Response to Letter Regarding “Comparison of 2 Assays for Measuring Serum Total Thyroxine Concentration in Dogs and Cats”

We thank Dr. Wun for his interest in our 2020 article, “Comparison of 2 assays for measuring serum total thyroxine concentration in dogs and cats” and for his questions and illustrative figure notations regarding our work.

Dr. Wun makes reference to two issues regarding the data from cats: (1) Dr. Wun states that the article did not present the regression lines for the broken stick regression analysis in the manuscript. These results were, in fact, reported in Results 3.2.1. The non-linear pattern can also be seen in the Bland–Altman plot for Figure 3. (2) Dr. Wun makes reference to the potential difference of MTT4 and CTT4 and says that the 3rd sentence in the discussion is “‘misleading and requires correction.” However, the example that Dr. Wun provides is for a hyperthyroid patient that was being treated with methimazole and having follow up monitoring. We did not consider every variation where CTT4 versus MTT4 might fall above or below some specified value, but it was clearly stated in the discussion that in the context of repeated assays, the same assay method should be used for follow-up testing. For the data from dogs, Dr. Wun references the second sentence in the Discussion section and Figure 1 in Results. He indicates the wording of “slightly higher concentrations” toward the end of the sentence should have been “slightly lower concentrations.” The authors acknowledge that Dr. Wun is correct and regret the error.