Recently changes in thyroid physiology during acute and chronic medical illness were demonstrated. The serum fT3, rT3, T4, T3 fT4 and TSH concentration were measured by radioimmunoassy method in 49 patients with critical illness and 10 normal subjects to assess the change of thyroid function in critical illness. The results were as follows; 1) The mean serum fT3 concentration was 6.68±1.05 pmol/ml in normal subjects while in patients with critical illness the serum fT3 concentration was significantly lowered to 1.55±1.15 pmol/ml (p〈0.001). 2) The mean serum rT3 concentration was 0.22±0.44 ng/ml in normal subjects and 0.42±0.37 ng/ml in patient with critical illness. The were was increment in critically ill patients as compared to normal subjects but no statistically significant difference (p〉0.05). 3) The mean serum T3 concentration was 1.24±0.25 ng/ml in normal subjects and 0.56±0.56 ng/ml in patients with criticial illness and there was significant difference in each other (p 〈0.005). 4) The mean serum T4, fT4, and TSH concentrations were 7.80±1.02 μg/dl, 1.26±0.39 ng/d1, 1.87±0.45 μU/ml in normal subjects respectively and 6.02±3.06 μg/dl, 1.46±0.80 ng/d1, 1.74±0.79 μU/ml in patients with critical illness and there wqs no significant difference between critically ill patients and normal subjects. 5) The ratio of mean serum concentration of fT3 and rT3 (fT3/rT3), 30.42±5.58 in normal subjects was significantly higher (p〈0.005) than the coresponding value, 6.39±6.76 in patients with critical illness. 6) The mean serum fT3 concentration in expired cases (n=12) during admission was significant difference between expired and survived cases (p〈0.005). The mean serum rT3 concentration was 0.67±0.58 ng/ml in expired cases and 0.34±0.22 ng/ml in survived cases with significant difference (p 〈0.005). Half of the cases who showed less than 3 μg/dl of serum T4 level were expired.