In the last year, results of T4 measurements occasionally have been spuriously too high (~20%). These results are verified on repeat analysis but apparently do not accurately reflect the basal thyroid levels or clinical/therapeutic status of the patient. The reason for this randomly, but consistently, seen problem is not understood. It occurs nationwide with the standard methodology and T4 antibody reagents most commonly used by clinical diagnostic labs.

This confounding situation serves to emphasize the importance of using other tests like the free T4 along with T4 testing, rather than relying solely on T4 for diagnostic evaluation of thyroid function or thyroid post-pill therapeutic monitoring. [For a more in-depth discussion of thyroid function testing, see November 1998 Antech News.]

The TgAA assay is a valuable screening test for the heritable form of canine autoimmune thyroiditis. Results are reported as a percentage with the normal reference range being < 200%. Low-grade false positive results may occur if the dog has been vaccinated in the previous 30–40 days. In these cases and for results just below the 200% cut-off, the test should be repeated in 2–4 months. Discontinue use of thyroid supplement for at least 90 days to obtain an accurate thyroiditis assessment.

Positive results are consistent with the presence of autoimmune thyroiditis, which usually will progress to clinical hypothyroidism within months to several years, especially in dog families or breeds at high risk for thyroid disease. Because of the heritable nature of thyroiditis, it is recommended that these dogs not be used in a breeding program.

From Antech's large data base, the dog breeds at highest risk are: Golden retriever, Shetland sheepdog, American cocker spaniel, Boxer, Doberman pinscher, Labrador retriever, German shepherd, Akita, Irish setter, English setter, Old English sheepdog, Collie, Standard poodle, Skye terrier, Bull mastiff, Scottish terrier, and Siberian husky. Only 4% of cases in the thyroiditis data base were dogs of mixed breeding.

Although endogenous TSH levels should be elevated in primary hypothyroidism, this assay used alone has relatively poor predictive value of true hypothyroidism in the dog. This is in contrast to humans where endogenous TSH levels are very accurate in predicting thyroid function status.

Because of the significant (up to 40%) discordancy found with canine TSH testing (i.e. high in euthyroid dogs, and low or normal in hypothyroid dogs), Antech Diagnostics does not recommend using this test as a sole predictor of thyroid function in dogs, and accords less significance to the TSH result than to other thyroid analytes measured and the clinical status of the patient.

References: Nachreiner et al, AJVR 59: 951-955, 1998; Antech News, November 1998; Bruner et al, JAVMA 212: 1572-1575, 1998; Dodds, Proc AHVMA, pp. 77-79, 1999.