Antithyroid antibodies

Anti-thyroid autoantibodies (or simply anti-thyroid antibodies) are autoantibodies targeted against one or more components on the thyroid. The most clinically relevant anti-thyroid autoantibodies are anti-thyroid peroxidase antibodies (anti-TPO antibodies), thyrotropin receptor antibodies (TRAbs) and thyroglobulin antibodies. TRAbs are subdivided into activating, blocking and neutral antibodies, depending on their effect on the TSH receptor. Anti-sodium/Iodide (Anti–Na⁺/I⁻) symporter antibodies are a more recent discovery and their clinical relevance is still unknown. Graves' disease and Hashimoto's thyroiditis are commonly associated with the presence of anti-thyroid autoantibodies. Although there is overlap, anti-TPO antibodies are most commonly associated with Hashimoto's thyroiditis and activating TRAbs are most commonly associated with Graves' disease. Thyroid microsomal antibodies were a group of anti-thyroid antibodies, they were renamed after the identification of their target antigen (TPO).

Anti-thyroid antibodies can be subdivided into groups according to their target antigen.

Anti-thyroid peroxidase (anti-TPO) antibodies are specific for the autoantigen TPO, a 105kDa glycoprotein that catalyses iodine oxidation and thyroglobulin tyrosyl iodination reactions in the thyroid gland. Most antibodies produced are directed to conformational epitopes of the immunogenic carboxyl-terminal region of the TPO protein, although antibodies to linear epitopes have been seen. Anti-TPO antibodies are the most common anti-thyroid autoantibody, present in approximately 90% of Hashimoto's thyroiditis, 75% of Graves' disease and 10-20% of nodular goitre or thyroid carcinoma. Also, 10-15% of normal individuals can have high level anti-TPO antibody titres. High serum antibodies are found in active phase chronic autoimmune thyroiditis. Thus, an antibody titer can be used to assess disease activity in patients that have developed such antibodies. The majority of anti-TPO antibodies are produced by thyroid infiltrating lymphocytes, with minor contributions from lymph nodes and the bone marrow. They cause thyroid cell damage by complement activation and antibody dependent cell cytotoxicity. However, anti-TPO antibodies are not believed to contribute to the destruction of the thyroid.

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