Changes of hypothalamo-pituitary-thyroid function after open heart surgery--especially evaluated by TRH test

Changes of pituitary and thyroid functions after operation were studied in the cases with cardiopulmonary bypass (ECC group) and without bypass (Control group). To know mechanisms of these changes, reaction of hypothalamo-pituitary-thyroid axis by using thyrotropin-releasing hormone (TRH) test and influence of serum cortisol level were examined. Serum concentrations of free T3, free T4 and TSH decreased after open heart surgery. The levels of TSH and free T4 were recovered at the second or third post operative day in Control group, but remained low level at the third POD in ECC group. The level of free T3 continued low level at the 7th POD. The feedback mechanism of the hypothalamo-pituitary-thyroid system was not seen in this series. The response of TSH and free T3 due to TRH test decreased in ECC group, and it was suppressed more clearly in the cases of longer bypass than in those of shorter ones. Serum cortisol level was significantly increased at the 3-hours after the operation in ECC group, and was more increased in the cases of longer bypass than in those of shorter ones. The results indicate that the pituitary and thyroid function on the post bypass period remains euthyroid sick syndrome and hypothalamo-pituitary axis remains suppressed state. Those phenomenon suggest that TRH test may be useful in evaluation of the intensity of surgical stress, especially in those who undergo the cardiopulmonary bypass.