Thyroidspecific antigens Thyroglobulin

Thyroglobulin autoantibodies can be demonstrated by several procedures, such as precipitation in agar, IIF, indirect hemagglutination of cells coated with thyroglobulin, RIA and ELISA. At present, hemagglutination is the most commonly employed technique for the detection of antibodies against thyroglobulin. The ELISA is increasingly used for the measurement of antibodies to thyroglobulin and will additionally detect antibodies that are nonagglutinating. Thyroglobulin antibodies are found in individuals with chronic thyroiditis, myxedema, Graves disease, goiter, thyroid tumors and other polyendocrine disorders. The highest titers are found in patients with autoimmune thyroid disorders. Low titers of thyroglobulin antibodies may be found in normal individuals. The prevalence of such antibodies in subjects without overt thyroid disease is higher in women. The incidence also increases with age, so that 18% of women over 40 years old may have antibodies to thyroglobulin. Total thyroglobulin antibody is not a specific marker of autoimmune thyroiditis as so many other conditions also show the presence of this antibody. Recent evidence suggests, however, that there may be disease-related epitope specificity of the autoantibody. The disease-related antibodies were detected by competitive inhibition ELISA experiments using murine monoclonal antibodies.

Microsomal antigens (TPO) of thyroid epithelial cells

The microsomal antigen has been identified chiefly as the thyroid peroxidase enzyme (TPO) of the thyroid epithelial ceil. However, purified TPO is not readily available for use in diagnostic tests. The most commonly used tests for the microsomal autoantibodies are either IIF, using primate thyroid tissue, or hemagglutination. ELISA, RIA or a chemiluminescence-based assay may also be employed. The antibodies to the microsomal antigens belong predominantly to the immunoglobulin G (IgG) class and, when detected by IIF, stain the cytoplasm of thyroid cells. The IIF test for antibodies to thyroid microsomal antigens is positive in approximately 70-90% of patients with chronic thyroiditis. It is also positive in 64% of patients with primary hypothyroidism, 50% of patients with thyrotoxicosis, 10% of patients with simple goiters, and 17% of patients with thyroid tumor. It can also be found in normals at a low frequency and titer.

Thyrotropin receptor

Autoantibodies binding to the thyrotropin (TSH) receptor on the surface of thyroid epithelial cells are believed to be the direct cause of the hyperthyroid state in Graves' disease. The thyroid-stimulating autoantibodies (TSAbs) bind near to or with the TSH receptor and mimic TSH activity, resulting in uncontrolled stimulation as there is no negative feedback control. Tests for TSAbs are primarily of two types: bioassays or binding assays. Most bioassays have limited utility because they are cumbersome and require scarce materials, such as viable slices of fresh human thyroid tissue. Bioassays, however, evaluate functional aspects of the antibodies as their end-point. Binding assays are based on competition between ,25I-labeled TSH and patient autoantibodies for binding to thyroid preparations. Some confusion has been created by the heterogeneity of the autoantibodies and the diversity of the assays for their

Table 1 Detection of autoantibodies

Autoantibody to:

Primary method of detection a Primary associated disorders

Thyroid Thyroglobulin

Microsomal (thyroid peroxidase)

TSH receptor

Parietal cell

Adrenal

Ovary

Testes, sperm

Pancreas islet cell

Parathyroid

Pituitary

Skin

Basement membrane zone Intercellular substance Muscle Smooth

Cardiac striated Skeletal striated Endomysial Acetylcholine receptor Neutrophil cytoplasm (C-ANCA, proteinase 3, P-ANCA, myeloperoxidase) Nuclear antigens (ANAs)

Mitochondria Phospholipids Rheumatoid factor

Hemagglutination ELISA

IIF (mk-thyroid) Hemagglutination Bioassay Binding assay IIF (ro-stomach) IIF (mk-adrenal) IIF (mk-ovary) IIF (mk-testes) Agglutination Immobilization

IIF (hu group O or mk-pancreas) IIF (mk-parathyroid) IIF (hu- or mk-pituitary)

IIF (hu-skin, mk-esophagus) IIF (hu-skin, mk-esophagus)

IIF (ro-stomach) IIF (ro-heart) IIF (ro-skeletal) IIF (mk-esophagus) RIA, ELISA IIF (hu-PMNs), ELISA

IIF (ro-liver, Hep-2 cells), ELISA Hemagglutination

Immunodiffusion IIF (ro-kidney) ELISA

Latex agglutination Nephelometry

Autoimmune thyroiditis, primary myxedema Autoimmune thyroiditis, primary myxedema Graves' disease

Atrophic gastritis, pernicious anemia Idiopathic Addison's disease Infertility Infertility

Insulin-dependent diabetes mellitus Idiopathic hypoparathyroidism Growth disorders

Bullous pemphigoid Pemphigus vulgaris

Chronic active hepatitis Autoimmune cardiomyopathy Myasthenia gravis Celiac disease Myasthenia gravis

Wegener's granulomatosis, microscopic vasculitides

Systemic lupus erythematosus, connective tissue disorders

Primary biliary cirrhosis Antiphospholipid syndrome Rheumatoid arthritis

PMN, polymorphonuclear leukocyte.

'Species and tissue substrate used: mk, monkey; ro, rodent; hu, human.

demonstration. Different specificities of autoantibodies to the thyroid epithelial cell surface on or near the TSH receptor may be represented.

The detection of these antibodies in Graves' patients ranges from 55 to 95%, depending on the assay. The TSAb assay is of greatest utility in conditions where conventional test results are equivocal or where clinical signs and symptoms are not readily apparent (e.g. ophthalmopathy in the absence of other features). Moreover, the TSAb assay is usually negative in patients with thyrotoxicosis in association with thyroid nodules, with cancer, or in patients with subacute thyroiditis. Therefore, a positive test confirms a diagnosis of Graves' disease with a high degree of confidence. The assay can be used to monitor patient therapy, as successful treatment of Graves' disease with propylthiouracil may result in lowering levels of TSAb antibodies. The assay has also proven useful in the diagnosis of neonatal thyrotoxicosis.

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