Sentence:  In the presence of a low iodine intake , thyroglobulin <WSD>labelled</WSD> in vivo with * * f is found to contain more mono-iodotyrosine than normal , the amounts of di-iodotyrosine and iodothyronines being correspondingly reduced . 
Meaning of the word marked with <WSD> tags:  distinguish (as a compound or molecule) by introducing a labeled atom