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Objective: To determine if triiodothyronine alters lactate, glucose, and pyruvate metabolism, and if serum pyruvate concentration could serve as a predictor of low cardiac output syndrome in children after cardiopulmonary bypass procedures., Methods: This study was ancillary to the Oral Triiodothyronine for Infants and Children undergoing Cardiopulmonary bypass (OTICC) trial. Serum pyruvate was measured in the first 48 patients and lactate and glucose were measured in all 208 patients enrolled in the OTICC study on the induction of anaesthesia, 1 and 24 hours post-aortic cross-clamp removal. Patients were also defined as having low cardiac output syndrome according to the OTICC trial protocol., Result: Amongst the designated patient population for pyruvate analysis, 22 received placebo, and 26 received triiodothyronine (T3). Lactate concentrations were nearly 20 times greater than pyruvate. Lactate and pyruvate levels were not significantly different between T3 and placebo group. Glucose levels were significantly higher in the placebo group mainly at 24-hour post-cross-clamp removal. Additionally, lactate and glucose levels peaked at 1-hour post-cross-clamp removal in low cardiac output syndrome and non-low cardiac output syndrome patients, but subsequently decreased at a slower rate in low cardiac output syndrome. Lactate and pyruvate concentrations correlated with glucose only prior to surgery., Conclusion: Thyroid supplementation does not alter systemic lactate/pyruvate metabolism after cardiopulmonary bypass and reperfusion. Pyruvate levels are not useful for predicting low cardiac output syndrome. Increased blood glucose may be regarded as a response to hypermetabolic stress, seen mostly in patients with low cardiac output syndrome.