Sentence:  Since the circulating thyroid hormones are the amino acids <WSD>thyroxine</WSD> and tri-iodothyronine ( cf. Section C ) , it is clear that some mechanism must exist in the thyroid gland for their release from proteins before secretion . 
Meaning of the word marked with <WSD> tags:  hormone produced by the thyroid glands to regulate metabolism by controlling the rate of oxidation in cells