THYROID AUTOANTIBODIES

CLINICAL UTILITY:

Antibodies to thyroid antigens are circulating markers of autoimmune thyroid disease. They react with thyroid microsomes or the glycoprotein, thyroglobulin, which is the molecular storage form of the hormones T3 and T4. Laboratory test procedures for the two antibodies are relatively simple, adding to their usefulness in diagnosis. When testing patients' sera for both antibodies, almost all cases of Hashimoto goiter and about 90% of primary myxedema cases will be detected. Since it is not always possible clinicaly to distinguish autoimmune thyroiditis from other types of goiter, these tests may be helpful in differential diagnosis.

In patients with unilateral exophthalmos, the presence of anti-thyroid antibodies will help differentiate between a diagnosis of local orbital lesions and a disorder of autoimmune endocrine etiology. About 70-90% of patients with variants of Graves' disease develop anti-thyroglobulin and/or anti-microsomal antibodies. Although most thyrotoxic patients have relatively low titers of antibody, approximately 20% have moderate to high titers (thyroglobulin: >1:640; microsomal: >1:6400.) The higher level of antibody may indicate a more severe form of the disease, with a tendency to relapse, or a concomitant destructive thyroiditis. Presence of anti-thyroid antibodies may suggest possible thyroid dysfunction in patients with other autoimmune endocrine disorders, such as Addison's disease or insulin-dependent diabetes mellitus.

METHOD DESCRIPTION:

Passive hemagglutination in microtiter plates is the method used to detect anti-microsomal and anti-thyroglobulin antibodies in patients' sera. Red blood cells coated with antigen (either microsomal fraction or thyroglobulin protein) are incubated with dilutions of patient serum. If antibody against the antigen is present in the serum, the red cells will agglutinate. The highest titer in which agglutination is still visible is the reported result.

REFERENCE RANGE:

Negative. Low titers of both antibodies are seen in a small percentage of the normal population, with greater frequency in women. Aprroximately 20% of middle aged females have low titers of antibody, with incidence increasing with age.

SPECIMEN REQUIREMENTS:

0.5 ml serum. Freeze.