Anti-TPO Antibody
Anti-thyroid peroxidase antibodies target the enzyme that makes thyroid hormone. They are the hallmark of Hashimoto's thyroiditis (the most common cause of hypothyroidism) and are also found in Graves' disease and postpartum thyroiditis. Anti-TPO is reported numerically but interpreted as positive or negative against a cutoff. Many people with positive anti-TPO are clinically euthyroid but carry a higher lifetime risk of developing hypothyroidism.
Reference Ranges
Male
0 – 34
IU/mL
Female
0 – 34
IU/mL
Child
0 – 34
IU/mL
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
Anti-TPO below 34 IU/mL is considered negative and makes autoimmune thyroid disease unlikely. A negative result in someone with hypothyroidism points to non-autoimmune causes such as iodine deficiency, post-surgical or post-ablation hypothyroidism, or medications.
What High Levels Mean
Anti-TPO above 34 IU/mL confirms autoimmune thyroid disease — most commonly Hashimoto's thyroiditis. Levels of several hundred to several thousand IU/mL are typical. In pregnancy, positive anti-TPO raises the risk of postpartum thyroiditis (tested at 6–12 weeks postpartum) and miscarriage, and usually triggers closer thyroid monitoring during and after pregnancy.
Frequently Asked Questions
I have positive anti-TPO but normal thyroid — should I worry?
Up to 10–15% of healthy adults have positive anti-TPO without thyroid dysfunction. It does not need treatment, but it raises the risk of developing hypothyroidism later — roughly 2–4% per year. Annual TSH testing is reasonable, as is testing during pregnancy because subclinical hypothyroidism in pregnancy has consequences for the fetus.
Does the level of anti-TPO predict disease severity?
Not directly. A very high anti-TPO (thousands of IU/mL) confirms autoimmunity but does not correlate tightly with how low your thyroid hormone will fall. TSH and Free T4 track disease severity; anti-TPO mainly establishes the cause. Levels often decline with time and do not need repeat testing once positive.
Can anti-TPO be checked during pregnancy?
Yes — and it is recommended in women with a history of thyroid disease, recurrent miscarriage, or a TSH in the upper-normal range (above 2.5 mIU/L). Positive anti-TPO in pregnancy is associated with higher miscarriage risk and postpartum thyroiditis. Close TSH monitoring and occasionally low-dose levothyroxine are used based on TSH trajectory.
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This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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