Serum FT3 levels and FT3/FT4 ratio as predictors for poor prognosis in hospitalized COVID-19 patients

Background: The coronavirus disease 2019 (COVID-19) can involve multiple organs and systems, including the endocrine system. In particular, thyroid dysfunctions are frequently seen in COVID-19 patients. The aim of this study was to evaluate thyroid function in hospitalized COVID-19 patients and to correlate thyroid function with inflammatory status, blood count parameters and mortality. Materials and methods: Data of COVID-19 patients admitted to the hospital of Pescara between October 2020 and March 2021 were retrospectively evaluated. Serum thyrotropin (TSH), free thyroxine (FT4), free triiodothyronine (FT3), FT3/FT4 ratio, thyroid antibodies (TgAb, TPOAb), inflammatory and blood count parameters (C-reactive protein, CRP; interleukin-6, IL-6; red blood cell, RBC; white blood cells, WBC; platelets, PLT; neutrophil to lymphocytic ratio, NLR) were analyzed and compared between survivors and non-survivors. Results: Three hundred thirty-four adult COVID-19 patients were considered for potential enrollment, and after assessment of inclusion and exclusion criteria, 264 were enrolled. The median age was 74.4 (20.6) years, and 167 patients (63.5%) were males. The average hospital stay was 9 days. Of the 264 enrolled patients, 101 (38.2 %) died of COVID-19 complications. The characteristics of survivors and non-survivors are shown in table 1. Serum FT3 levels and FT3/FT4 ratio were significantly lower in non-survivors compared to survivors. Instead, inflammatory and blood count parameters, except for RBC, were significantly higher in survivors. Notably, FT3 levels and FT3/FT4 ratio negatively correlated with CRP and NLR (r=-0.2, P<0.05). In Kaplan-Meier and Cox regression analyses, low FT3 levels (FT3 less than 2.5 pg/ml) were independently associated with mortality (H.R. 1.7, CI 95 % 1.01- 2.96, P=0.042). Conclusions: FT3 levels and FT3/FT4 ratio correlate negatively with inflammatory markers and may be predictive for poor prognosis in hospitalized COVID-19 patients.