Background and aim: Critically ill patients in the Intensive Care Unit (ICU) have high mortality rates, requiring accurate prognostic tools. The Acute Physiology and Chronic Health Evaluation (APACHE) II score is widely used to assess disease severity, while free triiodothyronine (FT3) has emerged as a potential biomarker reflecting metabolic stress. This study aimed to evaluate the relationship between APACHE II scores, FT3 levels, and mortality in ICU patients. Methods: A cross-sectional analytical study was conducted from May to July 2025 at Dr. Wahidin Sudirohusodo Hospital and Hasanuddin University Teaching Hospital, Makassar. Thirty critically ill adults meeting inclusion criteria were assessed. APACHE II scores were calculated within 24 hours of ICU admission, and thyroid hormones were measured using ECLIA or CMIA. Associations between APACHE II, FT3, and mortality were analyzed using appropriate statistical tests, with p<0.05 considered significant. Results: Participants had a mean APACHE II score of 16.93±5.10 and mean FT3 level of 1.63±0.20 pg/mL. A negative but non-significant correlation was observed between APACHE II score and FT3 level (r = –0.297, p>0.05). Low FT3 was significantly associated with higher mortality (66.7% vs. 11.1% in normal FT3; p<0.05; OR 16.0; 95% CI 1.66 154.6). Higher APACHE II scores were also significantly associated with mortality (p<0.05). Conclusions: Although no significant correlation was found between APACHE II scores and FT3 levels, both higher APACHE II scores and low FT3 were independently associated with increased mortality. Routine FT3 monitoring may complement APACHE II in prognostic assessment of critically ill patients. (www.actabiomedica.it)