Objective To investigate the therapeutic effects of thyroid hormone replacement on critically ill COPD patients with low serum thyroid hormone. Methods Sixty-seven critically ill patients with acute exacerbation of COPD ( AECOPD) , and complicated with respiratory and/ or heart failure and low serum thyroid hormone, admitted from July 2008 to June 2011, were recruited for the study. They were randomly divided into an intervention group ( n = 34) and a control group ( n = 33) . The control group received conventional treatment and the intervention group received conventional treatment plus additional thyroid hormone replacement therapy. Results Compared with the control group, the overall efficacy of the intervention group was not significantly different ( 88. 2% vs. 81. 8% , P gt; 0. 05) , while average effective time was significantly shorter [ ( 9. 6 ±2. 5) d vs. ( 12. 3 ±2. 8) d, P lt; 0. 05] . The post-treatment serum FT3 , FT4 , TT4 , and h-TSH levels were significantly higher in the intervention group than those in the control group, and significantly higher than baseline ( P lt;0. 05) . Conclusions For AECOPD patients complicated with respiratory and/or heart failure and low serum thyroid hormone, thyroid hormone supplement at low dosage will help to improve serumthyroid hormone level, and promote early recovery.