Recently changes in thyroid physiology during acute and chronic medical illness were demonstrated. The serum $fT_3,;rT_3,;T_4,;T_3,;fT_4$, and TSH concentration were measured by radioimmunoassay 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 $fT_3$ concentration was $6.68{pm}1.05pmol/ml$ in normal subjects while in patients with critical illness the serum $fT_3$ concentration was significantly lowered to $1.55{pm}1.15pmol/ml$(p<0.001). 2) The mean serum $rT_3$ concentration was $0.22{pm}0.44ng/ml$ in normal subjects and $0.42{pm}0.37ng/ml$ in patient with critical illness. There was increment in critically ill patients as compared to normal subjects but no statistically significant difference(p>0.05). 3) The mean serum $T_3$ concentration was $1.24{pm}0.25ng/ml$ in normal subjects and $0.56{pm}0.56ng/ml$ in patients with criticial illness and there was significant difference in each other(p<0.005). 4) The mean serum $T_4,;fT_4$, and TSH concentrations were $7.80{pm}1.02{mu}g/dl,;1.26{pm}0.39ng/dl,;1.87{pm}0.45{mu}U/ml$ in normal subjects respectively and $6.02{pm}3.06{mu}g/dl,;1.46{pm}0.80ng/dl,;1.74{pm}0.79{mu}U/ml$ in patients with critical illness and there was no significant difference between critically ill patients and normal subjects. 5) The ratio of mean serum concentration of $fT_3$ and $rT_3(fT_3/rT_3)$, $30.42{pm}5.58$ in normal subjects was significantly higher(p<0.005) than the coresponding patients with critical illness. 6) The mean serum $fT_3$ concentration in expired cases(n=12) during admission was significant difference between expired and survived cases(p<0.005). The mean serum $rT_3$ centration was $0.67{pm}0.58ng/ml$ in expired cases and $0.34{pm}0.22ng/ml$ in survived cases with significant difference(p<0.005). Half of the cases who showed less than $3{mu}g/dl$ of serum $T_4$ level were expired.