High affinity receptor for triiodothyronine (T3). Binds to thyroid hormone receptor element (TRE) weakly as homodimers and monomers, but binds TRE with much higher affinity as heterodimers with retinoid X receptors. Can bind DNA as a heterodimer with either rxra or rxrg. Expression peaks at the climax of metamorphosis. Composed of three domains: a modulating N-terminal domain, a DNA-binding domain and a C-terminal ligand-binding domain. A number of compounds can compete with and disrupt triiodothyronine (T3)-binding including 3,3',5-triiodothyroacetic acid. Belongs to the nuclear hormone receptor family. NR1 subfamily.