Anti-TPO Antibody Test: Normal Range & Thyroid Autoimmunity

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

What a Positive Result Means

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

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.

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

References

Sources cited on this page. PubMed links open the original abstract.

1. Stagnaro-Green A, Abalovich M, Alexander E, et al. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2011;21(10):1081–1125. PMID 12490590 · doi:10.1089/thy.2011.0087

What TPO antibodies mean – and when they don't cause symptoms

Anti-thyroid peroxidase antibodies (anti-TPO, also called thyroid microsomal antibodies) are the immune system's attack on thyroid peroxidase – the key enzyme that builds thyroid hormones. Their presence marks autoimmune thyroid disease, but the relationship between antibody level and symptoms is not straightforward.

Anti-TPO antibodies are found in:

• Approximately 95% of patients with Hashimoto's thyroiditis (autoimmune hypothyroidism)
• 80–90% of patients with Graves' disease (autoimmune hyperthyroidism)
• 10–15% of the general population with no thyroid dysfunction at all – "euthyroid anti-TPO positivity"

This last group is clinically important: a positive anti-TPO result does not mean a person has or will develop thyroid disease. However, euthyroid anti-TPO positivity does increase the lifetime risk of developing hypothyroidism – estimated at 2–4% per year – warranting annual TSH monitoring rather than no follow-up.

Hashimoto's thyroiditis – the most common autoimmune disease

Hashimoto's thyroiditis is the most common autoimmune condition in developed countries and the leading cause of hypothyroidism. The autoimmune process slowly destroys thyroid follicles over years to decades, progressively reducing thyroid hormone output. Characteristically:

• Early disease: TSH may be borderline elevated, with normal free T4. Antibodies are detectable but symptoms may be absent.
• Established hypothyroidism: TSH clearly elevated (above 10 mU/L in clinical hypothyroidism), free T4 low.
• Fluctuating phase ("hashitoxicosis"): Rarely, the gland releases stored hormone as follicles are destroyed, causing temporary hyperthyroid symptoms before the longer-term hypothyroid state establishes.

Anti-TPO levels do not predict the rate of progression to hypothyroidism and do not need to be repeated once the diagnosis is established – ongoing management is guided by TSH, not antibody titres. Thyroid antibody levels naturally tend to fall once levothyroxine treatment begins, but this has no clear clinical significance.

Anti-TPO in pregnancy

Anti-TPO positivity in pregnancy – even with a normal TSH – is associated with an increased risk of miscarriage, premature birth, and postpartum thyroiditis. The 2017 American Thyroid Association guidelines recommend TSH measurement in pregnant women who are anti-TPO positive. Postpartum thyroiditis occurs in approximately 5–7% of all women (rising to 25–50% in anti-TPO-positive women) – typically presenting as a transient hyperthyroid phase 1–4 months after delivery followed by a hypothyroid phase, usually resolving within a year.