BACKGROUND: Patients with congestive heart failure (CHF) often have low serum 
triiodothyronine (T(3)) concentrations. In a rodent model of myocardial 
infarction-induced CHF and low serum T(3), we hypothesized that replacing T(3) 
to euthyroid levels would improve left ventricular function without producing 
untoward signs of thyrotoxicosis.
METHODS AND RESULTS: Adult male Sprague-Dawley rats were subjected to left 
anterior descending coronary artery ligation (myocardial infarction). One week 
post-myocardial infarction, left ventricular fractional shortening was 
significantly reduced to 22+/-1% in CHF animals versus 38+/-1% for sham-operated 
controls (P<0.001). Serum T(3) concentration was also significantly reduced 
(80+/-3 versus 103+/-6 ng/dL; P<0.001), in CHF animals versus Shams. At 9 weeks 
post-myocardial infarction, systolic function (+dP/dt max) was significantly 
attenuated in CHF animals (4773+/-259 versus 6310+/-267 mmHg/s; P<0.001) as well 
as diastolic function measured by half time to relaxation (15.9+/-1.2 versus 
11.1+/-0.3 ms; P<0.001). alpha-myosin heavy chain expression was also 
significantly reduced by 77% (P<0.001), and beta-myosin heavy chain expression 
was increased by 21%. Continuous T(3) replacement was initiated 1 week 
post-myocardial infarction with osmotic mini-pumps (6 microg/kg/d), which 
returned serum T(3) concentrations to levels similar to Sham controls while 
resting conscious heart rate, arterial blood pressure and the incidence of 
arrhythmias were not different. At 9 weeks, systolic function was significantly 
improved by T(3) replacement (6279+/-347 mmHg/s; P<0.05) and a trend toward 
improved diastolic function (12.3+/-0.6 ms) was noted. T(3) replacement in CHF 
animals also significantly increased alpha- and reduced beta-MHC expression, 
(P<0.05).
CONCLUSIONS: These data indicate that T(3) replacement to euthyroid levels 
improves systolic function and tends to improve diastolic function, potentially 
through changes in myocardial gene expression.