Cardiopulmonary bypass and thyroid function: a "euthyroid sick syndrome".

The purpose of this prospective study was to define the effect of cardiopulmonary bypass on the concentrations of thyroid hormones and metabolites. Blood samples were obtained from 14 patients preoperatively, at specific times throughout cardiopulmonary bypass, and serially to 24 hours postoperatively. Thyroid-stimulating hormone, thyroid-binding globulin, total thyroxine, triiodothyronine (T3), and reverse T3, an inactive metabolite of thyroxine, were measured by radioimmunoassay. Free T3 was assayed by equilibrium dialysis. Values of total T3 and free T3, the active hormone, were significantly depressed (75% and 50%, respectively) up to 24 hours after bypass (p less than 0.05). Reverse T3 demonstrated a greater than fourfold rise at 8 and 24 hours postoperatively (p less than 0.05). Thyroid-binding globulin was decreased at all sampling times (p less than 0.05). Thyroid-stimulating hormone, thyroxine, and free thyroxine levels remained within normal ranges at all sampling times. These results indicate that cardiopulmonary bypass simulates the "euthyroid sick syndrome" as seen in severely burned patients and critically ill patients, which is characterized by depression of T3 and free T3 concentrations with a concomitant increase in reverse T3 levels and normal concentrations of thyroid-stimulating hormone, thyroxine, and free thyroxine. The hemodynamic effects of primary hypothyroidism are well established. These data provide further support for investigational trials of intravenous administration of T3 in the prevention or treatment of low cardiac output syndrome after cardiopulmonary bypass.