Thyroid-pituitary response to cardiopulmonary bypass.

The effects of open heart surgery on various parameters of thyroid-pituitary function were studied in 11 patients. The pattern of hormonal changes was variable. T4 concentration increased during anaesthesia and bypass, following which it returned to normal once haemodynamic stability had been regained. The pattern of change in T3 concentration did not follow consistently that of T4 and the proportional increase in concentration was not as great. There was a significant reduction in T3 concentration with the onset of haemodynamic stability. For individual patients there was no obvious correlation between changes in the concentrations of TSH, T3 and T4. The increases may be explained by release of T3 or T4 from extrathyroid tissue.