Nuclear hormone receptor that can act as a repressor or activator of transcription. High affinity receptor for the thyroid gland hormone triiodothyronine (T3). Transactivating activity is ligand-dependent, and is repressed in the absence of T3. Interacts (via the ligand-binding domain) with ncoa2. Widely expressed in a range of adult tissues including the brain, eye, fin, gill, intestine, liver, swim bladder and ovary. In the eye, expressed in the outer nuclear layer of the retina. Expressed both maternally and zygotically. Expressed at the 1 cell stage (0 hpf) but disappears by the 2 cell stage. Expressed again from the 16 cell stage onwards. Embryonic expression increases dramatically around the hatching period. High expression then continues until 6 dpf, before declining. By triiodothyronine (T3) and L-thyroxine (T4). Composed of three domains: a modulating N-terminal domain, a DNA-binding domain and a C-terminal ligand-binding domain. Belongs to the nuclear hormone receptor family. NR1 subfamily. In contrast to PubMed:10687864 and PubMed:11963654, PubMed:17418841 found that triiodothyronine (T3) did not induce expression in the embryo.