To the author’s knowledge, there are no reports about biomarkers of the redox status in serum or saliva of dogs with hypothyroidism. In this study, changes in various antioxidant and oxidant biomarkers were found in hypothyroid dogs that could indicate an altered redox status in this disease.
An increase in three antioxidant biomarkers (TEAC, PON-1 and GPx) was found in serum of dogs with hypothyroidism. The increases in GPx are in line with what was described in humans with Hashimoto’s thyroiditis [16, 23–25]. GPx is an important selenoprotein that scavenges and detoxifies H2O2, protecting thyroid cells from oxidative damage [26–28]. Thyroid hormone stimulation can influence GPx activity by increasing its concentrations, therefore it could be hypothesized that the increase of GPx activity in hypothyroid dogs is associated with an increased TSH stimulation [29].
The increase in PON-1 in the diseased dogs of our study differs from the decreases described in humans with hypothyroidism; where low PON-1 values are considered a risk of atherosclerosis associated with this disease [30–33]. Dogs carry cholesterol in HDL where PON-1 is bounded protecting it from oxidation [34] instead of in LDL, as it occurs in humans. Therefore, the elevation of cholesterol that usually appears in canine hypothyroidism could imply an increase in HDL and subsequently in PON-1. This could explain the increase of PON-1 found in dogs with hypothyroidism in our study, and this increase in PON-1 could also be one of the reasons why dogs have lower risks of atherosclerosis than humans [35–37]. In humans, TEAC was found to be decreased in various thyroid disorders, including multinodular goiter, contrary to our results [38]. Since TEAC is an analyte that represents the effect of various antioxidants, we believe further studies should be made to evaluate if PON-1 and/or GPx have an influence in the concentrations of TEAC. In addition, the alpha-tocopherol, one of the molecules measured by TEAC, is known to be transported attached to lipoproteins which are increased in hypothyroidism. Although it was not evaluated in this study, increases in this antioxidant are associated to increases in lipoproteins which could explain the high TEAC levels found in this study [39].
On the other hand, in serum, three oxidant biomarkers were found to be increased in dogs with hypothyroidism, namely, TOS, POX-Act and d-ROMs. TOS measures the total oxidant status, as it is integrated by different oxidant molecules [40]. In humans, serum TOS concentrations were found to be increased in subclinical hypothyroidism [41] and Hashimoto’s thyroiditis [42]. The increases in POX-Act and d-ROMs could be related to the increase in TOS concentrations, as both oxidants are part of the whole oxidant status of an organism [40, 43, 44].
In our study, AOPP concentrations without albumin correction were decreased in hypothyroid patients, however, when values were corrected, no significant changes were found, as reported in humans [1, 45]. In general, it is recommended to correct serum AOPP concentrations by albumin concentrations, as AOPP are carried by oxidized plasma proteins like albumin [46, 47], so it could be postulated that the corrected AOPP values should be considered in our study.
In saliva, FRAS and AOPP were decreased in hypothyroid dogs. However, when the salivary biomarkers were corrected by protein no significant changes were found in these analytes. On the other hand, the salivary TBARS : protein ratio increased in dogs with hypothyroidism. Although this is a topic that should be discussed more deeply in the future, it seems that the results of saliva after correction would be more in line with what has been described in humans since TBARS concentrations have been shown to be increased in plasma [23] and erythrocytes [48], possibly due to an increase in lipid peroxidation that is associated with the hyperlipidemia consistently seen in hypothyroidism [49, 50]. In our case, no significant changes were observed in TBARS serum or in saliva without correction of dogs with hypothyroidism. Therefore, it could be postulated that for TBARS, the correction of salivary results by protein could be more sensitive to detect the oxidant overproduction occurring in dogs with hypothyroidism, which is evidenced by the increases in TOS, POX-Act and d-ROMs in serum. However, further studies should be performed using a larger number of animals and ideally in dogs going through levothyroxine replacement therapy to confirm the results of our report. The divergences observed between serum and salivary TBARS are in line with the lack of correlation shown between TBARS, as well as, the other biomarkers of redox status in serum and saliva of our study, in addition divergences between serum and saliva have been described in other reports in dogs [51] pigs [52] or humans [53].
This report should be considered a preliminary study, and, as previously mentioned, further trials with larger populations should be made to evaluate the changes of the oxidative status analytes used in this study throughout treatment. Also, an evaluation of the biomarkers in other sample types, such as whole blood and red blood cell lysates should be performed since in a previous study these samples provided additional interesting information about the redox status in dogs [54].