Canis ISSN: 2398-2942

Thyroid: T4 autoantibody assay

Synonym(s): T4AA

Contributor(s): David Bruyette, Peter Graham

Overview

  • Antibodies that cross react with thyroxine (T4) and tri-iodothyronine (T3) are collectively known as thyroid hormone antibodies (THAA).
  • THAA are subsets of thyroglobulin autoantibody (TgAA).
  • During the process of inflammatory thyroid pathology, antibodies are generated against the largest protein mass within the thyroid gland which is thyroglobulin. The thyroglobulin molecule is the molecular location where T3 and T4 are manufactured and stored. When TgAA develops at an epitope within the thyroglobulin molecule that contains a T3 or T4 storage site, the resulting thyroglobulin autoantibody will have cross reactivity to T3 or T4 respectively. Consequently, only a proportion of TgAA will cross-react with the thyroid hormones.
  • These antibodies have no functional significance in vivo but in a laboratory immunoassay designed to measure serum T3 or T4 they can interfere and cause false results to be generated.
  • In most assay systems for serum thyroid hormone concentrations, THAA cause false high results to be generated but in some other assay systems a false low could be the result.
  • The analysis of Free T4 by equilibrium dialysis is free from interference by these antibodies as the dialysis membrane does not allow the large antibody molecules to cross into the dialysate that is used for the measurement of Free T4.
  • One survey suggests that THAA are present in around 30% of hypothyroid dogs and that T4AA are present in around 10%.

Sampling

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Tests

Methodologies

  • The principle of the THAA tests is the identification of a factor in the patient’s serum that binds T3 or T4 despite the presence of a chemical (8-anilino-1-napthalene sulfonic acid; ANS) that would be expected to displace thyroid hormones from normal binding proteins. In simple terms, radiolabeled thyroid hormone is added to the patient sample and incubated, the proportions of radiolabeled hormone that can then not be easily recovered back out of the serum in the presence of ANS gives an indication the amount of THAA. The test is conducted separately for T3 and for T4. The nature of this test means that it can be applied to any species (unlike TgAA which is species specific).

Availability

  • Some specialist endocrine laboratories.

Validity

Sensitivity

  • Not used in isolation as a diagnostic test - used to determine whether another test is likely to have suffered analytical interference.

Specificity

  • Not used in isolation as a diagnostic test - used to determine whether another test is likely to have suffered analytical interference.

Result Data

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Further Reading

Publications

Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Graham P A, Refsal K R, Nachreiner R F (2007) Etiopathologic findings of canine hypothyroidism. Vet Clin North Am Small Anim Pract 37(4), 617-631, v (Review) PubMed.
  • Kemppainen R J, Behrend E N (2001) Diagnosis of canine hypothyroidism. Perspectives from a testing laboratory. Vet Clin North Am Small Anim Pract 31(5), 951-962, vii (Review) PubMed.
  • Gaschen F, Thompson J, Beale K, Keisling K (1993) Recognition of triiodothyronine-containing epitopes in canine thyroglobulin by circulating thyroglobulin autoantibodies. Am J Vet Res 54(2), 244-247 PubMed.
  • Thacker E L, Refsal K R, Bull R W (1992) Prevalence of autoantibodies to thyroglobulin, thyroxine, or triiodothyronine and relationship of autoantibodies and serum concentrations of iodothyronines in dogs. Am J Vet Res 53(4), 449-453 PubMed.


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