Antibodies That Contribute to Thyroid Disease

Thyroid disease has several causes, but the most common ones are autoimmune diseases. These occur when your immune system attacks your own tissues. When this is the case, blood tests will detect autoantibodies, specific immune proteins the body produces to target cells for destruction.

If you have symptoms of thyroid disease, antibody tests may be ordered to look for these proteins. Based on the presence and amount of these antibodies, a lab can determine if an autoimmune disease is likely to be involved and, if so, which one.

For example, the thyroid peroxidase antibody (TPOAb) test can detect autoantibodies strongly suggestive of Hashimoto's disease, which causes hypothyroidism (underactive thyroid). The thyroid stimulating hormone receptor antibody (TSHR-Ab) test can detect autoantibodies strongly suggestive of Graves' disease, which causes hyperthyroidism (overactive thyroid).

This article discusses anti-thyroid antibodies, what they do, and why you may need to undergo testing. It will also help you understand what your test results mean.

Anti-Thyroperoxidase Antibodies (TPOAb)

Anti-thyroperoxidase antibodies (TPOAb) target thyroid peroxidase (TPO). TPO is an enzyme in the thyroid gland that helps produce important thyroid hormones called triiodothyronine (T3) and thyroxine (T4).

TPO uses iodine, an important nutrient, to produce these hormones once given the instruction to do so thyroid stimulating hormone (TSH) released from the pituitary gland, a small organ located in your brain.

Anti-TPO antibodies stop TPO from using iodine. This causes hypothyroidism, in which your thyroid gland doesn't produce enough thyroid hormones.

Anti-TPO antibodies are found in around 90% of people with Hashimoto's disease. However, they are also found in 75% of people with Graves' disease and 10% to 20% of those with thyroid cancer.

To make a definitive diagnosis, other blood tests and examinations may need to be performed if a positive TPOAb test result is received.

By way of example, people with Hashimoto's disease often have low T3 and T4 levels but high TSH levels. This is because the pituitary gland will produce more TSH in an effort to "tell" the thyroid to produce more T3 and T4.

At the same time, it can take time for anti-TPO antibodies to effect changes in TSH levels. In some cases, you can test positive for anti-TPO antibodies for months or years before you are diagnosed with hypothyroidism. Some people with anti-TPO antibodies never become hypothyroid.

Thyroid Stimulating Hormone Receptor Antibodies (TSHR-Ab)

TSH initiates the production of T3 and T4 by binding to molecules on the thyroid gland called TSH receptors. TSH receptor antibodies (TSHR-Ab) imitate the action of TSH and, by targeting these receptors, can trigger hyperthyroidism.

TSHR-Abs are present in over 85% of people with Graves' disease and less commonly with Hashimoto's disease. As such, high levels of TSHR antibodies are strongly suggestive of Graves' disease.

The TSHR-Ab test is also sometimes referred to as the thyroid-stimulating immunoglobulin (TSI) test.

Anti-Thyroglobulin (Anti-Tg) Antibodies

Thyroglobulin (Tg) is a protein made by the thyroid gland that is essential to the production of T3 and T4. Anti-Tg antibodies attack thyroglobulin, reducing the output of thyroid hormones and leading to hypothyroidism.

Anti-Tg antibodies are found in 70% of people with Hashimoto's disease and 30% of people with Graves' disease. Even so, they tend to be more indicative of Hashimoto's when paired with a positive anti-TPO result.

Thyroid Antibody Test Results

Antibody levels can be tested with a blood sample. The antibody test measures the amount of antibody per milliliter (mL) or liter (L) of blood in international units (IU).

Normal values are:

• Thyroid peroxidase antibody (TPOAb) test: Less than 9 IU/mL
  
• Thyroid-stimulating immunoglobulin (TSI) test: Less than 1.75 IU/L
• Thyroglobulin antibody (TgAb) test: Less than 4 IU/mL

What Anti-Thyroid Antibodies Mean for You

Thyroid treatment is not based on antibody levels. Treatment is based on symptoms and TSH, T4, and T3 levels.

Antibody tests are useful in finding the cause of your thyroid disease. They can also help identify subclinical thyroid disease, which is thyroid disease with mild or no symptoms.

Positive thyroid antibodies suggest you could have autoimmune thyroid disease. Still, they are only a piece of the picture. They can help healthcare providers decide if treatment is needed. Healthcare providers will also consider your symptoms, family history, and other blood test results.

The presence of antibodies may confirm subclinical hypothyroidism. Early therapy might prevent disease progression, but this has not been proven.

Summary

Some thyroid conditions are caused by autoimmune disorders. These are conditions where the body produces antibodies that interfere with the thyroid hormone-making process.

Thyroid antibodies may cause the thyroid to make too much or not enough thyroid hormone. An antibody test can determine the cause of your condition.

Thyroid treatment is based on your symptoms. If you do not have symptoms, you may not need treatment.

A Word From Verywell

Autoimmune diseases usually only affect one or a few organs. Still, if you have one autoimmune disease, you have a greater chance of having another one.

Autoimmune thyroid disease can be associated with other conditions thought to have autoimmune causes. This includes diabetes and inflammatory bowel disease.