Abstract
The human heart expresses the type 2 deiodinase (D2) that activates thyroxine (T4) to triiodothyronine (T3). At the same time, the inactivating type 3 deiodinase (D3) has been found in a rat model of right ventricular hypertrophy. It is not known whether the human myocardium metabolizes thyroid hormone. This study examined myocardial thyroid hormone metabolism in patients with aortic valve stenosis (AS) undergoing aortic valve replacement and in patients with coronary artery disease (CAD) undergoing coronary artery bypass grafting surgery. Myocardial thyroid hormone metabolism was assessed by analyzing the difference in serum thyroid hormone levels between the aortic root (incoming blood) and the coronary sinus (outgoing blood) of patients undergoing cardiac surgery. A total of 23 patients with AS and 35 patients with CAD were included. Patients received a pre-surgical echocardiogram, and pre-, during and post-surgical thyroid hormone serum levels were collected in the myocardial and peripheral circulations. Patients with AS exhibited the expected left ventricle (LV) hypertrophy (i.e., 20-30% increase in LV posterior wall and interventricular septum thickness and ∼10% increase in AS in LV diastolic diameter). Immediately before cardiopulmonary bypass, blood flowing through the AS myocardium exhibited a 4.6% reduction in T3 and 6.9% increase in rT3 levels, decreasing the serum T3/rT3 ratio by 9.6%. T4 and thyrotropin serum levels remained similar between the aortic root and coronary sinus. In contrast, no myocardial thyroid hormone metabolism was observed in CAD patients. Notably, the AS myocardium lost the ability to inactivate thyroid hormone after cardiopulmonary bypass, possibly due to myocardial stunning. There is accelerated thyroid hormone inactivation in the AS myocardium, which is likely the result of D3 expression. No evidence to suggest thyroid hormone activation in the myocardium was obtained in the present study.
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