Overview of thyroid hormone action. T3 enters the cell via thyroid hormone transporters, or is generated locally by cytoplasmic deiodinases (not shown). In the nucleus T3 binds to THR-containing dimers, which bind to genomic TREs to regulate gene transcription. Abbreviations: RXR, retinoic acid receptor; THR, thyroid hormone receptor; TRE, thyroid-hormone responsive element.