Thyroid hormone-binding protein. Probably transports thyroxine 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. Detected in plasma and cerebrospinal fluid (at protein level). Highly expressed in the choroid plexus. Detected in liver. Belongs to the transthyretin family.