Sentence:  Since the circulating <WSD>thyroid</WSD> 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 proteins before secretion . 
Meaning of the word marked with <WSD> tags:  any of several closely related compounds that are produced by the thyroid gland and are active metabolically