Thyroid hormone metabolism and level of illness severity in pediatric cardiac surgery patients.

To define better the temporal relationship between alterations in thyroid hormone metabolism and changes in illness severity, we studied thyroid function in 12 children preoperatively and at 2, 12, 24, 48, and 72 hours after cardiac surgery. We then compared these findings with illness severity as assessed by a modification of the Therapeutic Intervention Scoring System (TISS). All patients had significant (p less than 0.01) reduction in serum T3 (mean 35% of baseline levels) and elevation in serum rT3 (mean 237% of baseline levels) in the postoperative period. Average correlation coefficients for the combined data were as follows: TISS and T3 levels, -0.63 +/- 0.17; TISS and rT3 levels, 0.51 +/- 0.18. Examination of individual patient data normalized to preoperative T3 and rT3 levels and compared with TISS scores demonstrated a lag in alterations of serum rT3 and T3 levels relative to illness severity during recovery from cardiac surgery. We conclude (1) the euthyroid sick syndrome occurs in all pediatric cardiac surgery patients, regardless of procedure complexity, (2) changes in rT3 levels parallel but follow changes in degree of therapeutic intervention, indicating that these alterations result from, and do not cause, increasing severity of illness, (3) rT3 levels return toward normal before T3 levels, which remain low beyond the critical postoperative period, and (4) clinical indicators of illness severity are fairly well inversely correlated with T3 levels, supporting an adaptive reduction in 5'-deiodinase activity during the postoperative period.