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 (By similarity). Homotetramer. Dimer of dimers. In the homotetramer, subunits assemble around a central channel that can accommodate two ligand molecules. Interacts with rbp4 (By similarity). The N-terminus strongly influences thyroid hormone-binding properties. Belongs to the transthyretin family.