Anti-triiodothyronine antibodies associated with hypothyroidism and lymphocytic thyroiditis in a dog.

A 5-year-old spayed Shetland Sheepdog had clinical signs consistent with hypothyroidism. Laboratory findings, including subnormal serum T4 concentration, also were suggestive of hypothyroidism; however, the apparent serum T3 concentration, as determined by a solid phase assay, was extremely high. Thyroid scintigraphy revealed ectopic radionuclide uptake in the neck and thyroid biopsy revealed lymphocytic thyroiditis. Determination of T3 autoantibody titer confirmed that the previously measured high serum T3 concentration was spuriously high because of autoantibodies to T3.