Clinical and laboratory aspects of thyroid autoantibodies.
This review describes the aetiology of the major thyroid antigens. Iodination of thyroglobulin produces multiple antigen configurations which are functionally active but immunologically distinct. The thyroid stimulating hormone (TSH) receptor is a two-subunit glycoprotein; the extracellular A subunit is recognized by thyroid stimulating antibodies, while those antibodies recognizing the B subunit, located much nearer the cell surface, appear to function as blocking antibodies. Thyroid peroxidase (TPO), originally described as thyroid microsomal antigen, is present on the apical surface of thyroid follicular cells and is the antigen involved in cell-mediated cytotoxicity. Multiple B-cell-reactive epitopes exist, each giving rise to different antibodies. The aetiology and mechanics of the autoimmune cellular and antibody responses involves a combination of human leucocyte antigen (HLA) linkage, genetics and environmental factors to determine the initial and subsequent stages of the development of autoimmune thyroid disease. Depending on the antibody, a combination of enzyme-linked immunosorbent assay for TPO and thyroglobulin and bioassays and/or radioimmunoassay for TSH receptor antibodies are used to estimate their concentrations. The other conditions with which autoimmune thyroid diseases are associated include, for example, pernicious anaemia, connective tissue disorders, diabetes, coeliac disease, mood disorders like depression and fertility-related problems such as miscarriage, infertility, in vitro fertilization failure, pre-term delivery and postpartum thyroiditis. Often, there is no cause-and-effect relationship between them and it is debatable in some cases whether it is worthwhile monitoring patients with autoimmune thyroid disease for other conditions or vice versa. The review also itemizes the circumstances in which it might be useful to measure each antibody (i.e. the use of TPO antibodies in investigation of goitre, diagnosis of Graves' and Hashimoto's disease and the prediction of risk of developing hypothyroidism during subclinical thyroid disease; TSH receptor antibodies in maternal and neonatal hyperthyroidism and thyroglobulin antibodies in the monitoring and treatment of thyroid carcinoma). Finally, taking the current literature into account, an algorithm is recommended for the most effective use of these antibodies in the investigation of autoimmune thyroid disease.
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