Thyroid hormone alterations during and after cardiopulmonary bypass

The haemodynamic effects of thyroid hormones are well known and include those on heart rate, contractility and myocardial oxygen consumption. Cardiopulmonary bypass produces various alterations in endocrine homoeostasis and may exert important haemodynamic effects postoperatively. The purpose of this prospective study was to determine the relation of cardiopulmonary bypass to changes in thyroid function. Blood samples were obtained from 20 patients preoperatively, at specific times before, during and after cardiopulmonary bypass. Total thyroxin (TT4), total triiodothyronine (TT3), free thyroxin (fT4), free triiodothyronine (fT3), thyroid-stimulating hormone (TSH), thyroid-binding globulin (TBG) and albumin were measured by radioimmunoassay and competitive immunoassay. Values of TT4, TT3 and fT3 were significantly depressed up to 24 h after cardiopulmonary bypass (P < 0.05). TSH and fT4 levels remained within normal ranges at all sampling times. It is increasingly evident that cardiopulmonary bypass affects thyroid hormone metabolism, leading to a transient depression characterized by low levels of circulating TT4, TT3 and fT3.