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. Fluoride was used to treat hyperthyroidism (or an overactive thyroid) in the 1950s. It may put a damper on the gland's activities by suppressing the activity of various enzymes, causing physical damage or interfering with the absorption and use of iodine, a substance that is critical for thyroid health. In 2006, the U.S. Environmental Protection Agency convened a panel that recommended lowering the maximum allowable level of fluoride in water. Nine years later, the EPA is still considering whether or not to revise its fluoride standards.