What does it measure?
Thyroid peroxidase is an enzyme used to make thyroid hormones. Anti-TPO measures antibodies directed against it. Clinicians often use it to help understand the cause of an abnormal thyroid-function result or a suspected autoimmune thyroid condition.
Before the test
- Sample
- A blood sample from a vein.
- Companion results
- Bring your TSH and free T4 results, if available.
What can the result mean?
A positive result can support autoimmune thyroid disease, especially Hashimoto thyroiditis, but it can also occur when thyroid hormone levels are still normal. A higher antibody value does not directly tell us how underactive the thyroid is or whether treatment is needed. [1]
When is it useful, and how is it read?
Use anti-TPO to help explain a suspected autoimmune cause, then read it with thyroid function, symptoms, and pregnancy plans. Positive antibodies with normal thyroid function have a different meaning from antibodies alongside clear hypothyroidism. [1]
What can affect it?
TSH, free T4, symptoms, and pregnancy or pregnancy plans are important context. Laboratory cutoffs differ. Some supplements can interfere with certain assay systems, so provide your supplement list before testing.
Myth: «More antibodies means a more underactive thyroid»
The antibody level and thyroid hormone output are different things. TSH and free T4 are more useful for showing how the thyroid is functioning and whether hormone treatment is needed. [1]
Questions you might have
Do positive antibodies always mean I need thyroid hormone?
No. Treatment depends on thyroid function and the clinical setting, not the antibody number alone. [1]
Do antibodies need repeating to follow Hashimoto’s?
Often not. After the diagnosis, TSH monitoring is usually more useful than repeatedly measuring antibody levels. [1]
What comes next?
If antibodies are positive, agree on an appropriate thyroid-function follow-up with your clinician. Decisions about thyroid medicine depend on the functional results and your situation rather than an aim to make the antibody number disappear. [1]
Are my thyroid hormones working normally, and what monitoring makes sense with these antibodies?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
