Non-Thyroidal Illness Syndrome: A Predictor of Non-Invasive Ventilation Failure and Mortality in Critically Ill Patients

<b><i>Aim:</i></b> This study aimed to investigate the effect of Euthyroid Sick Syndrome (ESS) on non-invasive mechanical ventilation (NIV) failure and in-hospital mortality. We also examined the impact of prolonged low fT3 levels before intensive care unit (ICU) admission on mortality and NIV failure. <b><i>Methods:</i></b> The study included 386 ICU patients who received NIV. The patients were categorized into two groups: those with ESS and those without (Non-ESS). Additionally, retrospective fT3 levels were examined in the ESS group over 6 months before ICU admission, and 61 patients with prolonged ESS were identified. The primary endpoints of this study (NIV failure and mortality) were compared between study groups. <b><i>Results:</i></b> Of the 386 patients included in the study, comprising 332 survivors and 54 deceased individuals, ESS was observed in 49.0% of the total patient population, with a significantly higher prevalence (<i>p</i> = 0.005) among deceased patients (66.7%) compared to survivors (46.1%). Among the 189 patients with low fT3 levels, there was a significant disparity between survivors and deceased patients (<i>p</i> < 0.001). Of these, 32.3% had low fT3 levels both before and during ICU admission (prolonged ESS), with 26.1% survivors and 58.3% deceased (<i>p</i> < 0.001). NIV failure was significantly more common in the ESS group (44.4%) than in the non-ESS group (26.4%; <i>p</i> < 0.001). <b><i>Conclusion:</i></b> Our study demonstrates that ESS, mainly characterized by low fT3 levels, was significantly associated with increased mortality rates in ICU patients and a higher failure rate of NIV.