The Prognostic Role of Non-Thyroidal Illness Syndrome in Critically Ill Patients: A Retrospective Cohort Study

Aim: Non-thyroidal illness syndrome (NTIS), characterized by reduced triiodothyronine (T3) levels in the absence of intrinsic thyroid disease, is common among critically ill patients.However, its independent association with intensive care unit (ICU) mortality remains uncertain.This study investigated the association between NTIS and ICU outcomes in adult patients admitted to a tertiary-level medical ICU. Study Design:We retrospectively analyzed adult patients (18 years) admitted to a tertiarylevel medical ICU between May 2021 and May 2023.NTIS was defined as reduced serum T3 with normal or low thyroid-stimulating hormone (TSH) and thyroxine (T4) levels.Patients with known thyroid disease, corticosteroid therapy within the preceding seven days, SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2) positivity, prior ICU admission, ICU stay <24 hours, or recent radiocontrast exposure were excluded.Demographic, clinical, and laboratory characteristics were compared between patients with and without NTIS.Results: Of 109 patients, 85 (78%) had NTIS.ICU mortality was numerically higher among NTIS patients (30.5%) compared with those without NTIS (16.6%), although the difference was not statistically significant.The NTIS group had higher Sequential Organ Failure Assessment (SOFA) scores (median 8.5 vs. 6; p=0.054) and more frequent need for mechanical ventilation (55.2% vs. 41.6%;p=0.124).No significant differences were observed in ICU length of stay (p=0.17) or hospital mortality (p=0.178).Conclusions: Non-thyroidal illness syndrome was not independently associated with ICU mortality but showed a strong correlation with markers of illness severity.These findings suggest that NTIS may serve as a biomarker of critical illness severity rather than an independent predictor of mortality.