Thyroid Peroxidase Antibodies (TPO)
Antibodies aimed at an enzyme in the thyroid. They are linked to autoimmune thyroid conditions such as Hashimoto's thyroiditis.
AI-verified against cited sources. Awaiting expert reviewAlso called TPOAb, TPO Ab, Anti-TPOLOINC 8099-4
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What it is
Thyroid peroxidase (TPO) is an enzyme the thyroid uses to make its hormones. In autoimmune thyroid conditions, the immune system makes antibodies that target TPO. This test measures how much of these antibodies is in your blood. A positive result tells you the immune system has been reacting to the thyroid. It does not tell you how well the thyroid is working. TSH and free T4 do that. Some older reports call this test the antimicrosomal antibody test.
Why it matters
In people with a mildly raised TSH, TPO antibodies are the most sensitive blood test for thyroid autoimmunity, and they help explain why TSH is raised. Among people who already have a mildly raised TSH, those with positive TPO antibodies are more likely to go on to an underactive thyroid (about 4.3% a year, compared with 2.6% a year in those without). But TPO antibodies are also found in about one in ten people with no known thyroid disease, so a positive result by itself does not mean something is wrong.
If it’s high
An antibody level above your lab's cutoff is reported as positive. It is linked to autoimmune thyroid conditions, most often Hashimoto's thyroiditis. Many people with a positive result have normal thyroid function at the moment, and thyroid function can stay normal for years. How high the number is does not, on its own, tell you how well your thyroid is working.
Common causes
- Hashimoto's thyroiditis, the most common autoimmune thyroid condition
- No known thyroid disease at all: about one in ten people in a US survey without known thyroid disease had positive TPO antibodies
- Being a woman or being older: positive results are more common in women and become more common with age
- Graves' disease, an autoimmune cause of an overactive thyroid
- Postpartum thyroiditis, an inflammation after pregnancy
- Having another autoimmune condition, such as type 1 diabetes or celiac disease
What can help
- Ask how often your TSH should be checked, since TPO antibodies are linked to a slow drift toward an underactive thyroid
- Ask whether free T4 or other thyroid tests are needed now
- Tell your clinician if you are pregnant or planning to be
- Tell your clinician about any other autoimmune conditions you have
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on age. Choose an age to see the one that applies.
- 20+0–34 IU/mL
Sources: Labcorp, ARUP Laboratories, Journal of Clinical Endocrinology & Metabolism
See TPO Ab by age and sex, with the U.S. survey numbers where they exist.
If this applies to you
TPO Ab means a little more for some people. Open the one that fits you.
Thyroid condition
TPO antibodies are a sign of thyroid autoimmunity. NICE suggests measuring them once when TSH is raised, and not repeating them. When TSH is only mildly raised, raised thyroid antibodies are one of the things NICE weighs in deciding whether to treat and how often to recheck.
Pregnant or postpartum
TPO antibodies are a sign of thyroid autoimmunity. The ATA says people who have them have a much higher risk of postpartum thyroiditis, a thyroid inflammation in the year after birth that is often short-lived, and its 2026 guideline notes the result can help with counseling about that risk.
What can affect your result
- Sex: positive results are more common in women
- Age: positive results become more common with age
- Ethnicity: US survey data show positive TPO antibodies are less common in Black Americans than in white Americans
- Test method: cutoffs and calibration differ between labs, so results are not interchangeable
- Biotin supplements, on some immunoassays
- Pregnancy and the period after birth, when thyroid autoimmunity is looked at more closely
- High-dose biotin can interfere with some immunoassays. Labcorp asks patients to stop biotin at least 72 hours before the draw; check with your clinician or lab first.
- Results from different assays are not interchangeable, because cutoffs and calibration differ between methods.
- Some labs print only Positive or Negative, and a few use an equivocal band.
Antibody levels change slowly, over months to years, and can fade over time even without treatment.
When to talk to a clinician
A positive TPO antibody result is not urgent by itself. Talk with a clinician about what it means alongside your TSH and free T4, especially if you have symptoms such as unusual tiredness, feeling cold, constipation, weight changes, a racing heart or swelling at the front of the neck. If you are pregnant or planning a pregnancy, tell your prenatal clinician about a positive result.
Questions you could ask
- Is it useful to repeat TPO antibodies, or is once enough?
- Should thyroglobulin antibodies be checked as well?
- What does this result mean for my thyroid, given my TSH and free T4?
- How often should we recheck my TSH?
- Does this matter for pregnancy plans or for my other conditions?
Questions people ask
What does a positive TPO antibody result mean?
It means your immune system has made antibodies against an enzyme in the thyroid. This is linked to autoimmune thyroid conditions such as Hashimoto's thyroiditis. It does not show how well your thyroid is working right now, so your clinician will read it together with your TSH and free T4.
Does a positive result mean I will develop thyroid disease?
Not necessarily. About one in ten people with no known thyroid disease have positive TPO antibodies. In people who already have a mildly raised TSH, a positive result goes with a somewhat higher yearly chance of an underactive thyroid (about 4.3% versus 2.6%), so TSH is usually followed over time.
Why do different labs use different cutoffs?
TPO antibody tests are made by different manufacturers, and each one sets its own positivity cutoff. A result that is positive on one method can be negative on another. Compare your number with the cutoff printed on your own report.
Do I need to repeat this test?
Usually not. Antibody levels change slowly, and repeating the test does not help follow an individual, according to a European Thyroid Association guideline. Your clinician may repeat it if there is a specific reason.
Can I have Hashimoto's thyroiditis with a negative result?
Yes. In about one in five people with a mildly raised TSH, no thyroid antibodies are found, and an ultrasound can show changes of thyroid inflammation before antibodies appear. A negative result lowers the likelihood but does not rule it out.
Show the science and sources
Units
- 1 IU/mL = 1 IU/mL
- 1 IU/mL = 1 kIU/L
Is a change real?
TPO Ab naturally varies about 11.3% from test to test in the same healthy person. Between two results from the same lab, a rise of 53% or more, or a fall of 35% or more, is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Biological variation estimates of thyroid related measurands - meta-analysis of BIVAC compliant studies (Fernandez-Calle P et al., EFLM WG-BV).
For clinicians and researchers
Thyroid peroxidase is a membrane-bound heme glycoprotein on the apical side of thyrocytes. It catalyzes iodide oxidation, tyrosyl iodination and coupling in thyroid hormone synthesis. TPO is the main target of what was historically called the microsomal antigen, so TPOAb replaced the older agglutination 'antimicrosomal' titers. TPOAb are the most sensitive serological marker of autoimmune thyroiditis (ETA 2013). TPOAb are probably more a marker of the autoimmune process than a direct cause of damage, and levels can wane with time. In NHANES III, TPOAb were positive in 11.3 +/- 0.4% of people without known thyroid disease and TgAb in 10.4 +/- 0.5%. Prevalence was higher in women, increased with age, and was lower in Black Americans (4.5%) than in white Americans (12.3%), and TPOAb were significantly associated with both hypothyroidism and hyperthyroidism (Hollowell 2002). In subclinical hypothyroidism, progression to overt hypothyroidism is about 4.3% a year with positive TPOAb versus 2.6% a year with negative TPOAb, and about 20% of subclinical hypothyroidism patients have neither TPOAb nor TgAb detected (ETA 2013). Assays are immunoassays reported in IU/mL (equivalent to kIU/L), but the positivity cutoff is assay-specific, and Labcorp lists an age-specific cutoff series on a Roche ECLIA. The Labcorp adult cutoff is 34 IU/mL, while ARUP uses 9.0 IU/mL on a different chemiluminescent immunoassay. Biotin can interfere with streptavidin-biotin platforms, so Labcorp advises stopping it at least 72 hours before the draw.
LOINC 8099-4 (primary), 56477-3, 32042-4, 5383-5. Specimen: serum-or-plasma.
Sources
- Thyroid Peroxidase (TPO) Antibodies (test 006676), Labcorp
- Thyroid Peroxidase (TPO) Antibody (test 0050075), ARUP Laboratories
- Serum TSH, T4, and thyroid antibodies in the United States population (1988 to 1994): NHANES III, Journal of Clinical Endocrinology & Metabolism (2002)
- 2013 ETA Guideline: Management of Subclinical Hypothyroidism, European Thyroid Journal (European Thyroid Association) (2013)
- Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association, Endocrine Practice (2012)
- Biotin Interference with Routine Clinical Immunoassays: Understand the Causes and Mitigate the Risks, Endocrine Practice (2017)
- Thyroid function tests (MedlinePlus Medical Encyclopedia), MedlinePlus, National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.