Cardiac surgery using cardiopulmonary bypass produces a generalized systemic 
inflammatory response, resulting in increased postoperative morbidity and 
mortality. Under these circumstances, a typical pattern of thyroid abnormalities 
is seen in the absence of primary disease, defined as sick euthyroid syndrome 
(SES). The presence of postoperative SES mainly in small children and neonates 
exposed to long bypass times and the pharmacological profile of thyroid hormones 
and their effects on the cardiovascular physiology make supplementation therapy 
an attractive treatment option to improve postoperative morbidity and mortality. 
Many studies have been performed with conflicting results. In this article, we 
review the important literature on the development of SES in paediatric 
postoperative cardiac patients, analyse the existing information on thyroid 
hormone replacement therapy in this patient group and try to summarize the 
findings for a recommendation.