Thyroid hormone-binding protein, with a much higher binding affinity for triiodothyronine (T3) than for thyroxine (T4). Probably transports triiodothyronine from the bloodstream to the brain. Homotetramer. Dimer of dimers. In the homotetramer, subunits assemble around a central channel that can accommodate two ligand molecules. Interacts with RBP4. Strongly expressed in the brain, and to a lesser extent in the eye. The N-terminus strongly influences thyroid hormone-binding properties. Belongs to the transthyretin family.