TPO antibodies (anti-TPO) test (ATPO)

Anti-TPO are antibodies that the immune system mistakenly produces against thyroid peroxidase — an enzyme needed to produce thyroid hormones. It is one of the two thyroid autoantibodies tested most often, together with antibodies against thyroglobulin (anti-Tg). According to the American Thyroid Association (ATA), positive anti-TPO and/or anti-Tg antibodies together with hypothyroidism allow Hashimoto's thyroiditis to be diagnosed — the most common cause of hypothyroidism in countries where iodine is not in short supply. Anti-TPO matters for establishing the reason why TSH or free T4 has changed, but the ATA notes an important limit: following the antibody level over time does not help to predict whether hypothyroidism will develop, or to assess the response to treatment. TSH and FT4 are used for that, not the antibodies themselves. Anti-TPO can also be raised in Graves' disease or other autoimmune disorders, although there antibodies against the TSH receptor (TRAb) are tested more often. A small proportion of healthy people, especially women, have a slightly raised anti-TPO without any disorder of thyroid function.

• When a general thyroid test (TSH, free T4) shows hypothyroidism — to establish whether the cause is autoimmune (Hashimoto's thyroiditis)
• When TSH is slightly raised while free T4 is still normal (subclinical hypothyroidism) — the antibodies help to assess whether the condition may progress
• When an autoimmune thyroid disease is suspected because of family history or other autoimmune diseases (for example type 1 diabetes, coeliac disease)
• When planning a pregnancy or at the beginning of pregnancy, especially if there have been thyroid disorders or miscarriages before
• When the thyroid is enlarged (goitre) or its structure has changed on ultrasound, and an autoimmune cause has to be distinguished

A raised anti-TPO points to an autoimmune process in the thyroid. According to the American Thyroid Association, positive anti-TPO antibodies together with hypothyroidism mean Hashimoto's thyroiditis — the most common cause of hypothyroidism. MedlinePlus states that the more antibodies there are, the greater the likelihood of having an autoimmune thyroid disorder, but a small proportion of healthy people also have a raised anti-TPO.

What can lie behind a raised result

• Hashimoto's thyroiditis — the most common autoimmune cause of hypothyroidism
• Graves' disease and other autoimmune thyroid disorders
• Postpartum thyroiditis
• Other autoimmune diseases, in which the body more often produces various autoantibodies

What to discuss with a doctor

• TSH and free T4 (FT4) — it is these, and not the anti-TPO level itself, that show the actual thyroid function and the response to treatment (ATA)
• Thyroid ultrasound, if changes in structure are suspected
• Repeated monitoring of TSH over time, especially if anti-TPO is positive but function is still normal
• When a pregnancy is being planned — discussing with a doctor how often to monitor

Whether anti-TPO is repeated to monitor treatment

Not according to the ATA recommendation — following the antibody level over time does not help to assess the response to treatment or to predict whether hypothyroidism will progress. TSH and free T4 tests are used for that, and anti-TPO is usually tested once, during the initial diagnosis.

How anti-TPO differs from anti-Tg

Both are the most commonly tested thyroid autoantibodies and are often tested together. The ATA states that a positive result for either of them (or both) together with hypothyroidism allows Hashimoto's thyroiditis to be diagnosed. In some laboratories anti-TPO is considered a more sensitive marker than anti-Tg.

A raised anti-TPO when the thyroid works normally

MedlinePlus states that a small proportion of healthy people have a raised anti-TPO without any disorder of thyroid function. In that case a doctor usually recommends monitoring TSH periodically, because the autoimmune process may affect function over time.