Thyroid hormone status was assessed in 132 children with congenital heart 
defects undergoing cardiac surgery (median age 3.1 y; range 2 d to 16.2 y). 
Plasma TSH, thyroxine (T4), free thyroxine (fT4), triiodothyronine (T3), reverse 
triiodothyronine (rT3), thyroglobulin (Tg), and urinary iodine excretion were 
measured before and every other day after cardiac surgery (d 1-21). After 
surgery we observed strikingly low plasma concentrations of TSH (0.4 mU/L; 
0.2-1.3), T3 (0.6 nmol/L; 0.3-1.2), T4 (48.9 nmol/L; 12.9-82.4), IT4 (12.9 
pmol/L; 5.1-19.3), and Tg (9.4 micrograms/L; 1.5-20.6), whereas rT3 plasma 
concentrations increased (0.13 pmol/L; 0.05-0.3; n = 40). The maximal 
post-operative changes of TSH and rT3 preceded changes of T3, T4, fT4, and Tg. 
Postoperative urinary iodine excretion increased significantly (n = 109). 
Thyroid hormone plasma concentrations were lowest after cardiopulmonary bypass 
operations and in patients treated with dopamine. In patients with postoperative 
T3 plasma concentrations less than 0.6 nmol/L (n =52) the period of mechanical 
ventilation and intensive care treatment was significantly prolonged. 
Furthermore, the cumulative doses of inotropic and vasoactive catecholamines and 
furosemide were significantly higher in this patient group. Our results 
demonstrate transient secondary hypothyroidism in children after cardiac surgery 
that may contribute to postoperative cardiac and respiratory dysfunction and may 
delay recovery. Possible benefits of thyroid hormone replacement therapy need to 
be thoroughly examined.