BACKGROUND: Cardiopulmonary bypass (CPB) is an established cause of nonthyroidal 
illness syndrome (NTIS). Off-pump coronary artery bypass (OPCAB) has been 
reported to be less invasive than coronary artery bypass grafting (CABG) with 
CPB. We prospectively evaluated thyroid metabolism in OPCAB patients.
METHODS: We analyzed free thyroid hormones (FT3 and FT4), thyroid-stimulating 
hormone (TSH), and reverse T3 (rT3) in 20 consecutive patients undergoing CABG 
surgery. Nine patients underwent CABG with CPB, and 11 underwent OPCAB. Blood 
samples were taken on admission, on the day of surgery (7:30 AM), after 
sternotomy, at the end of the operation, and at 2, 6, 12, 24, 36, 48, 72, 96, 
120, and 144 hours postoperatively. The concentrations of FT3, FT4, and TSH were 
determined on each sample. Reverse T3 concentration was measured in 10 patients 
up to 48 hours and at 144 hours postoperatively.
RESULTS: Baseline, operative, and postoperative variables were similar in the 
two groups. FT3 concentration dropped significantly (p < 0.0001), reaching its 
lowest value 12 hours postoperatively. There were no significant differences 
between CPB and OPCAB patients. FT4 varied significantly in both groups (p < 
0.0001), but remained in the normal range. TSH variation was not significant. 
rT3 concentration rose significantly (p = 0.0002) in both groups, peaking 24 
hours after surgery. CONCLUSIONS. OPCAB induces a NTIS similar to that observed 
after CPB, probably due to the inhibition of T4 conversion to T3. This finding 
suggests that NTIS is a nonspecific response to stress. CPB should not be 
considered as the sole trigger of NTIS in cardiac surgical patients.