What is the Thyroid Peroxidase Normal Range?

Thyroid peroxidase (TPO) is an enzyme found in the thyroid gland that plays an essential role in the production of thyroid hormones T3 and T4. A blood test measuring thyroid peroxidase antibodies, or TPO antibodies, is commonly used to help diagnose and monitor autoimmune thyroid conditions like Hashimoto’s thyroiditis. But what constitutes a normal TPO antibody level, and what do your results mean? This in-depth guide examines the normal reference range, factors that influence TPO antibodies, and the significance of abnormal test findings.

What is Thyroid Peroxidase?

Thyroid peroxidase (TPO) is an enzyme located on the follicle cells of the thyroid gland. Its primary function is to catalyze reactions that are crucial for the synthesis of the thyroid hormones triiodothyronine (T3) and thyroxine (T4).

Specifically, TPO facilitates the iodination of tyrosine amino acid residues within the thyroglobulin protein produced by thyroid follicular cells. This iodination of tyrosine is an essential step leading to the production of T3 and T4 hormones. These hormones then enter the bloodstream and regulate metabolism, growth, development, and various cellular processes.

About the TPO Antibody Blood Test

The thyroid peroxidase antibody blood test specifically measures the concentration of antibodies directed against thyroid peroxidase. The presence of TPO antibodies indicates an autoimmune response targeting the thyroid.

Elevated TPO antibody levels are associated with autoimmune thyroid disorders like Hashimoto’s thyroiditis and Graves’ disease. The test can aid in the diagnosis of these conditions and help monitor antibody titer trends during treatment.

What is the Normal TPO Antibody Reference Range?

While the upper limit of the normal reference range for TPO antibodies can vary slightly between laboratories, it is generally considered normal when levels are below 35 IU/mL.

TPO antibody levels above the normal reference range indicate the likely presence of an autoimmune thyroid disorder. The higher the antibody titer, the more severe the autoimmune attack on the thyroid tissue is likely to be.

Can TPO Antibody Levels Be Too Low?

Extremely low TPO antibody levels are not typically a cause for clinical concern on their own. The focus is more on elevated antibody levels and what they signify.

In most cases, clinicians will be monitoring for elevations in TPO antibodies rather than decreases below the normal threshold. However, in select clinical contexts, such as the family history of autoimmune disease, persistently low levels may warrant continued monitoring.

What Factors Can Impact TPO Antibody Levels?

Certain factors and conditions can lead to fluctuations in TPO antibody levels:

  • Hashimoto’s thyroiditis drives increased TPO antibody production.
  • Graves’ disease can also increase TPO antibodies, although TSI antibodies are more specific.
  • Pregnancy can cause temporary elevation of thyroid autoantibodies like TPO.

What Does an Abnormal TPO Antibody Test Mean?

The implications of abnormal TPO antibody levels are:

  • Elevated TPO Antibodies: This indicates you likely have an autoimmune thyroid disorder such as Hashimoto’s thyroiditis or Graves’ disease. Your doctor will investigate further based on your symptoms and other test results.
  • Low TPO Antibodies: This is typically normal and not a cause for concern alone. But your doctor can likely monitor levels periodically if you have a family history of thyroid disease or symptoms warrant continued surveillance.

Monitoring TPO Antibodies Over Time

When monitoring a patient with autoimmune thyroid disease, a physician may check TPO antibody levels multiple times over months or years. This allows them to:

  • Track antibody titer trends over time in an individual patient.
  • Evaluate the immune response and autoimmune activity.
  • Assess the patient’s response to treatment such as thyroid hormone replacement.
  • While absolute values can vary, looking at an overall pattern is most informative when tracking TPO antibodies longitudinally in a patient.


Always discuss your thyroid peroxidase antibody test results with the doctor who ordered them. They can interpret your levels within the context of your medical history, symptoms, and other thyroid function tests.

Remember that the “normal” reference range only indicates typical values for the general population. Your personal optimal TPO antibody level depends on your individual health status. Monitoring trends over time provides the most clinical value when evaluating thyroid autoimmunity. With appropriate medical guidance, you can understand the implications of your TPO antibody levels.


  1. “Thyroid Peroxidase Antibody Test: Purpose, Procedure, Results “
  2. Thyroid Peroxidase Antibodies (TPO) Range & Symptoms | Ada
  3. Thyroid peroxidase antibody test: What is it? – Mayo Clinic
  4. Anti-TPO Ab (RDL) – Lab Results explained |
  5. Hashimoto’s Thyroiditis

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