We believe that a correction for the hemodilution which occurs during CABS is critical and that hormone values measured during surgery must be interpreted in light of this correction. Serum T3, when corrected in this manner, is unchanged throughout CABS. These data are in contrast to recent findings by Robushi et al, who describe a low serum T3 during CABS; however, no correction for hemodilution was made in that study. While serum FT3 levels have been noted to increase during euther-mic abdominal surgery, others have reported no change during euthermic abdominal or euthermic CABS. Our data show both an increase in free T3 levels and an increase in the dialyzable fraction of T3 after the onset of bypass and hypothermia. This increase in the dialyzable fraction of T3 likely reflects a change in hormone binding, as seen with aspirin, heparin, or anesthesia. Similarly, our findings of increased FT4 during CABS are consistent with other reports. Since FT4 and the dialyzable fraction of thyroxine increased very quickly after bypass in the present study, a likely explanation is decreased binding of thyroxine, as occurs with heparin or opiates, rather than a rapid change in the production or degradation of this hormone. Decreased binding of thyroxine to plasma proteins also occurs during various forms of nonthyroidal illness. The slight elevation in the corrected T4 and a rise in the T/T3 ratio may represent decreased peripheral disposal during deep hypothermia and CABS, or possibly the release of T4 from a thyroidal or peripheral tissue reservoir. add comment
Our data do not allow us to differentiate between the various speculated mechanisms. The increases in serum FT3, FT4 and T4 concentrations occur rapidly after the onset of bypass and persist during hypothermia. Any end organ effects of these acutely elevated iodothyronine levels are completely unknown and require further investigation.