Non-Thyroidal Illness and Its Impact on Outcomes in Critically Ill Patients: A Prospective Cohort Study

Background: Non-thyroidal illness syndrome (NTIS) is a common condition in critically ill patients, yet its prognostic value remains unclear. This study aimed to assess the incidence of NTIS among critically ill patients and evaluate its association with clinical outcomes and mortality. Methods: A prospective cohort study was conducted in a tertiary hospital between 2019 and 2025. Adult intensive care unit (ICU) patients without known thyroid disease were included. Thyroid function tests (thyroid-stimulating hormone (TSH), free thyroxine (T4) and free triiodothyronine (T3) were measured within 24 hours of admission. Clinical data, comorbidities, and outcomes were recorded. Patients were classified as having low T3 (< 3.0 nmol/L) or normal T3 (≥ 3.0 nmol/L). Logistic regression and ROC analyses were performed to assess associations with mortality. Results: Among 180 patients (mean age 57.7 ± 18 years; 62% male), 52% had NTIS. Patients with low T3 were older and had higher rates of diabetes, cardiovascular, and pulmonary diseases (p < 0.05). They more frequently developed shock, respiratory failure, and required mechanical ventilation. ICU mortality was significantly higher among the low T3 group (43% vs. 7.5%, p < 0.001). After adjustment for age, comorbidities, and illness severity, both low T3 (OR 0.23, 95% CI 0.07–0.79; p = 0.007) and low T4 (OR 0.80, 95% CI 0.65–0.99; p = 0.03) remained independently associated with mortality, whereas TSH showed no significant association. ROC analysis demonstrated that T3 had the strongest inverse correlation with survival (AUC = 0.208, p < 0.001). Conclusion: NTIS is highly prevalent in critically ill patients and is associated with increased mortality and adverse outcomes. Low T3, in particular, reflects disease severity and may serve as a prognostic marker in the ICU.