This paper reviews studies on the synthesis and secretion of the thyroid hormone-binding protein, transthyretin by the choroid plexus. The secretion of transthyretin by the choroid plexus into the cerebrospinal fluid may have an important function in the transport of thyroxine from the blood to the brain. The transthyretin gene is expressed in the choroid plexus of most vertebrates and synthesis of this protein may have evolved in the brain before the liver.