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 (By similarity). Interacts with RBP4. Detected in choroid plexus (at protein level). Detected in choroid plexus. The N-terminus strongly influences thyroid hormone-binding properties. Belongs to the transthyretin family.