[Diagnosis of autoimmune thyroid disease]

Abstract

Autoimmune thyroid disease (AITD) is the most common organ specific autoimmune disorder usually resulting in dysfunction (hyperfunction, hypofunction or both) of the thyroid gland. The syndromes comprising autoimmune thyroid disease are many intimately related illnesses: Graves' disease with goitre, hyperthyroidism and, in many patients, associated ophthalmopathy, Hashimoto's thyroiditis with goitre and euthyroidism or hypothyroidism but also thyroid dysfunction occurring independently of pregnancy and in 5-6% of postpartum women and thyroiditides induced by different drugs and other environmental influences. The immunological mechanisms involved in these diseases are closely related, while the phenotypes probably differ because of the specific type of immunological response that occurs. The syndromes are connected together by their similar thyroid pathology, similar immune mechanisms, co-occurrence in family groups, and transition from one clinical picture to another within the same individual over time. In some patients, other organ specific and nonorgan specific autoimmune syndromes are associated with autoimmune thyroid disease, including pernicious anemia, vitiligo, myasthenia gravis, primary adrenal autoimmune disease, celiac disease, rheumatoid arthritis or lupus. Thyroid peroxydase, TPO, the primary enzyme involved in thyroid hormonogenesis, was initially identified in 1959 as the 'thyroid microsomal antigenn. It is uncertain whether TPO autoantibodies or TPO-specific T cells are the primary cause of thyroid inflammation, which can lead, in some individuals, to thyroid failure and hypothyroidism. TPOAbs are the hallmark of AITB and are present in almost all patients with Hashimoto's thyroiditis, in two-thirds of patients with postpartum thyroiditis and also in 75% of patients with Graves' hyperthyroidism. The antibodies are mainly produced by lymphocytic infiltrate in the thyroid gland and only to a small extent by regional lymph nodes or the bone marrow. Unlike antibodies against thyroglobulin (Tg), TPO antibodies are capable of inducing antibody-dependent cell-mediated cytotoxicity. Antibodies to TSH-R mimic the function of TSH, and cause disease by binding to the TSH-R and stimulating (or inhibiting) thyroid cells. The TSHR, a member of the G protein-coupled receptor family with seven membrane-spanning segments. Patients with autoimmune thyroid disease may have both stimulating and blocking antibodies in their sera, the clinical picture being the result of the relative potency of each species; blocking antibodies seem more common in Graves' patients with ophthalmopathy compared to those without this complication. The major T cell epitopes are heterogeneous and T cell reactivity against certain TSH-R epitopes has been present in high proportion in normal subjects. More diversified response to TSH-R, with heterogeneity of epitope recognition by TSAb, is predictive of likely remission after antithyroid drug treatment for Graves' disease.