Sentence:  It has been suggested that the plasma thyroxine binding <WSD>proteins</WSD> , which have an extremely high affinity for thyroxine , compete with thyroglobulin for thyroxine ( Ingbar and Freinkel , 1957 ) . 
Meaning of the word marked with <WSD> tags:  any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of polymers of amino acids; essential in the diet of animals for growth and for repair of tissues; can be obtained from meat and eggs and milk and legumes