Sentence:  If tyrosine and a system generating hydrogen peroxide are <WSD>added</WSD> to a cell-free homogenate of the thyroid , large quantities of free mono-iodotyrosine can be formed ( Alexander , 1959 ) . 
Meaning of the word marked with <WSD> tags:  make an addition (to); join or combine or unite with others; increase the quality, quantity, size or scope of