Thyroid hormone directly affects the heart and peripheral vascular system. The 
hormone can increase myocardial inotropy and heart rate and dilate peripheral 
arteries to increase cardiac output. An excessive deficiency of thyroid hormone 
can cause cardiovascular disease and aggravate many preexisting conditions. In 
severe systemic illness and after major surgical procedures changes in thyroid 
function can occur, leading to the "euthyroid sick syndrome." Patients will have 
normal or decreased levels of T4, decreased free and total T3, and usually 
normal levels of thyroid stimulating hormone. This syndrome may be an adaptive 
response to systemic illness that usually will revert to normal without hormone 
supplementation as the illness subsides. Recently, however, many investigators 
have explored the benefits of thyroid hormone supplementation in those diseases 
associated with euthyroid sick syndrome. Thyroid hormone's effects on the 
cardiovascular system make it an attractive therapy for those patients with 
impaired hemodynamics and low T3. Thyroid hormone has also been considered a 
treatment for patients with congestive heart failure, for patients undergoing 
cardiopulmonary bypass and heart transplantation, and for patients with 
hyperlipidemia. At present there is no evidence suggesting a favorable treatment 
outcome using thyroid hormone supplementation for any systemic condition except 
in those patients with documented hypothyroidism.