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An acute decrease in cardiac performance can result from a reduced free triiodothyronine (FT3) level following (i) brain death (euthyroid sick syndrome), (ii) a period of cardiopulmonary bypass, and possibly (iii) regional or global myocardial ischemia. The two major pathophysiologic effects of brain death are (i) vascular injury associated with the hemodynamic consequences of the autonomic 'storm', and (ii) a generalized inhibition of mitochondrial function, which results in diminished organ function from the loss of energy stores from a rapid loss of circulating FT3. Deterioration of donor organ function can be reversed by hormonal replacement therapy, in which T3 plays a critical role. This results in (i) an increased number of organs being functionally acceptable, and (ii) increased early and intermediate graft survival. Cardiopulmonary bypass is associated with a reduction in the circulating level of FT3, and this can be associated with deterioration in cardiac function. The administration of T3 at the time of discontinuation of cardiopulmonary bypass reverses this state. In patients undergoing heart transplantation, T3 therapy to both donor and recipient is beneficial.