The cardiovascular system is an important target for thyroid hormones. The 
present study evaluates the changes affecting thyroid hormone metabolism during 
and 6 days after coronary artery bypass and their relationship with the 
post-operative outcome of the patients. Thirty-three patients were enrolled in 
the study; their thyroid hormone profiles were determined at 13 sampling points 
during surgery and for 6 days afterwards. Serum total tri-iodothyronine (T3) and 
free T3 (FT3) concentrations decreased significantly after surgery (P<0.001) and 
they remained significantly low until the end of the study. Free thyroxine (FT4) 
and T4 declined significantly immediately after surgery (P<0.05 for FT4, P<0.001 
for T4) but they returned to baseline values (24 h and 96 h post-surgery 
respectively). Serum reverse T3 increased remarkably 36 h after surgery 
(P<0.001) and remained significantly higher than the baseline value throughout 
the study. A relevant finding was that the days of post-operative 
hospitalization (10+/-3 days, means+/-S.D.) was inversely correlated with the 
slope of the recovery of T3 concentration (P<0.001) or with the area under the 
plasma curves of T3 (P=0.024, time range 72-144 h) and the FT3/FT4 ratio 
(P=0.037, time range 72-144 h) during the post-operative period. Our data 
suggest a prolonged reduction of T4 to T3 conversion in patients undergoing 
cardiac surgery and indicate that the recovery period is the most critical in 
the evaluation of a possibly successful approach for T3 substitutive therapy.