The presence of thyroid antibodies may falsely increase measured T3 and T4 concentrations, thereby making it more difficult without complete thyroid profiling to identify affected animals. In the MSU cohort population study, 57 of 1000 hypothyroid dogs had falsely increased T3 concentrations, and 17 of 1000 hypothyroid dogs had falsely increased T4 concentrations.

The thyroglobin autoantibody (TgAA) concentration is expected to be elevated (positive, >200) in all cases of autoimmune thyroiditis that have elevated circulating T3 and/or T4 autoantibodies. However, in a few cases from Antech's case study (16 of 465, 3.4%), the TgAA test was negative in the presence of elevated T3AA (all 16 cases) and elevated T4AA (1 case).

The total T4 concentration was > 2.0 µg/dL in 48 (10%) of the above 465 cases of thyroiditis. English Setters and Golden Retrievers were overrepresented in this group. These findings underscore the importance of measuring more than just a total T4 concentration when evaluating a case for the presence of hypothyroidism and/or thyroiditis.

While many dogs with primary hypothyroidism have elevated cTSH concentrations, up to one-third of affected dogs have normal or low (<0.10) cTSH concentrations, for reasons that are unclear. Conversely, some euthyroid dogs (7-18% in published studies) have elevated cTSH concentrations.