Sentence:  Since the circulating thyroid hormones are the amino acids thyroxine and tri-iodothyronine ( cf. Section C ) , it is clear that some mechanism must exist in the thyroid gland for their release from <WSD>proteins</WSD> before secretion . 
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