What are TPO antibodies?
TPO antibodies, also known as thyroid peroxidase antibodies or anti-TPO antibodies, are autoantibodies directed against the enzyme thyroid peroxidase (TPO) in the thyroid gland. TPO plays an important role when the thyroid gland produces the hormones thyroxine (T4) and triiodothyronine (T3).
When the immune system produces antibodies against TPO, it may be a sign of an autoimmune reaction against the thyroid gland. TPO antibodies are most commonly associated with Hashimoto's thyroiditis, but they can also occur in other types of autoimmune thyroid disease.
Why are TPO antibodies analyzed?
TPO antibodies are analyzed primarily to determine whether altered thyroid function may be caused by an autoimmune thyroid disease.
In Hashimoto's thyroiditis, the immune system attacks the tissue of the thyroid gland. The resulting inflammation can gradually impair the thyroid gland's ability to produce hormones and lead to hypothyroidism, meaning an underactive thyroid gland.
TPO antibodies may therefore be analyzed together with, for example, TSH and free T4 when investigating suspected thyroid disease. TPO antibodies may also occur in Graves' disease, although other antibodies, particularly TSH receptor antibodies (TRAb), are more specific when evaluating Graves' disease.
What do elevated TPO antibodies mean?
Elevated TPO antibodies suggest that there is an autoimmune reaction directed against the thyroid gland. This is particularly common in Hashimoto's thyroiditis.
However, a positive TPO antibody result does not automatically mean that thyroid function is reduced. Some people have TPO antibodies despite having normal levels of TSH and thyroid hormones and may be completely free of symptoms.
The presence of TPO antibodies is also associated with an increased risk of developing thyroid dysfunction in the future. The result should therefore be assessed together with factors such as TSH, free T4, symptoms, and medical history.
What do normal or low TPO antibody levels mean?
A normal or negative TPO antibody result means that no TPO antibodies, or only low levels, have been detected. This makes autoimmune thyroid disease less likely, but does not completely rule it out.
TPO antibodies primarily indicate whether there are signs of autoimmunity against the thyroid gland. They do not, by themselves, show whether the thyroid gland is producing too much or too little hormone. TSH and free T4 are primarily used to assess thyroid function itself.
TPO antibodies and Hashimoto's thyroiditis
Most people with Hashimoto's thyroiditis have TPO antibodies, which makes them an important marker in the diagnostic evaluation. In Hashimoto's thyroiditis, inflammation can gradually damage the thyroid gland and lead to reduced production of thyroid hormones.
Common symptoms of established hypothyroidism may include fatigue, sensitivity to cold, dry skin, constipation, weight gain, and difficulty concentrating. However, these symptoms are caused by altered thyroid function rather than by the level of TPO antibodies itself.
Do TPO antibodies need to be monitored over time?
Once TPO antibodies have been detected in autoimmune thyroid disease, there is usually limited value in repeatedly measuring the antibody level itself. The level may vary without directly reflecting how the thyroid gland is functioning in practice.
During follow-up, TSH and free T4 are instead primarily used to assess thyroid hormone production and any development of hypo- or hyperthyroidism.
