Sentence:  Israel is one of the few countries that widely fluoridates, besides the United States, Canada, Ireland, New Zealand and Australia. Fluoridation, the addition of fluoride to public drinking water supplies to reduce cavities, is the subject of intense controversy, especially outside of the United States. But opposition to the practice, on the grounds that fluoride has adverse effects on the <WSD>thyroid</WSD>, brain and bones, and is an unethical form of mass-medication, appears to be growing. 
Meaning of the word marked with <WSD> tags:  The thyroid gland.