A large study that looked at data from nearly every general medical practice in England suggests that water fluoridation may increase the risk of developing hypothyroidism, or underactive thyroid. This condition, in which the thyroid gland doesn’t produce enough hormones, is associated with symptoms such as fatigue, obesity and depression.
The study found that locations with fluoridated water supplies were more than 30 percent more likely to have high levels of hypothyroidism, compared to areas with low levels of the chemical in the water. Overall, there were 9 percent more cases of underactive thyroid in fluoridated places.
Fluoride is added to the water of about 10 percent of England’s population—and to the taps of about two-thirds of Americans—for the purpose of preventing cavities. It has proved controversial ever since being adopted by American public health authorities in the 1950s, and then spreading to some other countries; supporters say it is a boon for dental health, while critics say it may lead to a variety of health problems.