Good for Teeth, Bad for Brains? Fluoride is supposed to help prevent tooth decay, but could it also be causing neurological illnesses in children?   Maybe, report scientists who are studying the effects of fluoride. Their research links too much fluoride -- available not only in drinking water, but in many toothpastes, other dental products, and virtually all foods and beverages -- to brain abnormalities and deficits in IQ.   The researchers say over-consumption of fluoride can lower thyroid hormone levels. In women who are pregnant, these decreased thyroid levels may adversely affect the neurodevelopment of the fetus, and that could lead to problems like autism and attention deficit disorder (ADD).   "The question is, what level of exposure results in harmful effects to children?" write the authors of the statement. "The primary concern is that multiple routes of exposure, from drinking water, food and dental care products, may result in a high enough cumulative exposure to fluoride to cause developmental effects."   Paul Beeber, an attorney and president of the New York State Coalition Opposed to Fluoridation, believes the time has come to end the fluoridation of public water supplies. "With many neurological diseases, such as autism and ADD, afflicting too many American children, fluoride's dubious promises of less cavities no longer outweigh fluoride's serious health risks," he was quoted as saying.   Currently, about two-thirds of the nation's public water supplies are fortified with fluoride.