Serum reverse triiodothyronine in dogs with hypothyroidism, nonthyroidal illness syndrome and healthy dogs

Background Diagnosing canine hypothyroidism remains challenging because decreased total thyroxine (tT4) concentrations are not specific for hypothyroidism and are frequently observed in dogs with non-thyroidal illness syndrome (NTIS). Although additional thyrotropin (TSH) and free thyroxine (fT4) measurements may improve diagnostic accuracy, differentiation between hypothyroidism and NTIS remains difficult in clinical practice. In human medicine, reverse triiodothyronine (rT3), an inactive metabolite of thyroxine, has been proposed as a useful marker to distinguish hypothyroidism from NTIS. However, studies evaluating rT3 concentrations in dogs with hypothyroidism and NTIS are currently lacking. Objective To establish and validate a canine-specific HPLC-MS/MS assay for reverse triiodothyronine (rT3), determine a reference interval in healthy dogs, and evaluate the diagnostic utility of rT3 for differentiating canine hypothyroidism from NTIS in comparison with free thyroxine measured by equilibrium dialysis (fT4d). Materials and methods Reverse triiodothyronine (rT3) concentrations were measured by HPLC-MS/MS in healthy dogs, dogs with hypothyroidism, and dogs with NTIS. A canine-specific reference interval for rT3 was established using healthy adult dogs. Hypothyroidism was diagnosed based on compatible clinical signs, decreased total thyroxine (tT4), increased thyrotropin (TSH), and clinical improvement following levothyroxine supplementation. Dogs with moderate to severe systemic disease, decreased tT4 concentrations, and no clinical signs consistent with hypothyroidism were included in the NTIS group. The diagnostic value of rT3 in distinguishing between dogs with hypothyroidism and NTIS was investigated and compared with the diagnostic value of fT4 measured by equilibrium dialysis (fT4d) using ROC analysis. Results 47 healthy dogs, 18 hypothyroid dogs, and 23 dogs with NTIS were included. The reference interval for rT3 was 113–439 pg/ml. Median rT3 concentrations were significantly lower in hypothyroid dogs (2.5 pg/ml; [IQR 2.5–18.5 pg/ml], range 2.5–106 pg/ml) compared with dogs with NTIS (229 pg/ml; [IQR 148.7–538.3 pg/ml], range 52.2–700 pg/ml; p < 0.0001) and healthy dogs (281 pg/ml; [IQR 220–344.5 pg/ml], range 111–817 pg/ml; p < 0.0001). The area under the ROC curve (AUC) for rT3 to differentiate hypothyroidism from NTIS was 0.981 (95% CI: 0.951–1.010), compared with 0.896 (95% CI: 0.798–0.994) for fT4d. Using cut-off values of ≤52.2 pg/ml and ≤106 pg/ml, rT3 showed sensitivities and specificities of 83.4% (95% CI: 59–96%) and 100% (95% CI: 85–100%), and 100% (95% CI: 82–100%) and 82.6% (95% CI: 61–94%), respectively. Using the lower limit of the reference interval as a cut-off, sensitivity and specificity were 77.8% (95% CI: 53–92%) and 91.3% (95% CI: 72–98%). Conclusion and clinical relevance The establishment of a reference interval for rT3 will enable future interpretation of rT3 values in dogs. rT3 represents a valuable diagnostic parameter for differentiating canine hypothyroidism from NTIS.