Prognostic Significance of Dynamic Free T3 Changes in Critically Ill Patients

Background: Non-thyroidal illness syndrome is frequent in critically ill patients, but the prognostic value of dynamic changes in thyroid function tests remains unclear. This study evaluated whether serial measurements of thyroid-stimulating hormone (TSH) and free triiodothyronine (FT3) provide additional predictive value for 30-day mortality beyond conventional severity scores in ICU patients. Methods: This single-center retrospective observational study included 74 adult patients treated for ≥72 h in a general ICU who had TSH and FT3 measured within 24 h of admission and repeated at 48–72 h. Patients aged 18 years or above admitted to the intensive care unit were included in the study. Demographic characteristics, comorbidities, APACHE II, SOFA, modified NUTRIC (mNUTRIC) scores, and routine laboratory data (including albumin, CRP, and lactate) were recorded. The primary outcome was 30-day mortality. Between-group comparisons were performed using t-tests, Mann–Whitney U, and Chi-square tests. Variables significant in univariate analyses were entered into binary logistic regression models, and predictive performance was assessed using receiver operating characteristic (ROC) curves and the Youden index. Results: The mean age was 68.7 ± 14.7 years, and 41.9% (n = 31) of the patients died within 30 days. Non-survivors had higher APACHE II, SOFA, and mNUTRIC scores and lower albumin, lymphocyte count, and second FT3 levels compared with survivors (all p ≤ 0.003). Baseline FT3 and TSH were not associated with mortality, whereas both the subsequent FT3 measurements and the ΔT3 (variance in former to latter FT3) were remarkably predictive. The latter FT3 < 1.63 pg/mL produced an AUC of 0.835 (sensitivity: 77%, specificity: 74%), and a ΔT3 log ratio threshold of −0.09 (≈20% early FT3 decline) produced an AUC of 0.835 (sensitivity: 71%, specificity: 81%). The APACHE II + ΔT3 (numeric) model showed the best discrimination (AUC: 0.921; sensitivity: 87.1%, specificity: 81.4%), outperforming APACHE II alone (AUC: 0.861). Conclusions: In critically ill adult patients, dynamic T3 kinetics—particularly premature decline in FT3 within the first 72 h—provide incremental prognostic value for 30-day mortality beyond APACHE II. Serial FT3 monitoring may help identify high-risk patients whose endocrine adaptation to critical illness is failing.