Cardiopulmonary bypass and thyroid function: A “euthyroid sick syndrome”

Abstract

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 (T 3), and reverse T 3, an inactive metabolite of thyroxine, were measured by radioimmunoassay. Free T 3 was assayed by equilibrium dialysis. Values of total T 3 and free T 3, the active hormone, were significantly depressed (75% and 50%, respectively) up to 24 hours after bypass ( p < 0.05). Reverse T 3 demonstrated a greater than fourfold rise at 8 and 24 hours postoperatively ( p < 0.05). Thyroid-binding globulin was decreased at all sampling times ( p < 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 T 3 and free T 3 concentrations with a concomitant increase in reverse T 3 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 T 3 in the prevention or treatment of low cardiac output syndrome after cardiopulmonary bypass.