A Hypothesis about the Cause of Fatal Outcomes in Sars-Cov-2 Mohammad Reza Besharati 1 , Sharif University of Technology, Tehran, Iran Nafiseh Jafari, Malek Ashtar University of Technology, Tehran, Iran Dr. Ehsan Mostafavi 2 , Pasteur institute of Iran Dr. Mohammad Izadi 2 , Sharif University of Technology, Tehran, Iran Dr. Alireza Talebpour 2 , Shahid Beheshti University, Tehran, Iran There are some reports about T3 reduction in some Sars-Cov-2 patients[1][2][3][4]. Based on a rigorous machine learning classification[5], we have found that the T3-Level is the key-control-variable for fatal outcomes in reported metabolomics data[6] of Sars-Cov-2 patients (Please See the Figure-1 which represents the computed Decision Tree). Figure 1- The resulting decision tree of J48 classification algorithm suggests that the key control variable of “death” and “survive” in sever Covid-19 cases is the level of T3 thyroid Hormone in the blood [5]. Based on prior research literature, The Lower levels of triiodothyronine hormone (T3) is associated with unbalanced states of the immune system: down-regulation of T3 is associated with up-regulation of "CD4/CD8 ratio"[6]. This phenomenon could yield severe autoimmune reactions and could be responsible for fatal outcomes of Sars-Cov-2 [7]. So the people who have lower levels of T3 in their blood are at a higher level of mortality risk. Age, diabetes, obesity, ethnicity, gender, genetics, epigenetics, and 1 Responsible Author 2 Supervisor