Antibodies That Contribute to Thyroid Disease

While there are a number of different thyroid disorders, autoimmune antibodies against the thyroid gland are among the most well-understood causes of thyroid disease. Because of this, your doctor may order thyroid antibody tests for you. These results can help identify the root cause of your thyroid disease and may factor into a treatment plan for your condition.

Antibodies are proteins produced by your body to help protect you against infections. They can, however, mistakenly attack your own tissues, causing illness. The consequence is described as an autoimmune disease, and some thyroid conditions are caused by this immune system dysfunction.

The most common thyroid antibodies are:

• Anti-thyroperoxidase (TPO) antibodies
• Thyroid stimulating hormone (TSH) receptor (TSHR-Ab) antibodies
• Anti-thyroglobulin (anti-Tg) antibodies

The most common thyroid antibodies attack thyroid peroxidase. Also called thyroperoxidase (TPO), this enzyme functions in the thyroid gland to help produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3).

Autoimmune antibodies can interfere with TPO's ability to use iodine to produce these hormones, resulting in hypothyroidism. TPO antibodies cause inflammation, can eventually destroy all or part of your thyroid gland, and can also cause your thyroid gland to form nodules or to become enlarged.

The presence of anti-TPO antibodies is associated with pre-term labor and with Hashimoto's thyroiditis, an autoimmune thyroid condition.

It can take time for the destructive effect on your thyroid gland to be reflected in your thyroid-stimulating hormone (TSH) level. It's not uncommon to have positive TPO antibodies for months or years before your TSH level rises to a point where you are diagnosed with hypothyroidism. It's important to mention, as well, that some people never progress to being hypothyroid, despite having positive TPO antibodies.

TSH, a hormone released by the pituitary gland in the brain, stimulates the thyroid gland to make thyroid hormone. TSH initiates this process by binding to TSH receptors on the thyroid gland. TSH receptor antibodies (TSHR-Ab) can imitate the action of TSH, causing excess thyroid hormone production. High TSHR-Ab levels are associated with Grave's disease, an autoimmune condition that usually causes hyperthyroidism.

Thyroglobulin (Tg) is a protein that helps the thyroid gland function properly. Anti-Tg antibodies are associated with Hashimoto's thyroiditis.

Antibody levels can be determined by analyzing a blood sample. Normal values are as follows:

• TPO antibody: The measured serum level should be less than 9 IU/mL.
• Anti-Tg antibody: The measured serum level should be less than 4 IU/mL.
• Thyroid-stimulating immunoglobulin antibody (TSI): This value should be less than 1.75 IU/L.

Note, however, that normal range values may differ based on the laboratory where you are getting your test.

Generally speaking, your thyroid treatment is not based on antibody levels—it is based on your symptoms and thyroid hormone levels. However, antibody tests can be useful in assessing the cause of your thyroid disease and can help identify subclinical thyroid disease.

Positive thyroid antibodies suggest that you could have autoimmune thyroid disease, but they are only a piece of the picture. While they may influence the decision to move ahead with treatment, other factors in addition to your symptoms—family history, other blood test results—will be considered, too.

Sometimes, the presence of antibodies supports the diagnosis of subclinical hypothyroidism, which is thyroid disease with minimal symptoms or without symptoms. Early therapy for subclinical thyroid disease may prevent its progression, but this has not been validated.

Usually, autoimmune disease affects one or a few organs in the body. But if you have one autoimmune disease, it increases your chances of having another as well. Autoimmune thyroid disease can be associated with other conditions that are believed to have an autoimmune etiology, like diabetes and inflammatory bowel disease.