Effects of triiodothyronine supplementation after myocardial ischemia. 
Cardiopulmonary bypass causes a "euthyroid-sick" state characterized by low levels of circulating triiodothyronine.  Triiodothyronine supplementation in this setting has been postulated to improve postischemic left ventricular function by increasing the availability of myocardial high-energy phosphates.  These postulates have not been substantiated, however, using load-independent parameters of left ventricular function and analysis of high-energy phosphate metabolism.  To test these hypotheses, 14 healthy pigs (30 to 40 kg) were placed on cardiopulmonary bypass and instrumented with left ventricular minor-axis ultrasonic crystals and micromanometer-tipped pressure catheters.  Hearts were subjected to 30 minutes of global, normothermic ischemia.  Triiodothyronine (0.1 mg/kg; n = 7) or placebo (n = 7) was administered in a random, investigator-blinded fashion at the removal of the aortic cross-clamp and after 60 minutes of reperfusion.  Hemodynamic, metabolic, and ultrastructural data were obtained before ischemia and after 30, 60, 90, and 120 minutes of reperfusion.  By 90 minutes of reperfusion left ventricular contractility had returned to preischemic levels in hearts supplemented with triiodothyronine, despite postischemic myocardial adenosine triphosphate levels of 50% to 60% of baseline in both groups.  Ultrastructurally, the sarcoplasmic reticulum and mitochondria were significantly better preserved in the group treated with triiodothyronine.  This study suggests that triiodothyronine supplementation significantly enhances postischemic left ventricular functional recovery and that this recovery is due to mechanisms other than enhanced availability of myocardial high-energy phosphates.