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 (By similarity). Detected in plasma (at protein level). Detected in liver. The mouse protein shows increased stability and much reduced propensity to form amyloid fibrils, compared to the human protein. Belongs to the transthyretin family.