodies: What a High Result Means if Your TSH Is Normal
TPO antibodies (thyroid peroxidase antibodies, anti-TPO) show that your immune system is reacting to your thyroid. They mark Hashimoto’s thyroiditis, yet about 1 in 10 healthy people test positive. With a normal TSH, the American Thyroid Association says you don’t need thyroid hormone, only TSH checks over time. In the UK Whickham survey, women with positive antibodies and a normal TSH developed an underactive thyroid at about 2% a year. Each lab sets its own cut-off (we found upper limits from 5.6 IU/mL to 60 U/mL), so compare your number only with the range on your report.
Units: IU/mL, kIU/L and kU/L give the same number. Some US labs print U/mL. Report names: TPO Ab, TPOAb, anti-TPO, thyroid microsomal antibodies (older name).
- What they are
- Lab cut-offs
- Yearly risk
- Normal TSH
- Raised TSH
- When to recheck
- Pregnancy
- Selenium and gluten
- Urgent help
What TPO antibodies actually tell you
Thyroid peroxidase (TPO) is an enzyme your thyroid uses to make thyroid hormone. TPO antibodies are proteins your immune system makes against it, a sign of thyroid autoimmunity. In Hashimoto’s thyroiditis (Hashimoto’s disease), that reaction can slowly reduce how much hormone the gland makes.
Think of TPO antibodies as fingerprints at the scene. They show the immune system has been at the thyroid. They don’t show how well the gland is working. That is the job of TSH and free T4.
Mayo Clinic Laboratories reports TPO antibodies in about 90% of people with Hashimoto’s, 60–80% with Graves’ disease (an overactive thyroid) and 10–12% of healthy people. Labcorp adds that positives are more common in women and with age. Moderately raised levels also appear in other autoimmune conditions, such as type 1 diabetes and pernicious anemia, a cause of vitamin B12 deficiency. A negative result doesn’t fully rule out thyroiditis, because antibody production can stay inside the gland (Labcorp). More on thyroid hormones.
Why TPO antibody cut-offs differ between labs
There is no single normal range. Each manufacturer’s test measures TPO antibodies differently, and the lab uses the cut-off that goes with it. A 2015 multicentre study found “a pronounced difference” in manufacturers’ cut-offs, from 3.2 to 35.0 IU/mL, and results on the same samples still varied widely between methods after conversion to one unit.
Bars show values reported as in range (teal UK, blue US, purple doubtful). Sources: reference 15; Walsh 2010.
| Lab | Test | Negative or in range |
|---|---|---|
| Royal Cornwall Hospitals (UK) | Not stated | 0.0–5.6 IU/mL |
| Mayo Clinic Laboratories (US) | Beckman Coulter | Below 9.0 IU/mL, all ages |
| University Hospitals Sussex (UK) | Roche Anti-TPO II (from October 2026) | Below 20 IU/mL |
| Gloucestershire Hospitals (UK) | Not stated | Below 25; 25–35 doubtful; above 35 IU/mL clear positive |
| Labcorp (US) | Roche cobas | Adults 0–34 IU/mL; ages 1 to 5 0–13 |
| University of Vermont Medical Center (US) | Siemens Advia Centaur XPT | 60 U/mL or less |
So a result of 30 can be flagged high at one lab and sit in range at another. Even one maker’s cut-offs differ: Roche’s newer test at Sussex uses 20, Labcorp’s Roche test 34. On units, 1 IU/mL is 1,000 IU per liter, which is 1 kIU/L (kU/L), so 120 IU/mL is 120 kU/L. Our unit converter covers other tests.
Positive TPO antibodies with a normal TSH: the yearly risk
The best-known figures come from the Whickham survey in northeast England, which retested adults after 20 years. The figures are for women, and the survey used an older antibody test (antithyroid microsomal antibodies).
| Starting result (women) | Yearly risk of an underactive thyroid | Hypothyroid at 20 years |
|---|---|---|
| Antibodies positive, TSH normal | 2% | 27% |
| TSH raised, antibodies negative | 3% | 33% |
| TSH raised and antibodies positive | 4% | 55% |
| All women, for comparison | 3.5 per 1,000 (0.35%) | Men: 0.6 per 1,000 a year |
Most women with antibodies alone still had a working thyroid 20 years later, though their yearly risk was several times the average. Risk was higher in women with a starting TSH above 2.0 mU/L and high antibody levels.
The Australian Busselton study followed 1,184 people for 13 years with current antibody tests. Its definition of hypothyroidism includes mild cases. In antibody-positive women, the starting TSH mattered a lot.
Hypothyroidism: TSH above 4.0 mU/L or on thyroxine. Source: Busselton Health Study, Walsh JP et al., J Clin Endocrinol Metab 2010.
So positive antibodies raise the odds, and a low-normal TSH lowers them. Once TSH is above range (subclinical hypothyroidism), the 2012 AACE/ATA guideline gives progression of 4.3% a year with TPO antibodies and 2.6% without. These are group averages.
What your TPO result means alongside TSH
A TPO result is read with TSH and free T4. Each card follows the clues to a next lead.
Positive TPO antibodies, normal TSH
- TPO above your lab’s cut-off
- TSH and free T4 in range
- Thyroid autoimmunity with a thyroid that still works normally
- Another autoimmune condition (Mayo)
- A positive result in a healthy person
TSH checks over time (see when to recheck). Tell your doctor if you’re planning a pregnancy.
ATA: no thyroid hormone needed, keep monitoring TSH, and don’t repeat the antibody test. NICE (2019) also advises against repeating it.
Ask: How often should my TSH be checked? Is my TSH low-normal or high-normal? Should I have a coeliac test?
Positive TPO antibodies, TSH above range
- TSH above range
- Free T4 in range (subclinical) or low (overt hypothyroidism)
- Hashimoto’s thyroiditis
- A test taken during an acute illness, which NICE says can affect results
With a normal free T4, a repeat TSH and free T4, preferably after 2 to 3 months (ETA 2013). With a low free T4, your doctor will discuss treatment and set any dose.
NICE: if untreated, check TSH and free T4 yearly when antibodies are raised. Treatment is considered at a TSH of 10 mU/L or higher on 2 tests 3 months apart.
Ask: Is my TSH high enough to treat? When will we repeat it?
Positive TPO antibodies, low TSH
- TSH below range
- Free T4 or free T3 may be high
- Graves’ disease
- Postpartum thyroiditis, within 2 to 6 months of a birth or miscarriage
TSH receptor antibodies (TRAbs).
NICE (2019): measure TRAbs to confirm Graves’ disease in adults, and consider a thyroid scan if they are negative.
Ask: Is this Graves’ disease or thyroiditis? Do I need TRAbs?
When to recheck TSH: UK and US guidance
None of the UK or US guidance we reviewed sets a fixed interval for positive antibodies with a normal TSH. The ATA says you “do need to monitor” TSH, without a schedule.
| Situation | UK guidance | US guidance |
|---|---|---|
| Positive antibodies, normal TSH | No interval in NICE NG145; BTF: every 6 months while planning a pregnancy | ATA: monitor TSH, no interval given |
| Positive antibodies, TSH above range | NICE: yearly; UK 2006 guideline: yearly, or sooner if symptoms develop | ATA: once or twice a year for mild hypothyroidism |
| Negative antibodies, TSH above range | NICE: every 2 to 3 years | None found |
| Pregnant, positive antibodies | BTF summary of RCOG 2025: at 7 to 9 weeks and at 20 weeks | ATA 2017: at confirmation, then every 4 weeks through mid-pregnancy |
| Repeat the antibody test? | NICE: do not repeat; BTF: not needed | ATA: not needed |
As Gloucestershire’s lab puts it, “serial measurements are not helpful.” Ask for a TSH check sooner if symptoms of an underactive thyroid appear, such as extreme tiredness, feeling cold, weight gain or constipation (NHS). Our always tired case file follows the clues behind fatigue.
TPO antibodies in pregnancy and fertility
Thyroid antibodies are found in 2–17% of pregnant women (ATA 2017). They are linked with miscarriage, preterm birth and thyroid problems after the birth (British Thyroid Foundation). In the ATA’s pooled cohort studies, the odds of pregnancy loss were about 2.3 times higher and the risk of birth before 37 weeks about 1.4 times higher.
Testing. The ATA’s 2017 guideline strongly recommends TSH when pregnancy is confirmed and every 4 weeks through mid-pregnancy. The UK’s BTF, summarising 2025 RCOG guidance, advises tests every 6 months while planning, then at 7 to 9 weeks and at 20 weeks.
Treatment. The UK TABLET trial (2019) gave levothyroxine or placebo to 952 women with TPO antibodies, normal thyroid function and past miscarriage or infertility. Live birth rates were 37.4% with levothyroxine and 37.9% with placebo. The BTF now says levothyroxine is not recommended when thyroid function is normal. The earlier ATA 2017 guideline said it “may be considered” after a previous loss, on low-quality evidence. Your doctor will advise.
Fertility. The ATA recommends a TSH test for all women seeking care for infertility.
After the birth. The 2006 UK guideline says TPO antibodies in early pregnancy predict a 30–50% chance of postpartum thyroiditis within 2 to 6 months. It usually causes a brief overactive phase, then an underactive one, and often settles. The guideline advises TSH and free T4 at 6 to 8 weeks after the birth.
Selenium, gluten and iodine: what the evidence shows
Selenium. A 2024 meta-analysis of 35 studies (Huwiler et al.) found that in people with Hashimoto’s not on thyroid hormone, selenium lowered TPO antibodies and TSH slightly, with no change in free T4 and side effects similar to controls. The ATA’s summary says it isn’t yet known whether selenium delays or prevents hypothyroidism. The 2012 AACE/ATA guideline includes a recommendation against selenium to treat or prevent hypothyroidism, and the ATA 2017 guideline doesn’t recommend it for TPO-positive women in pregnancy. The NIH sets an adult upper limit of 400 micrograms a day from all sources; too much can cause hair and nail loss. See our selenium guide.
Gluten. A 2023 meta-analysis found only 4 small, non-randomized studies (87 patients) of gluten-free diets in autoimmune thyroiditis without coeliac disease. TPO antibodies fell, but not significantly. The authors judged the evidence “not yet sufficient” to recommend the diet to all patients with Hashimoto’s. Coeliac disease is a separate question: NICE (2015) advises a coeliac blood test for people with autoimmune thyroid disease at diagnosis, after eating gluten in more than one meal a day for at least 6 weeks.
Iodine. The AACE/ATA guideline warns that substantial kelp use may be linked with significant rises in TSH. More in our iodine guide.
What can throw off a TPO antibody result
- Biotin supplements. NICE (2023) warns high biotin intake can make thyroid tests falsely high or low; the University of Iowa lab notes falsely raised TPO results. Labcorp asks you to stop biotin 72 hours before, Mayo 12 hours.
- A different lab. Cut-offs and numbers differ by test.
- Other autoimmune conditions. Labcorp lists Sjögren’s syndrome, lupus, rheumatoid arthritis and pernicious anemia.
- Acute illness. NICE advises against thyroid tests during an acute illness, which affects the TSH you read the antibodies against.
Reading TPO antibodies on your lab report or app
- Names. Thyroid peroxidase (thyroperoxidase) antibodies, TPO Ab, TPOAb or anti-TPO. Labcorp also uses the older name, thyroid microsomal antibodies.
- Units. IU/mL, kIU/L or kU/L (same number), or U/mL on some Siemens tests. A low result may print with a less-than sign.
- Flags. High, positive or a gray zone, such as Gloucestershire’s “doubtful” 25–35 IU/mL.
- UK and US. NICE suggests the test for adults whose TSH is above range, so UK results often follow a raised TSH; the NHS App shows results from your GP record. US portals show each lab’s own range.
When to get help urgently
- Someone becomes very drowsy, confused or hard to wake: call 999 (UK) or 911 (US). Rarely, a severe underactive thyroid causes myxoedema coma, which the NHS calls an emergency needing hospital treatment.
- A fast or irregular heartbeat with chest pain, fainting or severe breathlessness: call 999 or 911.
- You’re pregnant with TPO antibodies and haven’t had TSH checked: contact your midwife or doctor within days. The ATA advises a TSH test once pregnancy is confirmed.
TPO antibody questions people ask
Do high TPO antibodies mean I have Hashimoto’s?
They show thyroid autoimmunity, the hallmark of Hashimoto’s, but 10–12% of healthy people test positive. With a normal TSH, no thyroid hormone is needed (ATA).
What is a normal TPO antibody level?
It depends on the test. Mayo’s range is below 9.0 IU/mL, Labcorp’s adult range 0–34 IU/mL and one Siemens test 60 U/mL or less. Use the range on your report.
How likely am I to develop hypothyroidism?
About 2% a year for women with positive antibodies and a normal TSH (Whickham). A low-normal TSH lowers the risk.
Should I repeat my TPO antibody test?
Usually not. NICE, the ATA and the British Thyroid Foundation say repeat antibody tests aren’t needed. TSH is the test that gets rechecked.
Can selenium or a gluten-free diet lower TPO antibodies?
Selenium lowered them slightly in trials, without proof it prevents hypothyroidism. Gluten-free diet studies are too small to recommend it.
Are TPO antibodies a problem in pregnancy?
They’re linked with miscarriage, preterm birth and postpartum thyroiditis, so TSH is checked in pregnancy. Levothyroxine didn’t raise live births in the TABLET trial.
How we wrote this page
Written and medically reviewed by Dr. Mustafa Sakr, MBBCh. Last reviewed: October 2026. The numbers come from UK, US and European thyroid guidance (NICE, BTA, BTF, RCOG, ATA, AACE, ETA), population studies, trials and published lab ranges, listed below. We update this page when guidance changes.
How we chose the numbers: TPO cut-offs belong to each test, so there is no single range to check a number against, and this page has no checker. Where sources disagree, such as Mayo’s 2.1% and AACE/ATA’s 2.6% yearly progression without antibodies, we use the guideline figure.
References
- NICE. Thyroid disease (NG145), 2019; updated 2023. nice.org.uk
- British Thyroid Foundation. Positive antibodies with normal thyroid function, 2025 (summarising RCOG Green-top Guideline No. 76, 2025). btf-thyroid.org; rcog.org.uk
- ACB, BTA, BTF. UK Guidelines for the Use of Thyroid Function Tests, 2006. PDF
- Alexander EK et al. ATA pregnancy and postpartum guidelines. Thyroid 2017;27:315–389. PDF
- American Thyroid Association. Hashimoto’s Thyroiditis (patient page). thyroid.org
- Garber JR et al. AACE/ATA guidelines for hypothyroidism in adults, 2012. PDF
- Pearce SH et al. 2013 ETA Guideline: Management of Subclinical Hypothyroidism. Eur Thyroid J 2013;2(4). summary
- NICE. Coeliac disease (NG20), 2015. nice.org.uk
- Vanderpump MPJ. The epidemiology of thyroid disease (Whickham data). Br Med Bull 2011;99:39–51. PDF
- Walsh JP et al. Busselton 13-year study. J Clin Endocrinol Metab 2010;95:1095–1104. abstract
- Dhillon-Smith RK et al. TABLET trial. N Engl J Med 2019;380:1316–1325. abstract
- Huwiler VV et al. Selenium in Hashimoto thyroiditis: meta-analysis. Thyroid 2024;34:295–309. PDF; ATA summary, 2024
- Piticchio T et al. Gluten-free diet and autoimmune thyroiditis: a meta-analysis. Front Endocrinol 2023;14:1200372. PMC
- Multicentre evaluation of automated TPO antibody immunoassays. Clin Chem Lab Med 2015. degruyterbrill.com
- Lab directories, October 2026: Mayo; Labcorp; Iowa; Vermont; Sussex; Gloucestershire; Cornwall
- NIH Office of Dietary Supplements. Selenium fact sheet, 2021. ods.od.nih.gov. NHS. Underactive thyroid: symptoms, 2025. nhs.uk
This page is for education only and isn’t medical advice, a diagnosis or a substitute for your own doctor, who knows your history. Reference ranges vary between laboratories. Always interpret results with a qualified clinician. See our medical disclaimer.
