Retrospective evaluation of circulating thyroid hormones in critically ill dogs with systemic inflammatory response syndrome

Critical illness can be associated with transient alterations in circulating thyroid hormone concentrations, indicating the presence of non-thyroidal illness (NTI). NTI is well described in humans, but there are few reports on its occurrence and prognostic significance in dogs. This retrospective study assessed the occurrence of NTI in a population of dogs with systemic inflammatory response syndrome (SIRS) and investigated its association with disease severity (APPLEfast scores). A total of 41 SIRS dogs were included and were divided by SIRS origin (non-septic SIRS, n = 10; septic SIRS, n = 41) and final outcome (survivors, n = 37; non-survivors, n = 4). Healthy, age-matched dogs (n = 15) were included as controls. Serum thyroid hormone levels including total T3, free T3, total T4, and reverse T3 were measured upon admission. Compared to controls, there were significant changes in serum thyroid hormone concentrations in SIRS dogs, suggesting the presence of NTI. Septic SIRS dogs had higher APPLEfast scores and lower serum thyroid hormones concentrations than those in non-septic SIRS and control dogs. In conclusion, NTI was frequent in dogs with SIRS and may be associated with the presence of sepsis or high illness severity.