A group of proteins that includes thyroxine-binding globulin, a glycoprotein that serves as the major and specific carrier of thyroxine in plasma, accounting for 70-75% of the bound thyroxine; thyroxine-binding prealbumin, an albumin that serves as the secondary carrier, accounting for between 20 and 25% of the bound thyroxine; and serum albumin, which accounts for the remaining bound thyroxine.