Changes in plasma free thyroid hormones during cardiopulmonary bypass do not indicate triiodothyronine substitution

Concentrations of free and bound thyroid hormone in plasma during cardiopulmonary bypass were determined in 10 patients. Ages ranged from 50 to 76 years (median 61 years). Apart from thyroxine and triiodothyronine, a number of other variables that might contribute to an altered thyroid state or binding of thyroxine and triiodothyronine to plasma proteins were measured (thyroid stimulating hormone, reverse triiodothyronine, albumin, nonesterified fatty acids, and cortisol). At the end of the observation, total triiodothyronine and total thyroxine concentrations were 86% and 70% (mean) of initial levels, respectively, and reverse triiodothyronine and albumin concentrations were both 74%. On the contrary, free triiodothyronine and free thyroxine levels rose to a maximum of 137% and 203% (mean), but normalized after heparin neutralization (86% and 102%). Thyroid-stimulating hormone concentration declined insignificantly during cardiopulmonary bypass, reaching a minimum at the time when body temperature was maximally depressed (74%), and then it normalized (101%). Plasma cortisol level did not rise above normal limits despite the major stress stimulus. In conclusion, total triiodothyronine and total T4 concentration in plasma of patients undergoing cardiac operations is lowered, while the exchangeable pool concentrations (plasma free triiodothyronine and free thyroxine) are elevated during bypass but return to control levels at the end of the operation. If serum changes truly reflect intracellular events, it does not seem likely that the myocardial cell is depleted of thyroid hormones after cardiopulmonary bypass, as recently claimed by others. This finding does not, however, exclude the possibility that a pharmacologic dose of triiodothyronine may be convenient for improvement of cardiac performance.