When people receive abnormal thyroid test results, the term “TPO antibodies” can be unfamiliar and concerning. TPO stands for thyroid peroxidase, an enzyme needed for normal thyroid hormone production.

TPO antibodies are immune proteins that mistakenly target thyroid peroxidase. Their presence can be an important clue that the immune system is reacting against the thyroid.

The most important cause to understand is Hashimoto’s thyroiditis. Hashimoto’s is an autoimmune thyroid disorder and is a major cause of hypothyroidism. TPO antibodies are commonly present in people with Hashimoto’s.

Another condition associated with elevated TPO antibodies is Graves’ disease. Graves’ disease is also autoimmune, but it generally causes the thyroid to become overactive.

There are also situations in which thyroid antibodies may be detected without obvious thyroid dysfunction. A person may have positive antibodies while TSH and free T4 remain within the laboratory reference range. This is one reason antibody results need to be interpreted in context.

Thyroid inflammation following pregnancy, sometimes called postpartum thyroiditis, can also involve thyroid autoantibodies.

It is important not to assume that a high TPO number automatically tells you how severe your thyroid condition is. Antibody levels and thyroid hormone levels are different measurements.

If TPO antibodies are elevated, healthcare professionals may review TSH, free T4, symptoms, family history, and other relevant information.

Knowing the causes of high TPO antibodies is useful, but the next step should always be understanding what the complete thyroid evaluation shows.