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When the body attacks certain parts of its own thyroid gland, evidence of this can be found in the presence of abnormal molecules called thyroid peroxidase (TPO) antibodies. Other terms for these substances are antithyroid microsomal antibodies or antiperoxidase antibodies. Elevated levels are most closely associated with Hashimoto's thyroiditis, but can also be seen with Graves' disease. A certain percentage of asymptomatic people without thyroid problems test positive for this antibody, and these people are at increased risk for developing thyroid abnormalities in the future.
In general, the presence of thyroid peroxidase antibodies in the blood is an abnormal finding. Antibodies are proteins made by the immune system that help fight off bacteria or other threats to the body's health. Although the body normally only makes antibodies against foreign substances, in a certain class of illnesses called the autoimmune disorders the body makes antibodies against elements of itself. With production of the TPO antibodies, the body is making a protein that attacks a molecule important to the function of the thyroid, the thyroid peroxidase enzyme. This substance is essential for creating thyroglobulin, which is a protein that carries the thyroid hormone through the blood to distant parts of the body.
A positive result for the presence of thyroid peroxidase antibodies is most often found in autoimmune thyroiditis, a condition also known as Hashimoto's thyroiditis. Patients with this disease create antibodies that attack the thyroid gland. They eventually experience a decrease in the production of the thyroid hormone, a condition known as hypothyroidism. Many doctors consider that finding positive TPO antibodies in a patient who has hypothyroidism to be diagnostic of Hashimoto's thyroiditis. The mainstay of treatment for this condition is to provide the patient with supplemental thyroid hormone in pill form.
Having thyroid peroxidase antibodies is also associated with a number of other conditions. Patients with Graves' disease, an autoimmune thyroid disorder that causes the thyroid to overproduce the thyroid hormone, can have thyroid peroxidase antibodies present in their blood. Some studies have suggested that women who produce this antibody are at increased risk of developing reproductive problems such as early delivery in pregnancy, miscarriages, and infertility.
Some normal, asymptomatic people can have detectable levels of thyroid peroxidase antibodies. As much as 5 to 10 percent of the general population would have positive results on this laboratory test, if checked. Although many times these people do not have any symptoms, they could be at risk for having problems with their thyroid function in the future. Some doctors check this antibody level before starting certain medications that could lead to hypothyroidism, including amiodarone and lithium, because they might choose not to use these drugs in patients already at risk for developing problems with thyroid hormone production.