Anti-TPO Antibody Test: Normal Range

What Is Anti-TPO?

Thyroid peroxidase (TPO) is an enzyme involved in making thyroid hormone. Anti-TPO antibodies are produced by the immune system mistakenly attacking the thyroid gland: a hallmark of autoimmune thyroid disease.

Normal Anti-TPO Range

Result	Range
Negative (normal)	Below 35 IU/mL
Positive (elevated)	35 IU/mL and above

What a Positive Result Means

Hashimoto's Thyroiditis

The most common cause of an underactive thyroid (hypothyroidism) in adults: anti-TPO is positive in roughly 90–95% of cases.

Graves' Disease

An autoimmune cause of overactive thyroid (hyperthyroidism); anti-TPO can also be positive, alongside TSI/TRAb antibodies.

When Is It Tested?

Abnormal TSH, T3 or T4 results
Goitre (enlarged thyroid)
Family history of autoimmune thyroid disease
Unexplained fatigue, weight changes or hair loss
During pregnancy screening for thyroid risk

Frequently Asked Questions

What does a positive anti-TPO result mean?

Raised anti-TPO antibodies indicate autoimmune thyroid disease: most often Hashimoto's thyroiditis (underactive thyroid) or, less commonly, Graves' disease. They confirm the cause is autoimmune rather than another thyroid problem.

Can anti-TPO be positive with normal thyroid function?

Yes. Many people have positive antibodies with a normal TSH: this signals an increased future risk of developing thyroid dysfunction, so periodic monitoring is usually advised.

Should anti-TPO be re-tested over time?

There's usually no need to repeat it: once positive, antibody status rarely changes management. Monitoring focuses on TSH and free T4 to track thyroid function itself.

Who should have anti-TPO tested?

Anti-TPO testing is not part of routine screening. It is most useful once a thyroid abnormality has already been found on TSH or free T4, to establish whether the cause is autoimmune, which changes the likely course and the need for long-term monitoring.

It is also considered in women with recurrent miscarriage or subfertility, in people with other autoimmune conditions such as type 1 diabetes or coeliac disease, and in pregnancy where borderline thyroid results are found, because antibody-positive women are more likely to progress to overt hypothyroidism and may warrant closer follow-up.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

Subclinical Hypothyroidism. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK536970

Anti-TPO Antibody Test: Normal Range & Thyroid Autoimmunity