Thyroid Autoantibody Tests

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Subject: Thyroid Autoantibody Tests

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Definition

  • Antithyroid peroxidase (TPO) antibodies are autoantibodies directed against the peroxidase enzyme. This enzyme catalyzes the iodination of tyrosine in thyroglobulin (Tg) during the biosynthesis of T3 and T4. Historically, these antibodies were referred to as antimicrosomal antibodies (AMAs) because the antibodies bind to the microsomal part of the thyroid cells. Recent research has identified thyroid peroxidase as the primary antigenic component of microsomes. Measurement of TPO antibodies has essentially replaced the measurement of antimicrosomal antibodies. In virtually all cases of Hashimoto disease and in the majority of cases of Graves disease, anti-TPO antibodies are elevated. High levels of anti-TPO antibodies, in the context of the clinical presentation of hypothyroidism, confirm the diagnosis of Hashimoto disease. Tg autoantibody measurements are most useful for evaluating samples submitted for Tg measurements because Tg autoantibodies can interfere with both competitive immunoassays and immunometric assays for Tg.

  • Normal range:

    • Tg antibodies: <40 IU/mL

    • TPO antibodies: <35 IU/mL

Use

  • To assess the thyroid autoantibody status in patients with thyroid disease

  • To distinguish subacute thyroiditis from Hashimoto thyroiditis, as antibodies are more common in the latter

  • Occasionally useful to distinguish Graves disease from toxic multinodular goiter when physical findings are not diagnostic

  • Thyroid receptor antibodies mainly used in Graves disease, especially as a predictor of relapse of hyperthyroidism

Interpretation

  • Positive in approximately 95% of cases of Hashimoto disease and approximately 85% of Graves disease. Very high titer is suggestive of Hashimoto thyroiditis but absence does not exclude Hashimoto thyroiditis. Less than 1:1,000 occurs virtually only in Graves disease or Hashimoto thyroiditis.

Increased In

  • Significant titer of microsome antibodies indicates Hashimoto thyroiditis or postpartum thyroiditis.

  • Significant titer of antibodies in euthyroid patient with unilateral exophthalmos suggests the diagnosis of euthyroid Graves disease. Elevated antibody titer in a patient with Graves disease should direct a surgeon to perform a more limited thyroidectomy to avoid late postthyroidectomy hypothyroidism.

  • Occasionally positive in papillary–follicular carcinoma of the thyroid, subacute thyroiditis (briefly), and lymphocytic (painless) thyroiditis (in approximately 60% of patients).

  • Primary thyroid lymphoma often shows very high titers. This result should suggest need for biopsy in elderly patient with a firm enlarging thyroid.

  • Low titers are present in >10% of normal population, increasing with age.

  • Other autoimmune diseases (e.g., PA, RA, SLE, myasthenia gravis).

Decreased In

  • In the absence of antibodies, Hashimoto thyroiditis is very unlikely cause of hypothyroidism.

Limitations

  • Tg antibodies may interfere with assay for serum Tg.

 
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