In primary hypothyroidism, as serum free T4 levels fall, pituitary output of thyroid stimulating hormone (TSH) rises in a regulatory, compensatory response. In human medicine, highly sensitive and accurate endogenous TSH assays are available which make diagnostic testing straightforward, as virtually all hypothyroid patients have elevated TSH levels. However, in veterinary medicine, canine endogenous TSH (cTSH) is poorly predictive of primary hypothyroidism in dogs (70%) versus > 95% in humans, and can give 20-40% discordant results [both false positive and false negative]. This finding has been verified by several published studies. The reason is unclear, but it appears that some dogs have a slightly different bioform of TSH that reacts poorly or unpredictably in the assay. Thus, the cTSH assay by itself is not recommended for diagnosing canine hypothyroidism, and spuriously low or high cTSH levels can be seen in some hypothyroid or euthyroid dogs, respectively.

An estimated 80% of cases of canine hypothyroidism result from heritable autoimmune (lymphocytic) thyroiditis. Many popular breeds are at increased risk for this disorder, with English Setters being the breed exhibiting the highest prevalence of thyroiditis today (>40% of those tested).

The presence of elevated TgAA levels confirms thyroiditis, promotes early recognition of the disorder, and facilitates genetic counseling. Low-grade false positive results can occur if the dog has been vaccinated recently, especially with rabies vaccine; retesting is recommended in 90 days. False negative results can occur in up to 8% of T3AA and/or T4AA confirmed positive thyroiditis cases, presumably because not all epitopes of TgAA are recognized by the assay reagent. Dogs on thyroid supplement should be off this medication for at least 90 days to obtain accurate TgAA results. Please note that reporting units for the TgAA normal reference range have changed recently from <200% to <20%. All equivocal or positive results are repeated with a confirmatory test to correct for non-specific binding (NSB TgAA); the normal reference range for this confirmatory test is <10%.

Whereas most cases of autoimmune thyroiditis (˜92%) have elevated TgAA in their serum, only about 20% have elevated serum T3 and/or T4 AA. Thus, the presence of elevated T3 and/or T4 AA supports a diagnosis of auto-immune thyroiditis but underestimates its prevalence, as negative (non-elevated) serum T3 and/or T4 AA levels do not rule out thyroiditis. On the other hand, positive results support the presence of thyroiditis, even if the TgAA level is normal. Most circulating antibodies are against T3 (~70%), some affect both T3 and T4 (˜25%), and only a few affect T4 alone (˜5%). When these autoantibodies are present, measurement of T4 and T3 levels will be spuriously high.