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 serum and cerebrospinal fluid (at protein level). Highly expressed in the choroid plexus. Detected at lower levels in the liver. This protein binds retinol-binding protein at levels similar to, and the thyroid hormones at levels much higher than, the human protein. Tetramer dissociation and partial unfolding leads to the formation of aggregates and amyloid fibrils. Small molecules that occupy at least one of the thyroid hormone binding sites stabilize the tetramer, and thereby stabilize the native state and protect against misfolding and the formation of amyloid fibrils (By similarity). Belongs to the transthyretin family.