The effect of a surgical stress on serum TSH, T4, free T4, T3, rT3 and TBG was 
measured before, during and after a coronary bypass operation. Precautions were 
taken to age, sex, pre-operative nutritional state, anamnestic illness, pre- and 
post-operative medication. Compared to the pre-operative period TSH increased 
during operation and normalized at the first post-operative day, while only 
minor alterations for total and free serum T4 were observed. T3 and rT3 declined 
at the start of extracorporal circulation, T3 remained reduced during the 
post-operative period, while rT3 reached its pre-operative value at the first 
post-operative day. It is concluded that the coronary bypass operation evokes a 
rapid decline in T3, which is not normalized by the TSH induced response of the 
thyroid gland, while the post-operative period is characterized by a "low T3 
state". The clinical relevance of this finding is discussed with respect to the 
cardiovascular and the nutritional state of the patients in the post-operative 
period.