Sentence:  Since the circulating thyroid hormones are the amino acids thyroxine and tri-iodothyronine ( cf. Section C ) , it <WSD>is</WSD> 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:  have the quality of being; (copula, used with an adjective or a predicate noun)