Sentence:  This appears to result from both a reduced amount of the <WSD>iodine</WSD> substrate and a more rapid secretion of newly iodinated thyroglobulin . 
Meaning of the word marked with <WSD> tags:  a nonmetallic element belonging to the halogens; used especially in medicine and photography and in dyes; occurs naturally only in combination in small quantities (as in sea water or rocks)