Sentence:  The mechanism of action of these drugs has not been completely worked out , but certain of them appear to act by reducing the oxidised form of <WSD>iodine</WSD> before it can iodinate thyroglobulin ( Astwood , 1954 ) . 
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)