High affinity receptor for triiodothyronine (By similarity). In the absence of thyroid hormone during late blastula stage development, acts as a transcriptional repressor. Whereas in the presence of thyroid hormone, can act as an activator of transcription. In addition, represses retinoic acid (RA)-signaling during blastula and gastrula stages of development. Interacts with ncoa2. After the mid-blastula transition (MBT), expressed throughout the deep cells, which give rise to the embryo proper. In adults, isoform 2 shows highest expression in the eye and liver. Expressed in adult gonads. Expressed both maternally and zygotically. Most abundant during early oogenesis during stages I and II, and expression levels drop by oocyte maturation. Expressed at the 1-cell stage and then again after the MBT from the blastula to early gastrula (shield) stages, during which expression levels decrease. Not detected after gastrulation until adults. Composed of three domains: a modulating N-terminal domain, a DNA-binding domain and a C-terminal ligand-binding domain. The C-terminal extension present in isoform 1 and absent in isoform 2 (the F-domain) regulates transcriptional activity by altering the selectivity of binding to cofactors. Belongs to the nuclear hormone receptor family. NR1 subfamily.