What are TPO antibodies?
Thyroid peroxidase (TPO) is an enzyme the thyroid uses to make thyroid hormone. In autoimmune thyroid disease the immune system makes antibodies against it, called TPO antibodies (TPOAb, also listed as anti-TPO or antithyroid microsomal antibodies). They are a marker of an immune attack on the thyroid.
The test tells you why the thyroid might be failing, not how well it is working. The American Thyroid Association (ATA) puts it plainly: TSH and free T4 tell you actual thyroid function; antibodies help diagnose the cause.
TPO antibodies normal range
| Result | Value (ARUP assay) | What it means |
|---|---|---|
| Negative (normal) | 0.0 to 9.0 IU/mL | No sign of thyroid autoimmunity |
| Positive | Above 9.0 IU/mL | Thyroid autoimmunity; higher values make it more likely |
Cut-offs differ between labs and assays, so the same number can be read differently at different labs. Read yours against the range on your report, and use the same lab if you retest. IU/mL and kIU/L are the same number.
How common is a positive result?
In the US NHANES III survey of more than 17,000 people, TPO antibodies were positive in 11.3% of the population. They were more common in women than men, became more common with age, and were less common in Black Americans (4.5%) than in white Americans (12.3%). TPO antibodies were significantly linked to both underactive and overactive thyroid.
TPO antibodies vs thyroglobulin antibodies
Thyroid panels often include a second antibody, against thyroglobulin (TgAb). In NHANES III, 10.4% of people had positive TgAb, but TgAb on its own, without TPO antibodies, was not significantly associated with thyroid disease. That makes TPO the more informative of the two. The ATA says either or both being positive in someone with hypothyroidism points to Hashimoto's.
Hollowell et al. 2002, US NHANES III survey of more than 17,000 people.
What high or positive TPO antibodies mean
MedlinePlus lists these causes of a positive result:
- Hashimoto's thyroiditis, the most common cause of an underactive thyroid. The ATA says positive TPO and/or thyroglobulin antibodies in someone with hypothyroidism make the diagnosis.
- Graves' disease can also raise TPO antibodies, although a different antibody (TSI) drives it.
- Granulomatous or subacute thyroiditis, often after an upper respiratory infection.
A positive result does not always mean you need treatment. MedlinePlus says it may simply mean a higher chance of developing thyroid disease in the future, often alongside a family history. The practical step is to check TSH and free T4 now and periodically. TPO antibodies are also seen in other autoimmune conditions, including rheumatoid arthritis, lupus, Sjogren syndrome, autoimmune hepatitis and autoimmune adrenal disease.
High TPO antibodies have been linked with a higher risk of miscarriage, preeclampsia, premature birth and IVF failure. Tell your doctor or fertility clinic about a positive result so they can monitor your thyroid.
Worked example: positive TPO with a normal TSH
Say a 31-year-old woman planning a pregnancy gets TPO antibodies of 85 IU/mL, about nine times ARUP's 9.0 IU/mL cut-off, with a TSH of 2.8 mIU/L and a free T4 of 1.2 ng/dL. Both function tests are inside their ranges, so her thyroid is working normally today. The antibodies show an immune reaction to the thyroid, which MedlinePlus frames as a higher chance of thyroid disease later.
Because she wants to conceive, the pregnancy note above applies: high TPO antibodies have been linked with miscarriage, preeclampsia and premature birth. MedlinePlus also says medicines that treat thyroid disease are safe to take during pregnancy.
Questions for the doctor or fertility clinic: How often should my TSH be checked before and during pregnancy? Is there a TSH level you want me under while trying to conceive? Does a family history of thyroid disease change anything? There is little point repeating the antibody test itself, since the ATA says tracking it adds little.
What a low or negative result means
There is no such thing as a TPO level that is too low. A negative result means no antibodies were found and your thyroid symptoms are probably not autoimmune. It does not rule out every thyroid problem: TSH and free T4 still decide whether your thyroid is working normally.
What can change TPO antibodies
Antibody levels vary, but the ATA notes that following TPO levels over time does not help detect the development of hypothyroidism or the response to treatment. There is no established treatment aimed at the antibody number itself; treatment, when needed, targets thyroid hormone levels and is decided with your doctor. What changes the picture is thyroid function, so if you are TPO positive the useful habit is checking TSH and free T4, not chasing the antibody value.
How to test TPO antibodies
- No fasting or special preparation is needed for the antibody test.
- Stop biotin for 2 days before, since it can distort thyroid immunoassays (ATA).
- Test with TSH and free T4. Antibodies alone cannot tell you whether you need treatment.
- Once you know your antibody status, repeating the test adds little (ATA).
Common questions about thyroid antibodies
Which thyroid antibody test do I need?
MedlinePlus lists four. TPO antibodies and thyroglobulin antibodies point to Hashimoto's disease, and most people with Hashimoto's have high levels of one or both. Thyroid stimulating immunoglobulin (TSI) and thyrotropin receptor antibodies (TRAb) point to Graves' disease. With a high TSH, TPO is the one to start with; with a low TSH, ask your doctor about TSI or TRAb, which SiPhox panels do not include.
Do I need to prepare for the test?
MedlinePlus says no special preparation is needed for a thyroid antibody test. If TSH and free T4 are drawn at the same time, stop biotin 2 days before, as the ATA advises for thyroid function tests.
Which at-home tests include TPO antibodies
SiPhox Health measures TPO antibodies on two panels: Thyroid Focus (included in the base $124 a test price) and Ultimate 360 (+$125 a test). Both also include thyroglobulin antibodies, TSH, free T4 and free T3. No other SiPhox panel includes thyroid antibodies.
Given the ATA's point that repeat antibody tests add little, a sensible plan is one Thyroid Focus test to learn your antibody status, then switch to a broader panel for later cycles while keeping an eye on TSH, which is on four of the other five SiPhox panels. See our at-home thyroid test guide and SiPhox review.
A note on home samples: SiPhox's partner lab tests capillary blood collected from the upper arm. SiPhox says its results show a 95% to 99% correlation with venous draws in its own quarterly comparisons against Labcorp and Quest, but it has not published a study of the EasyDraw device. If a result would change a treatment decision, confirm it with a venous test through your doctor.
Thyroid Focus
A full thyroid workup including both thyroid antibodies, plus core lipids.
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Ultimate 360
Everything on the four core panels plus iron studies, B12 and folate, GGT and PSA.
Check priceFrequently asked questions
What is a normal TPO antibody level?
Negative. On ARUP's assay that means 0.0 to 9.0 IU/mL. Other labs use different cut-offs, so read your result against the range on your own report.
Does a positive TPO test mean I have Hashimoto's?
Only if your thyroid is also underactive. Positive antibodies with hypothyroidism make the diagnosis (ATA). With normal TSH and free T4, a positive result means a higher chance of thyroid disease later, so your doctor may suggest periodic TSH checks.
Can TPO antibodies go away?
Levels can rise and fall, but the American Thyroid Association says tracking them over time does not help predict hypothyroidism or judge treatment. TSH and free T4 are the numbers to follow.
Are TPO antibodies dangerous in pregnancy?
High TPO antibodies are linked with higher risks of miscarriage, preeclampsia and premature birth (MedlinePlus). Tell your doctor if you are pregnant or planning to be, so your thyroid can be monitored.
How many people have positive TPO antibodies?
About 11% of the US population in the NHANES III survey, more often women and older adults.
Sources
- MedlinePlus Medical Encyclopedia: Thyroid peroxidase antibody
- MedlinePlus Lab Tests: Thyroid antibodies
- ARUP Laboratories Test Directory: Thyroid peroxidase (TPO) antibody
- American Thyroid Association: Thyroid function tests
- Hollowell JG et al. Serum TSH, T4 and thyroid antibodies in the United States population (NHANES III). J Clin Endocrinol Metab 2002
- SiPhox Health: panel comparison (checked 29 Sep 2026)
- SiPhox Health: Thyroid Focus panel