OBJECTIVE: The purpose of this study was to evaluate serum triiodothyronine 
levels as a trigger of postoperative atrial fibrillation (AF) in elderly 
patients undergoing cardiac surgery and to study the possible association of 
serum triiodothyronine levels with preoperative and postoperative hemodynamics.
DESIGN: Prospective study.
SETTING: University hospital.
PARTICIPANTS: Forty-six consecutive nonemergency patients 65 years or older 
undergoing cardiac surgery during 1999 to 2000 in Tampere University Hospital, 
Tampere, Finland.
INTERVENTIONS: Free serum T3 concentration was used as a measure of serum 
triiodothyronine levels. Samples were taken preoperatively, on the fourth 
postoperative day, and at the 3-month follow-up. The hemodynamic state of the 
patients was estimated by whole-body impedance cardiography preoperatively, 
during the intensive care unit period, daily until the fourth postoperative day, 
and at the 3-month follow-up.
MEASUREMENTS AND MAIN RESULTS: AF occurred in 43% of the patients. The patients 
in the AF group had significantly more grafts (3.9 v 3.1, p = 0.02), and there 
was a small difference in age between the AF and non-AF groups (73 years v 69 
years, p = 0.06). The free T3 concentration on the fourth postoperative day was 
significantly lower in the AF group (3.5 nmol/L v 4.6 nmol/L, p = 0.04). In 
logistic regression analysis, the independent predictors of AF were age, number 
of grafts, and serum free T3 concentration on the fourth postoperative day. In 
the group with low T3 concentration, the cardiac index was lower (1.4 v 1.8, p = 
0.05) and the systemic vascular resistance index was higher (4,064 v 2,969, p = 
0.04) but only immediately after the operation. Although the AF mostly appeared 
during the second to fourth postoperative days, there were no longer any 
differences in the hemodynamic state at that time.
CONCLUSIONS: In a group of elderly patients undergoing cardiac surgery, there 
was a strong association between a postoperative decrease of serum 
triiodothyronine levels and atrial fibrillation. The decrease of serum 
triiodothyronine levels was related to the changes of hemodynamic parameters 
only in the immediate postoperative period.