Anti Thyroid Peroxidase Antibody

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Contents 1 Introduction 2 Etiology 3 Reference-interval 4 Principle 5 Clinical-significance 6 Methods 7 Specimen 8 More General Terms 9 Additional Terms 10 References

Introduction

• Thyroid peroxidase is the main & possibly only autoantigenic component of microsomes.

Etiology

• Hashimoto's thyroiditis (90%)

• titers of 1:2500 are diagnostic of Hashimoto's thyroiditis

• idiopathic myxedema
• Graves' disease
• postpartum form of subacute lymphocytic thyroiditis (some)

Reference-interval

• negative < 85 U/ mL

Principle

• The purified microsomal antigen is isolated from human thyrotoxic glands by high speed centrifugation. The microsomal fraction is then bound to the surface of turkey erythrocytes which have been treated with tannic acid, & these 'sensitized' cells will agglutinate in the presence of specific autoantibodies. A small proportion of human sera is reactive against turkey cells, giving rise to non-specific agglutination of the sensitized cells. These non-specific reactions may be detected by means of unsensitized control cells. Both test & control cells are treated with formalin & freeze-dried to give long term stability on storage.
  
• The combination of thyroglobulin & microsomal hemagglutination tests will detect practically all Hashimoto goiters & about 90% of primary myxedema cases. The two tests should be performed together on all cases of goiter scheduled for operation as it is not always possible clinically to distinguish autoimmune thyroiditis from other types of goiter. Another important application of the two thyroid antibody tests is in the differential diagnosis of primary thyrotoxicosis & various tachycardias, anxiety states, unexplained weight loss or diarrhea. In cases with unilateral exophthalmos the test will help to differentiate between & endocrine etiology & local orbital lesions, obviating more invasive or expensive tests. About 70-90% of cases with variants of Graves' disease give positive thyroglobulin &/or microsomal hemagglutination titers compared with 10-15% of controls according to age & sex. Although most thyrotoxic subjects show relatively low levels of antibody, about 20% have moderate to high titers (Thyroglobulin >1:640, Microsomal >1:6400), & this indicates either a more severe form of the disease with a tendency to relapse, or a concomitant destructive thyroiditis, predisposing to postoperative myxedema or to spontaneous loss of thyroid function some years after the thyrotoxic episode. Similarly, thyroglobulin in combination with microsomal hemagglutination will distinguish between atrophic thyroiditis with mild or severe hypothyroidism & cases of depression or obesity due to other causes.
  
• Positive results in these two tests are not sufficient to exclude thyroid cancer, nor are low titers (Thyroglobulin < 1:160, Microsomal <1:1600) always indicative of severe thyroid lesions, as many cases of 'focal thyroiditis' remain subclinical & non-progressive. If a positive result is obtained, supplementary investigations such as thyroid scintiscans for cancer, TRH tests for thyroid autonomy & serum TSH estimations for suspected hypothyroidism are necessary, the choice of test being dependent on the clinical findings.
  
• Thyroglobulin & microsomal hemagglutination tests give useful predictive evidence of possible thyroid dysfunction in patients with other autoimmune endocrine disorders such as Addison's Disease, insulin-dependent diabetes mellitus or polyendocrine auto-immunopathies, & in members of families prone to organ-specific autoimmunity.

Clinical-significance

• principal circulating marker of autoimmune thyroiditis

• goitrous thyroiditis ( Hashimoto's disease),
• atrophic thyroiditis ( myxedema)
• thyrotoxicosis (Graves'/Basedow disease)

• anti- microsomal antibodies are directly largely against thyroid peroxidase
• in patients with subclinical hypothyroidism, presence of anti- thyroid peroxidase antibodies confers an increased risk of developing overt hypothyroidism [3]

Methods

• Immunometric assay

Specimen

• Blood collected by venipuncture should be allowed to clot naturally & the serum clarified by centrifugation before testing. If it should be necessary to store samples before testing, they should be kept frozen at about -20 degrees C.
• Avoid repeated freezing & thawing . All patients' sera should be inactivated by heating at 56 degrees C for 30 minutes prior to testing.
  
• Plasma samples are not suitable for testing.
• Infected serum samples are unsuitable for testing.
  
• serum, store at -20 degrees C.

More General Terms

• autoantibody
• serology

Additional Terms

• anti-thyroid peroxidase (microsomal) antibody
• Hashimoto's thyroiditis; chronic lymphocytic thyroiditis; struma lymphomatosa; lymphadenoid goiter; primary myxedema
• thyroid peroxidase

References

1. Saunders Manual of Medical Practice, Rakel (ed), WB Saunders, Philadelphia, 1996, pg 648
2. Clinical Guide to Laboratory Tests, 3rd edition, NW Tietz ed, WB Saunders, Philadelphia, 1995
3. Medical Knowledge Self Assessment Program (MKSAP) 15, American College of Physicians, Philadelphia 2009
4. Package Insert, Thymune-M, Wellcome Diagnostice, Research Triangle Park, N. C. 27709.
5. Tietz, Norbert W., Textbook of Clinical Chemistry, W. B. Saunders Co., Philadelphia, 1986. p.1136.

anti-thyroid peroxidase (microsomal) antibody