Anti-TPO Antibodies Test | What It Means for Your Thyroid

The anti-TPO antibodies test measures whether your immune system is producing antibodies against thyroid peroxidase (TPO), an enzyme your thyroid needs to manufacture its hormones. It is the most common blood test for diagnosing autoimmune thyroid disease, including Hashimoto thyroiditis and Graves disease.

What Thyroid Peroxidase Does

TPO is an enzyme located on the surface of thyroid follicular cells. It plays a critical role in combining iodine with the protein thyroglobulin to produce the hormones T3 and T4. When the immune system mistakenly produces antibodies against TPO, these antibodies can damage thyroid cells and impair hormone production over time.

What the Test Measures

The test reports the concentration of anti-TPO antibodies in the blood. Results are typically reported in IU/mL:

• Negative (below the laboratory threshold, usually <35 IU/mL) — no significant autoimmune attack on the thyroid
• Positive / elevated — autoimmune thyroid disease is present or developing

The absolute number matters less than its clinical context. Mildly elevated TPO antibodies in a person with normal thyroid function mean something very different from highly elevated antibodies in someone with a rising TSH.

Anti-TPO Antibodies and Hashimoto Thyroiditis

Hashimoto thyroiditis is the most common cause of hypothyroidism in iodine-sufficient countries. Anti-TPO antibodies are present in approximately 90 percent of people with Hashimoto disease, making the test highly sensitive for this condition. The autoimmune attack on the thyroid is gradual, so antibodies often appear years before thyroid function becomes abnormal.

On ultrasound, Hashimoto thyroiditis typically produces a heterogeneous, hypoechoic gland with a coarsened echotexture. Read more about this appearance in our abnormal thyroid ultrasound guide.

Anti-TPO Antibodies and Graves Disease

TPO antibodies are also elevated in approximately 70 percent of patients with Graves disease, though the antibody more specific to Graves is TSH receptor antibody (TRAb). The co-presence of anti-TPO antibodies in a hyperthyroid patient supports an autoimmune cause rather than a toxic nodule or thyroiditis.

High TPO Antibodies With Normal Thyroid Function

Many people, particularly women, test positive for TPO antibodies while maintaining entirely normal TSH and thyroid hormone levels. This is called euthyroid autoimmune thyroiditis. It indicates that the autoimmune process has started but has not yet impaired the gland enough to affect hormone production.

These patients are typically monitored with periodic TSH testing because they carry a higher risk of developing overt hypothyroidism over time, particularly during pregnancy or periods of metabolic stress.

Related Tests

Anti-TPO is usually ordered alongside TSH, T3, and T4. For thyroid cancer monitoring, see thyroglobulin. For medullary thyroid cancer screening, see calcitonin.

Frequently Asked Questions

What are anti-TPO antibodies?

Anti-TPO antibodies are immune system proteins that target thyroid peroxidase, an enzyme essential for thyroid hormone production. Their presence indicates an autoimmune response against the thyroid gland and is the hallmark blood test for autoimmune thyroid disease.

What does a high TPO antibody level mean?

High TPO antibodies indicate that your immune system is producing antibodies against your own thyroid tissue. This is most commonly associated with Hashimoto thyroiditis but can also be found in Graves disease. It does not automatically mean your thyroid function is impaired.

Can you have high TPO antibodies and normal thyroid function?

Yes. Many people, particularly women, have elevated TPO antibodies with completely normal TSH and thyroid hormone levels. This is called euthyroid autoimmune thyroiditis. It indicates an autoimmune process is present but has not yet affected gland function.