Testing for thyroid antibodies, such as thyroid peroxidase antibody (TPO), is primarily ordered to help diagnose anautoimmune thyroid disease and to distinguish it from other forms of thyroid dysfunction. Thyroid autoantibodies develop when a person's immune system mistakenly targets components of the thyroid gland or thyroid proteins, leading to chronic inflammation of the thyroid (thyroiditis), tissue damage, and/or disruption of thyroid function. One or more of the following tests may be performed to help establish a diagnosis of and/or monitor an autoimmune thyroid disorder:

Thyroid peroxidase antibody (TPO)—the most common test for autoimmune thyroid disease; it can be detected inGraves disease or Hashimoto thyroiditis.
Thyroglobulin antibody (TGAb)— this antibody targets thyroglobulin, the storage form of thyroid hormones.

Thyroid stimulating hormone receptor antibodies (TSHRAb)—includes two types of autoantibodies that attach to proteins in the thyroid to which TSH normally binds (TSH receptors):

Thyroid stimulating immunoglobulin (TSI) binds to receptors and promotes the production of thyroid hormones, leading to hyperthyroidism.

Thyroid binding inhibitory immunoglobulin (TBII) blocks TSH from binding to receptors, blocking production of thyroid hormones and resulting in hypothyroidism.
These tests may be ordered to help investigate the cause of an enlarged thyroid (goiter) or other signs and symptoms associated with low or high thyroid hormone levels. Testing may be performed as a follow-up when other thyroid test results such as total or free T3, free T4, and/or TSH indicate thyroid dysfunction.

One or more thyroid antibody tests may also be ordered to determine if a person with an autoimmune condition is at risk of developing thyroid dysfunction. This may occur with disorders such as systemic lupus erythematosus, rheumatoid arthritis, or pernicious anemia.

A person who is receiving treatment for thyroid cancer may be monitored with a thyroglobulin test. In this case, the thyroglobulin antibody test is used to determine if the antibody is present in the person's blood and likely to interfere with the test to measure the thyroglobulin level.

Testing may be ordered when an individual has abnormal TSH and/or free T4 test results or signs and symptoms of a low or high level of thyroid hormones or the presence of a goiter, especially if the cause is suspected to be an autoimmune disease.

If a pregnant woman has a known autoimmune thyroid disease (such as Hashimoto thyroiditis or Graves disease) or has another autoimmune disorder and thyroid involvement is suspected, then one or more of the thyroid antibodies may be ordered early in the pregnancy and then again near the end. These tests are used to help determine whether the baby may be at risk of thyroid dysfunction. Thyroid antibodies can cross the placenta and cause hypothyroidism or hyperthyroidism in the developing baby or newborn.

Negative test results means that thyroid autoantibodies are not present in the blood at the time of testing and may indicate that symptoms are due to a cause other than autoimmune. However, a certain percentage of people who have autoimmune thyroid disease do not have autoantibodies. If it is suspected that the autoantibodies may develop over time, as may happen with some autoimmune disorders, then repeat testing may be done at a later date.

Mild to moderately elevated levels of thyroid antibodies may be found in a variety of thyroid and autoimmune disorders, such as thyroid cancer, type 1 diabetes, rheumatoid arthritis, pernicious anemia, and autoimmune collagen vascular diseases.

Significantly increased concentrations most frequently indicate thyroid autoimmune diseases such as Hashimoto thyroiditisand Graves disease.

In general, the presence of thyroid antibodies suggests the presence of an autoimmune thyroid disorder and the higher the level, the more likely that is. Levels of autoantibody that rise over time may be more significant than stable levels as they may indicate an increase in autoimmune disease activity. All of these antibodies, if present in a pregnant woman, can increase the risk of hypothyroidism or hyperthyroidism in the developing baby or newborn.

A certain percentage of people who are healthy may be positive for one or more thyroid antibodies. The prevalence of these antibodies tends to be higher in women, tends to increase with age, and for thyroid peroxidase antibodies, indicates an increased risk of developing thyroid disease in the future.