Background: Non-thyroidal illness syndrome (NTIS) is dysregulation of thyroid hormones despite normal thyroid gland function and is diagnosed by low serum triiodothyronine (T3) levels. This condition is associated with caloric deprivation, severe illness and deleteriously affects ICU patients. 
Objectives: This study tried to illustrate the relation between thyroid hormonal homeostasis at time of the surgical intensive care unit (SICU)" admission and oncoming complications i.e., in SICU complications rate (CR) and mortality rates (MR).
Patients and methods: Physiological derangement was assessed using the acute physiology and chronic health evaluation II (APACHE II) score. Serum levels of thyroid stimulating hormone (TSH), free and reverse triiodothyronine (fT3 and rT3) and free thyroxine (fT4). The fT3/fT4 ratio was calculated, and ratio <2 suggests the diagnosis of NTIS. Patients were categorized as survivors (SG) and non-survivors (NSG) groups. Study outcomes are the relation between thyroid hormonal levels and patients' outcomes.
Results: The CR and MR were 53.9% and 28.9%, respectively. ICU stay was significantly longer and CR was significantly higher among NSG patients. Mean fT3 levels were significantly lower, while rT3 levels were significantly higher in NSG patients. The mean fT3/fT4 ratio was significantly lower in NSG than in SG patients and 102 patients (44%) had a ratio of <2 with a significant intergroup difference. High APACHE II score, low fT3, and low ratio are the significant predictors for high CR, while low ratio was the highly significant negative predictor for survival.
Conclusion: Low at-admission levels of fT3, low fT3/fT4 ratio and high rT3 levels could predict the vulnerability for getting In-ICU complications and mortality.