TPO Antibodies
Antibodies against a key thyroid enzyme, the main marker of autoimmune thyroid disease.
| Typical reference range | below 9 IU/mL |
Ranges vary by laboratory, method, age and sex: always use the range printed on your own report. Why labs differ.
What it measures
Thyroid peroxidase is the enzyme that attaches iodine to make thyroid hormones. In autoimmune thyroiditis, the immune system produces antibodies against it.
Why it matters
Positive TPO antibodies identify Hashimoto's thyroiditis (and are common in Graves' disease) and predict future hypothyroidism, especially when TSH is borderline. They also matter in pregnancy planning.
What a high result can mean
Elevated antibodies indicate thyroid autoimmunity. About 10-15% of the population, more women, have positive antibodies; many have normal thyroid function for years. Levels can also rise transiently postpartum.
What a low result can mean
Negative or low results are normal.
What affects the result
Cut-offs vary substantially between assays (Quest under 9 IU/mL; Labcorp under 34 IU/mL), so compare results from the same lab. Selenium supplementation may modestly lower titers. Antibody levels do not need frequent repeating once positive.
Units and conversion
Usually reported in IU/mL.
- From kIU/L: multiply by 1 to get IU/mL
- From U/mL: multiply by 1 to get IU/mL
Also called on lab reports
Thyroid Peroxidase Antibodies, Thyroid Peroxidase (TPO) Ab, TPO Ab, Anti-TPO, Thyroid Peroxidase Antibodies (TPO), Antithyroid Peroxidase Ab, Thyroperoxidase Antibody, TPOAb
Related tests: Thyroid
This page is educational and does not replace advice from a healthcare professional.