Flouride is one of those things that is close to iodine on the Periodic table, and can block iodine receptors by taking iodine's place. From my site (kittyshark.net)
http://www.kittyshark.net/toxins :
If you have city water, you and your pet may be drinking flouride and chlorine, which are usually added to public water systems. I would strongly encourage you to use a water filtration system, both for your drinking water and your pet’s, especially in light of the following:
“Flouride is an enzyme and hormone inhibitor, affecting the nervous system as well as digestion. Flouride is the major cause of brittle bones and teeth,” and “may cause brain and kidney damage.” It has also been linked to cancer. In fact, many European countries “do not use water flouridation . . . Based on the grounds that its use is unethical and imposes unnecessary health risks.” 6
It displaces iodine in the thyroid, which can contribute to hypothyroidism. In addition to public water and toothpaste/dental products, it’s also in the certain medications. The ones that may also be used pets are Prevacid, Prozac, Prilosec, and the quinolone class of antibiotics.7
Chlorine and cholide do the same thing, and are usually added to public water systems.