Nonthyroidal illness (formerly called sick euthyroid syndrome) is a common condition in patients hospitalized in the intensive care unit, and it is caused by changes in the levels of thyroid hormones in the blood . These changes are correlated to the severity of the critical illness, morbidity and mortality. This condition is characterized by low serum T3 levels, high rT3 levels, normal or low T4 levels, and normal or low TSH levels. Previous studies have shown a relationship between NTIS (nonthyroidal illness) and poor therapeutic outcomes in patients admitted with sepsis, multi trauma, ARDS, acute respiratory failure, septic shock and mechanically ventilated patients in intensive care . In general, the recommendation is not to routinely take blood tests to evaluate thyroid function in critical patients in the intensive care unit, unless they were previously known to have a thyroid disease or there is a clinical suspicion that such a disorder developed during hospitalization in the intensive care unit. Also, studies so far have not shown an advantage for eltroxin administration to intensive care patients with low T3 and/or T4 levels when the laboratory tests indicate NTIS. However, there is a specific group of patients, with NTIS and extremely low T4 levels (less than 50% of the lower limit of the normal range) who have a particularly high mortality rate in intensive care (over 85%) and in whom it has not been investigated whether the administration of eltroxin will improve the therapeutic outcomes ( , i.e. was such a low T4 level in them considered a marker for the severity of their general critical illness, or is it another disorder in itself, the treatment of which would benefit the patient) . We would like to examine whether the administration of eltroxin in this specific group of patients, which has not been studied so far, will be beneficial in improving the therapeutic outcomes