PREVALENCE OF AUTOANTIBODIES TO THYROGLOBULIN IN DOGS WITH NONTHYROIDAL ILLNESS

Objective-To evaluate a thyroglobulin autoantibody (TgAA) assay and de termine a diagnostic threshold. Sample Population-Serum samples from d ogs with various endocrine abnormalities and from 30 obese adult femal e Beagles. Procedure-TgAA were determined by use of the ELISA. Six exp eriments were done: 1, definition of positive results for TgAA using s amples from normal and T3 autoantibody (T3AA) positive dogs; 2, establ ishment of prevalence of positive results in 91 clinically normal dogs ; 3, evaluation of positive results for sera from dogs with nonthyroid al illnesses; 4, testing of samples from dogs with primary hypothyroid ism but absence of T4AA or T3AA, or both; 5, determination of prevalen ce of false-negative results in dogs that are T4AA and/or T3AA positiv e, which were (18 dogs) or were not (22 dogs) receiving L-thyroxine re placement therapy; and 6, examination of thyroid biopsy specimens from 18 dogs (8 TgAA positive and 10 TgAA negative). Results-Positive resu lts were defined as at least twice (200%) the optical density of the n egative-control sample. False-positive results were obtained for only 3.4% of 146 dogs with nonthyroidal illness. Thirty-seven percent of do gs with primary hypothyroidism, but no evidence of T4AA or T3AA, or bo th, were TgAA positive. False-negative results were found in 1 of 22 a nd 2 of 18 T3AA-positive dogs with and without thyroid replacement the rapy, respectively. Thyroid biopsy specimens from 8 TgAA-positive dogs had evidence of lymphocytic thyroiditis, whereas those from 10 TgAA-n egative dogs did not. Conclusion and Clinical Relevance-The assay is s ensitive and specific for identification of lymphocytic autoimmune thy roiditis in dogs, and has potential for aiding early diagnosis of thyr oiditis in dogs and identifying dogs likely to perpetuate hypothyroidi sm in breeding programs.