The immune system may produce antithyroid antibodies that destroy thyroid tissue. An antithyroid antibody test determines whether you have these antibodies.

If you test positive for antithyroid antibodies, you may have Hashimoto's thyroiditis, the most common cause of hypothyroidism. However, many people with hypothyroidism never test positive for antithyroid antibodies. Also, people who test positive for antithyroid antibodies may never develop hypothyroidism. Some people who test positive for antithyroid antibodies develop hyperthyroidism (Graves' disease).

If you have mild (subclinical) hypothyroidism, the presence of antithyroid antibodies can help predict how likely it is that you will develop hypothyroidism that eventually causes symptoms.