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Tuesday, April 22, 2008
Autoimmune thyroiditis
Lymphocytic thyroiditis is the underlying cause in many cases of primary hypothyroidism in
dogs and the predisposition to its development is believed to be highly heritable. It is an immune mediated disorder characterized histologically by a diffuse infiltration of lymphocytes, plasma cells, and macrophages in the thyroid gland. Antibodies interact with the follicular cell, colloid, or thyroglobulin antigens and activate the complement cascade and cell-mediated cytotoxicity. The progressive destruction of follicles and secondary fibrosis eventually leads to a failure of thyroid hormone production. More than 60 or 70% of the thyroid tissue needs to be destroyed before we see changes in laboratory measures of thyroid function. This process can take months or years to cause classic hypothyroidism. In some animals it may not progress. Anti-thyroglobulin antibodies are released into the circulation in animals with this condition. A test for these antibodies in serum is included in the Michigan State University Canine Thyroid Diagnostic profile.
In addition to testing for serum levels of total thyroxine (TT4), total tri-iodothyronine (TT3), free thyroxine (FT4), and free tri-iodothyronine (FT3), this profile tests for T4 antibodies (T4AA), T3 antibodies (T3AA), canine thyrotropin (cTSH; thyroid stimulating hormone), and thyroglobulin antibodies (TgAA). The cTSH test provides much needed information in any attempt to diagnose hypothyroidism. Many non-thyroidal factors can cause decreases of TT4, TT3, FT4, and FT3 into the hypothyroid range in a dog with normal thyroid function making it difficult to
differentiate sick-but-euthyroid animals from those with hypothyroidism.
Some researchers have looked for and failed to find such an association of lymphocytic thyroiditis with vaccination.
(This information was extracted from Michigan State University Diagnostic Center for Population and Animal Health 517-353-1683 www.animalhealth.msu.edu
Frequently-Asked Questions - Endocrinology
Thyroid Function in Dogs)
Posted by terry35678 Tuesday, April 22, 2008
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