Thyroid hormone regulates cardiac metabolism through multiple mechanisms. Traditionally, most cardiac metabolic studies have focused on presumed transcriptional actions by defining thyroid hormone-induced changes in mRNA or protein levels. Recent studies have established metabolic pathways in heart that rapidly respond to thyroid hormone. Functions have also been implicated for thyroid hormone receptors, which are separate from their transcriptional actions. Finally, thyroid through ligand binding may play a direct role in transactivation of mitochondrial DNA. This review will explore these newly identified modes of thyroid action on metabolism in heart.