Low T3 Syndrome in severely ill patients with COVID-19 infection

Introduction The coronavirus disease (COVID-19) is an infectious disease, caused by the SARS-CoV-2 virus, which causes severe respiratory disease. Critical ill patients often experience a condition known as Low T3 Syndrome (LT3S). Previous studies showed an association between low FT3 levels and mortality among patients with COVID-19. Moreover, thyroid hormones might be altered by cigarette consumption. Τhe aim of this study was to investigate the association of LT3S with mortality and the severity and risk of intubation in critically ill patients with COVID-19 infection, and to explore whether this association is confounded by smoking. Methods A total of 105 critically ill patients aged ≥18 years, with laboratoryconfirmed (RT-PCR) COVID-19 were enrolled. The study was conducted between January 2021 and October 2021 in the Intensive Care Unit of the 1st Department Respiratory Medicine in ‘Sotiria’ Hospital and laboratory data and clinical information were retrieved retrospectively from the electronic patients record. LT3S was defined as serum levels of FT3 <2.3 pg/mL with low or normal TSH levels. Patients were divided into two groups according to serum FT3 values: group with LT3S and group without LT3S. Mortality in the ICU was the primary outcome of the study, while the risk of intubation was a secondary outcome. Results In all, 43 out of the 105 included patients were diagnosed with LT3S. Patients in the LT3S group were older than those with non LT3S [median (IQR): 62 (13.7) vs 52.8 (15.5), p=0.011]. Non-statistically significantly higher mortality rate, SOFA and APACHE II scores were observed in the LT3S group compared to no LT3S group (p=0.080, p=0.311 and p=0.079, respectively). Moreover, LT3S was not associated with high risk of intubation (HR=1.32; 95% CI: 0.78–2.22). Twenty-five patients (58.1%) in the LT3S group were never smokers, versus 41 (66.1%) patients in the non LT3S group. Never smokers with LT3S had significantly higher mortality rate than never smokers without LT3S (40% vs 17.1%, p=0.039), and LT3S in the never smoking subgroup was associated with an increased risk of intubation (HR=2.21; 95% CI: 1.18–4.16). Conclusions LT3S was found to be associated with mortality of patients with critical COVID-19 among never smokers but not among ex-smokers or active smokers. This finding may denote that smoking may act as a confounder of the association between LT3S and mortality. Further investigation is needed to demonstrate the impact of LT3S in critically ill patients with COVID-19 infection, as well as the role of smoking in the development of the syndrome