Fluoride and chlorine are both abundantly present in tap water in the US and much of the rest of the developed world. Both of these elements are in the halogen family. Halogens are a family of 5 non-metallic elements that readily combine with metals to form salts. Most of these resulting compounds are toxic. The five elements in this group are fluorine, chlorine, bromine, iodine and astatine. Fluorine and chlorine are the focus of this article.
Second, fluoride stimulates certain proteins which cause cells to be unable to take up thyroid hormones.
Third, fluoride decreases the amount of Thyroid Stimulating Hormone (TSH) made by the pituitary, thus reducing the amount of hormones put out by the thyroid gland.
Fourth, fluoride competes with TSH at receptor sites on the thyroid gland so less TSH reaches the thyroid and less thyroid hormone is manufactured.