Early total triiodothyronine, illness severity, and in-hospital mortality in critically ill adults.

Low T3 is common in critical illness, but whether it adds prognostic information beyond overall illness severity remains uncertain. We performed a retrospective cohort study of adults in the first ICU stay recorded in MIMIC-IV who underwent total triiodothyronine measurement within 48 h of ICU admission and had no documented thyroid disease. The primary outcome was in-hospital mortality. Of 201 included patients, 65 (32.3%) had low T3 (< 60 ng/dL). Compared with patients with normal T3, those with low T3 had higher SOFA scores, more frequent mechanical ventilation and vasopressor use, and higher in-hospital mortality (24.6% vs. 8.1%, P = 0.003). Low T3 was associated with mortality in the unadjusted model (OR 3.71, 95% CI 1.61-8.56) and after adjustment for age, sex, and Charlson Comorbidity Index (OR 2.83, 95% CI 1.17-6.81), but the association was attenuated after additional adjustment for SOFA score (OR 1.37, 95% CI 0.51-3.68). Cox regression showed a similar pattern (HR 1.06, 95% CI 0.42-2.67). Early low total T3 identified ICU patients with greater illness severity and higher crude mortality; however, its association with mortality was largely attenuated after SOFA adjustment.