Antithyroid antibodies are a type of antibody produced by the immune systems of people with a specific type of autoimmune disease. As a result of production of antithyroid antibodies, the immune system attacks cells of the thyroid gland, leading to impaired thyroid function and hypothyroidism. Certain autoimmune diseases can cause the development of these types of antibodies.
Bursa-dependent lymphocytes, commonly called B lymphocytes, are cells of the immune system that generate protein molecules called antibodies. These proteins are specifically targeted to attach to foreign proteins such as those from bacteria and viruses. The attachment of antibodies to proteins on bacteria or viruses makes the pathogens vulnerable to destruction by other cells of the immune system.
In a healthy immune system, B lymphocytes generate antibodies that are specific only for foreign cells. Sometimes, however, the immune system becomes dysfunctional in a way that causes B lymphocytes to generate antibodies to proteins in the body. These antibodies are called autoantibodies. In theory, almost any protein in the body can be targeted by autoantibodies. In practice, some proteins tend to become targets more frequently than others.
The reasons why the immune system becomes dysfunctional this way are not well understood. One possibility is that it occurs as the result of an immune response to a pathogen. This theory suggests that some of the proteins from the pathogen resemble certain body proteins closely enough that antibodies generated to combat the pathogen also react against the body proteins. After the infection is over, the immune system might be triggered to attack the body proteins.
Certain proteins of the thyroid gland are included in this category. The thyroid is a small gland located in front of the windpipe, which produces hormones that regulate metabolism. Antithyroid antibodies tend to interfere heavily in thyroid function, sometimes causing conditions called hyperthyroidism and hypothyroidism. In hyperthyroidism, the thyroid gland becomes overactive, which causes symptoms such as unintended weight loss, rapid heart rate, tremors, anxiety, insomnia and fatigue. Hypothyroidism reduces thyroid activity, causing weight gain, fatigue, cold intolerance, hair loss, dry skin and constipation.
There are some specific autoimmune disorders caused by antithyroid antibodies. These are Hashimoto’s thyroiditis and Graves’ disease. Hashimoto’s thyroiditis, a type of chronic thyroid inflammation, is a common cause of hypothyroidism. Graves’ disease causes hyperthyroidism and is the most common cause of an overactive thyroid gland. These thyroid autoimmune disorders are diagnosed using tests designed to detect levels of antithyroid antibodies in the blood.