Low serum free triiodothyronineis level predicts worse outcome of patients with severe fever with thrombocytopenia syndrome

Background Severe fever with thrombocytopenia syndrome (SFTS) caused by phlebovirus results in neuropsychiatric symptoms, multiorgan dysfunction and significant mortality. We aimed to evaluate the thyroid function in SFTS patients, elucidate its association with neuropsychiatric manifestations, disease severity, and prognosis, retrospectively. Methods Serum levels of free triiodothyronine (FT3), free thyroxine (FT4) and thyroid stimulating hormone (TSH) were compared between survivors and non-survivors, between those with and without nervous symptoms at baseline, and at baseline and remission. Logistic regression analysis was utilized to determine independent risk factors for mortality. A risk model based on risk factors was constructed and its prognostic value was evaluated by receiver operating characteristic (ROC) curve. Results A total of 207 SFTS cases with thyroid function data enrolled from January 2016 to January 2020 were included with 34 patients (16.4%) died. Baseline serum levels of FT3, TSH ( p < 0.001), and FT3/FT4 ratio ( p < 0.05) were significantly decreased in nonsurvivors than in survivors. Prevalence of low serum FT3 in nonsurvivors (81.8%) was greater than in survivors (41.3%). FT3 level ( p < 0.001) was markedly reduced in patients with central neurological symptoms than those without. FT3 and FT4 levels were increased in remission than at baseline ( p < 0.001). Logistic regression analysis showed that age (OR 0.92, 95% CI 0.868–0.958) and serum FT3 level (OR 3.055, 95% CI 1.494–6.248) were the independent risk factors for mortality. A risk model based on age and FT3 had a high predictive value for mortality (AUC = 0.818, 95% CI 0.795–0.868) at a cutoff value of > 3.39. Conclusions Low serum FT3 level was associated with a worse outcome of SFTS patients.