A therapeutic formulation of the primary physiologically active form of endogenous thyroid hormone. In vivo, triiodothyronine enters the nucleus and binds to nuclear thyroid hormone receptors that subsequently bind to thyroid response elements (TREs) located in target genes. Receptor binding by triiodothyronine in combination with recruited coactivators results in maximal transcriptional activation after binding to TREs; in general, binding of thyroid hormone receptor alone to TREs leads to repression of gene transcription.