Sentence:  At the present time we do <WSD>not</WSD> know by what biochemical mechanism TSH acts on the thyroid , but for bio-assay of the hormone there are a number of properties by which its activity may be estimated , including release of iodine from the thyroid , increase in thyroid weight , increase in mean height of the follicular cells and increase in the thyroidal uptake of * * f . 
Meaning of the word marked with <WSD> tags:  negation of a word or group of words