Thyroid hormone (TH) promotes cardiac mitochondrial maturation and substrate metabolism after birth. This regulation involves ligand dependent binding of nuclear TH receptors to target gene elements. TH also putatively controls genes indirectly by modulating transcription and/or translation of other nuclear steroid receptors and co-activators, such as peroxisome proliferator-activated receptor- (PPAR) and peroxisome proliferator-activated receptor- coactivator-1 (PGC-1). We tested the hypothesis that TH influences PPAR and PGC-1 regulation of metabolic genes during postnatal maturation in sheep heart in vivo. We measured their mRNAs and/or protein levels, and downstream targets in left ventricle from lambs: fetal (F), 30-day old after postnatal thyroidectomy (THY), and 30-day old euthyroid (CON). Both PPAR and PGC-1 mRNA expression decreased from F to C, while PGC-1 protein increased substantially, and PPAR did not change. THY limited this mRNA response and attenuated the paradoxical postnatal PGC-1 protein elevation, but did not alter mRNA levels for PPAR, Nuclear Res-piratory Factor-1 and Hypoxia Inducible Factor-1. THY promotion in PPAR mRNA did not change PPAR protein or mRNA for PPAR target genes, pyruvate-dehydrogenase kinase 4 and muscle type carnitine-palmitoyl-transferase I. THY reduction in PGC-1 protein, occurred while reducing cytochrome c oxidase and cytochrome c content, and decreasing cardiac maximal inherent respiratory capacity. These data imply that TH modulates mitochondrial maturation partly through posttranscriptional control of PGC-1, while any important regulation of PDK4 and mCPTI by change in PPAR protein expression remains doubtful. Also, the paradoxical expression pattern between mRNA and protein, particularly for PGC-1, suggests a feedback control mechanism.