Thyroid Peroxidase Antibody (Anti-TPO)

What is Anti-TPO?

Thyroid peroxidase (TPO) is an enzyme that the thyroid gland needs for hormone production; it enables iodine to be incorporated into the structure of thyroid hormones. In some people, the immune system perceives this enzyme as foreign and produces antibodies against it. The Anti-TPO test measures the level of these antibodies in the blood using an immunological method. In older sources, these antibodies are also referred to as antimicrosomal antibodies.

Anti-TPO is one of the most sensitive indicators that the immune system has started a process directed at the thyroid gland. The test does not show the amount of thyroid hormones but whether the gland is affected by an autoimmune process. For this reason, it is usually evaluated together with TSH and free T4. The antibodies can appear years before any disturbance in thyroid hormones begins.

When is the Anti-TPO test ordered?

Doctors most often order this test when an underactive thyroid gland has been detected, to find out whether it has an autoimmune cause. In people whose TSH is slightly high but whose free T4 is normal, it also gives an idea of the likelihood of developing overt hypothyroidism in the future. Because Anti-TPO positivity in pregnancy may call for closer monitoring of thyroid function, the test may also be ordered for some women who are planning a pregnancy. It can also be used for assessment before starting certain medicines that can affect the thyroid gland.

• Elevated TSH or a diagnosis of hypothyroidism
• Suspected Hashimoto's thyroiditis
• Goiter or enlargement of the thyroid gland
• Women planning a pregnancy, or pregnant women with a thyroid problem
• Investigation of recurrent miscarriage or infertility
• Thyroid assessment in autoimmune diseases such as type 1 diabetes and celiac disease

How do you prepare for the Anti-TPO test?

No fasting is required for the Anti-TPO test, and the blood sample can be given at any time of day. Because antibody levels do not change significantly during the day, you do not need to choose a specific time. If free T4 will also be run on the same sample, it is best to take your thyroid medication after your blood draw. Once the antibody test has been found positive, it usually does not need to be repeated often unless your doctor specifically requests it.

Supplements containing high-dose biotin (vitamin B7) can affect the antibody result in a misleading way with some immunological methods; if you use such a product, talk to your doctor before giving blood. The other medicines you take are not expected to have a significant effect on this test, but it is still useful to share your medication list with your doctor. You can also give your blood sample during the nurse's visit, without leaving your home.

What does the Anti-TPO result mean?

A high (positive) Anti-TPO indicates an autoimmune process affecting the thyroid gland. The most common cause is Hashimoto's thyroiditis; it is also frequently positive in Graves' disease. Even when thyroid hormones are normal, positivity may point to an increased risk of developing hypothyroidism in the future, and in pregnancy it calls for attention with regard to postpartum thyroid inflammation.

Mild positivity can also be seen in some people without thyroid disease, especially in women and older adults. A negative result does not completely rule out autoimmune thyroid disease, and a high antibody level does not indicate how severe the disease is. Cut-off values can differ between laboratories and, in part, by age and sex; the laboratory's own reference range is shown on this page. The meaning of the result should be determined by your doctor, who evaluates it together with your TSH and free T4 values and your symptoms.