Introduction: Despite numerous studies in human medicine, in the veterinary medicine the nonthyroidal illness syndrome (NTIS) has been not yet well studied. The few studies conducted on critically ill dogs show that the levels of thyroid hormones drop down during illnesses.

Objective: To evaluate serum total thyroxine (tT4) in hospitalized feline patients and its possible relationship with APPLEfast, SIRS and outcome.

Methods: Cats were included if they were hospitalized (HC) in our veterinary teaching hospital. On the contrary, cats that have had previously diagnosed thyroid diseases or cats with alterations on clinical or ultrasound examination of the thyroid were excluded. The tT4 was evaluated at the time of the admission (T0) using exceeding serum samples. A control group (CG) was composed by cats with non-thyroidal chronic diseases without the necessity of hospitalization in which the tT4 was evaluated. The APPLEfast score was calculated for each cat at the admission. Cats had systemic inflammatory response syndrome (SIRS) if they showed: (1) abnormal temperature (<37.8 or ≥39.2°C), (2) tachycardia (≥225 beats/min) or bradycardia (≤140 beats/min), (3) tachypnea (≥40 breaths/min) and (4) WBC abnormalities (WBC ≥19500 or ≤5000 or band neutrophils ≥5%). Cats were divided in survivors and non-survivors according to the 10-day outcome. Data were analyzed using statistical software. Results: Forty-seven cats were retrospectively included. Twenty-six cats (55%) died within 10-days from the admission. The CG was composed by 37 cats. Age was lower in HC than in CG, while the two groups were similar by gender. HC showed a significantly lower tT4 compared to CG (1.3±0.7 vs. 2±0.9; p<0.0001). Non-survivors had lower mean tT4 than survivors (1.1±0.65 vs. 1.58±0.8; p=0.03). The cut-off point for tT4 for mortality was 1.65 µg/dL (sensitivity 81%, specificity 57%, OR 5.6). There was no correlation between APPLEfast score and tT4. Sixteen cats (32%) has SIRS and no association between tT4 and the presence of SIRS was found.

Conclusions: NTIS in critically ill cats may occur and the evaluation of tT4 in hospitalized cats can help the clinician with prognostic information.