Thyroid Antibodies

Testing for thyroid antibodies, such as thyroid peroxidase antibody (TPO), is primarily ordered to help diagnose an autoimmune 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. (Read more in the "What is being tested?" section.)

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 in Graves 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.

TBII is not routinely tested, but TSI is often used to help diagnose Graves disease.

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.

A low level of thyroid hormones (hypothyroidism) can cause symptoms, such as:

• Weight gain
• Fatigue
• Dry skin
• Hair loss
• Intolerance to cold
• Constipation

A high level of thyroid hormone (hyperthyroidism) can cause symptoms, such as:

• Sweating
• Rapid heart rate
• Anxiety
• Tremors
• Fatigue
• Difficulty sleeping
• Sudden weight loss
• Protruding eyes

The following table summarizes when thyroid autoantibody tests may be done.

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.

Thyroid antibody testing may also be ordered when a person with another autoimmune disorder has symptoms of thyroid dysfunction and/or when she has reproductive difficulties that a healthcare provider suspects may be associated with autoantibodies.

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 thyroiditis and 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.

If a person with thyroid cancer has thyroglobulin antibodies, they may interfere with tests for thyroglobulin levels. This may mean that the thyroglobulin test cannot be used as a tumor marker or to monitor the individual's thyroid cancer. Some methods of testing, including one called mass spectrometry, are not affected by the presence of thyroglobulin antibodies. When measured these ways, the thyroglobulin test can be used as a tumor marker, regardless of whether or not thyroglobulin antibodies are present. However, if a method is used that is affected by thyroglobulin antibodies, then the levels of the antibodies themselves can be used as a tumor marker to monitor thyroid cancer. If they stay high or drop low initially but increase over time, then it is likely that the treatment has not been effective and the cancer is continuing or recurring. If levels are falling and/or have fallen to low or undetectable levels, then it is more likely that the therapy has been effective in eradicating the cancer.

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. If an individual with no apparent thyroid dysfunction has a thyroid antibody, the healthcare provider will track the person's health over time. While most may never experience thyroid dysfunction, a few may develop it.

The sensitivity and specificity of thyroid antibody testing is improving but is still not as good as health practitioners would like it to be. All of the thyroid antibody tests have changed over time. This is part of the reason that the tests have historically acquired many different names. There are also many distinct methodologies and each has different reference (normal) ranges. If someone is having several tests done at regular intervals for monitoring purposes, it is best to have test done by the same laboratory each time, using the same methodology.