Reverse triiodothyronine concentrations in hyperthyroid, healthy, and azotemic cats determined by ultra-performance liquid chromatography-tandem mass spectrometry

Without thyroid scintigraphy, diagnosing feline hyperthyroidism can be challenging. The primary aims of this study were the quantification of total 3,3',5'-triiodothyronine (total reverse T3, TrT3) concentrations in hyperthyroid, healthy, and azotemic cats using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) and to investigate its potential as a predictive marker for the diagnosis of hyperthyroidism. The hypothesis was that in feline hyperthyroidism rT3, representing the "inactivated" metabolite of T4, increases in relation to T4 and "active" T3. Left over samples of 138 hyperthyroid cats submitted for radioiodine treatment, 73 healthy cats and 83 cats with kidney disease were analyzed. Azotemic cats were included to represent cats with possible non-thyroidal illness syndrome. The healthy group was used to calculate reference limits and to investigate the influence of age and gender. UPLC-MS/MS detected both T3 isomers with high analytic sensitivity. TrT3 measurements correlated positively with total T4 (TT4, rSP = 0.937, P < 0.001), and total T3 concentrations (TT3, rSP = 0.866, P < 0.001). TT4 correlated positively with TT3 (rSP = 0.939, P < 0.001). Hyperthyroid cats had higher TT4, TT3, and TrT3 concentrations as well as TrT3/TT4 ratios compared to the other groups (P < 0.001), whereas the TT3/TrT3-ratios was lower (P < 0.001). TrT3 exceeded TT3 concentrations in 85.5 % of the hyperthyroid cats. The optimum cutoff to identify hyperthyroidism determined by ROC-curve analysis was TrT3 > 0.75 nmol/l (sensitivity 1, specificity 0.968). No effects of gender (P = 0.848) or age (P = 0.691) were observed. In conclusion, rT3 is the second most abundant thyroid hormone in feline hyperthyroidism, can be measured by UPLC-MS/MS with high diagnostic accuracy and its measurement opens new doors to investigate feline iodothyronine metabolism.