Diagnostic utility of basal serum T4 and TSH concentrations in dogs with hypothyroidism confirmed by TSH stimulation test

The objective of this study was to evaluate the diagnostic performance of a basal serum thyroxine (T4) and thyrotropin (TSH) combination test for the diagnosis of canine hypothyroidism in clinical practice. It was hypothesized that the usefulness of basal T4 and TSH combination test in dogs with suspected non-thyroidal illness syndrome would be different with that in previous studies. The data of this study were searched for dogs who underwent both TSH stimulation test and basal serum T4 and TSH combination test. Forty-four cases were classified into either of three groups (hypothyroid, intermediate, or euthyroid) according to the results of TSH stimulation test, and the basal T4 and TSH concentrations were compared among the three groups. The usefulness of the T4 and TSH combination test regarding the diagnosis made by the TSH stimulation test was evaluated. Laboratory findings were compared among the three groups. Seventeen cases were classified into the hypothyroid group, 11 into the intermediate group, and 16 into the euthyroid group. Of the 13 cases with low T4 and high TSH concentrations, only 7 cases were diagnosed with hypothyroidism and 3 cases were considered euthyroid. There were no significant differences in laboratory results among the three groups. The basal serum T4 and TSH combination test was useful to reject hypothyroidism but was not enough to confirm it. When dogs are suspected to have non-thyroidal illness syndrome, these test could lead to false positives. Although thyroid drug trial may be effective depending on the basal T4 and TSH combination test, TSH stimulation tests are encouraged to confirm canine hypothyroidism.