Introduction: Thyroid hormones play a key role in the maintenance of body growthby modulating metabolism and the immune system. In the 20th century,researchers found that thyroid dysfunction is associated with theincreased mortality of patients admitted to the intensive care units (ICU).This study was conducted to evaluate the prognostic value of the thyroid functions; free triiodothyronine(FT3), total triiodothyronine (TT3), free thyroxin (FT4), total thyroxine (TT4) and thyroid-stimulating hormone (TSH) in unselected ICU patients. Methods: A total of 183 unselected critically ill patients without known thyroid diseases were screened for eligibility and followed up during their ICU stay. Patient's baseline characteristics, the Acute Physiology and Chronic Health Evaluation II (APACHE II), thyroid hormones and C-reactive protein (CRP) levels were determined. The primary outcome was ICU mortality. The patients were divided into two groups; group (1) survivors and group (2) nonsurvivors. Potential predictors were analysed for possible association with outcomes. We also evaluated the ability of thyroid hormones together with APACHE II score to predict ICU. Results:Among thyroid hormone functions, FT3had the greatest power to predict ICU mortality, as suggested by the largest area under the curve (AUC) of 0.838. The AUC for FT3 was nearly the same for APACHE II score (0.822) but greater than that for CRP (0.722). Multiple regression analysis revealed that FT3 and TSH levels, APACHE II score and CRP level could independently predict primary outcome. The addition of FT3 and TSH levels to APACHE II score gave an NRI of 55.80%. The level of FT3 showed a significant negative correlation with APACHE II score (r =-0.382, p = 0.000) and with CRP (r =-0.244, p = 0.001).The level of TSH showed a significant negative correlation with APACHE II score (r =-0.194, p = 0.008). Conclusion: Among thyroid functions, the serum levels of both FT3 and TSH are the most powerful and independent predictors of ICU mortality. Moreover, the addition of FT3 and TSH to APACHE II score could significantly improve the ability to predict ICU outcome.