Incidence of Positive T3AA & T4AA with and without TGAA Elevations in Rhodesian Ridgebacks
There is conflicting opinion on the significance of elevated T3AA and T4AA and their role in lymphocytic thyroiditis (heritable thyroid disease).
Dr. Jean Dodds of Hemopet, who does many Ridgeback thyroid evaluations, includes the T3AA and T4AA circulating autoantibody data in assessing for lymphocytic thyroiditis. She says that in up to 8 percent of affected dogs are TgAA negative. (This false negative can result from a type of receptor on the thyroglobulin molecule not binding to the TgAA marker; see the parenthetical explanation below about specific and non-specific binding.) In this small subset of affected but TgAA-negative dogs, Dr. Dodds states that persistently elevated levels of T3AA and T4AA indicate the presence of lymphocytic thyroiditis.
In February 2006, the RRCUS Health and Genetics Senior Chair had a discussion on this topic with K.R. Refsal, DVM, PhD of the Diagnostic Center for Population and Animal Health at MSU. He indicated that T3AA and T4AA were originally done to give veterinarians more data to help interpret thyroid results. He added that increases in T3AA and/or T4AA might be due to nonspecific binding, where the marker specific for a particular antibody binds to something other than the target antibody. As a result, this can give a falsely high reading. Repeat testing at a later date often shows normal antibody levels. According to Dr. Refsal, the reasons for persistent elevations of T3AA and T4AA with negative/normal TGAA are not know.
Breeders must understand that a dog will received OFA clearance if T3AA and/or T4AA are elevated out of the normal range, so long as the TGAA value is negative. Due to conflicting opinions on the significance of elevated T3AA and T4AA antibodies, the RRCUS H&G committee is collecting data on the incidence of this phenomenon in the breed to determine whether or not Ridgebacks with these elevations progress to develop lymphocytic thyroiditis.
Please contact Susan Ralston if a dog you own has/has had elevated T3AA and/or T4AA with or without elevation in TGAA. As always, all information is kept in the strictest confidence.
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