A British study finds a correlation between the amount of fluoride in public drinking water and a rise in incidence of underactive thyroid.
While the study is only able to establish an association, not cause-and-effect, experts say the link deserves serious investigation.
"Clinicians in the United States should emphasize to patients this association and should test patients for underactive thyroid," said Dr. Spyros Mezitis, an endocrinologist at Lenox Hill Hospital in New York City.
"Patients should probably be advised to drink less fluoridated water and consume less fluoridated products, including [fluoridated] toothpaste," added Mezitis, who was not involved in the study.
But a representative of the American Dental Association took issue with the British report.
"Public health policy is built on a strong base of scientific evidence, not a single study," said Dr. Edmond Hewlett, ADA spokesman and a professor at the UCLA School of Dentistry. "Currently, the best available scientific evidence indicates that optimally fluoridated water does not have an adverse effect on the thyroid gland or its function."
The new study was led by Stephen Peckham of the University of Kent in Canterbury, England. They compared 2012 national data on levels of fluoride in drinking water to trends for hypothyroidism (underactive thyroid) as diagnosed by family physicians across England.
They found that in locales where tap water fluoride levels exceeded 0.3 milligrams per liter, the risk for having an underactive thyroid rose by 30 percent.
Peckhams's team also found that hypothyroidism rates were nearly double in urbanized regions that had fluoridated tap water, compared with regions that did not.
"Consideration needs to be given to reducing fluoride exposure," the researchers wrote. They believe that public efforts to strengthen dental health should move away from fluoridated water and instead "switch to topical fluoride-based and non-fluoride-based interventions."
Mezitis agreed that, while "fluoridation of the water supply is important for dental health, studies have also shown that iodine deficiency that may be caused by extra ingestion of fluoride is related to hypothyroidism."
He added that "drinking water is fluoridated in the United States, where hypothyroidism is a highly prevalent disorder -- affecting over 15 million individuals mainly female and greater than 40 years old."
Another thyroid expert agreed that attention should be directed at fluoride in the drinking supply.
"This dramatic increase in thyroid dysfunction associated with fluoridation of the water supply adds to previous studies indicating that fluoride has an inhibitory effect on the thyroid gland," said Dr. Terry Davies, a professor of medicine at the Icahn School of Medicine at Mount Sinai, in New York City.
The study "supports the argument that our water supply should be pure water and nothing else," said Davies, who is also an endocrinologist at The Mount Sinai Hospital.
But the ADA's Hewlett countered that other studies have not uncovered a link between fluoridated water and thyroid problems.
"A far more rigorous systematic evaluation of human studies conducted by the Scientific Committee on Health and Environmental Risks at the request of the European Commission does not suggest a potential thyroid effect at realistic exposures to fluoride," he said. "Additionally, the 2006 Report by the U.S. National Research Council found no adverse effects on the thyroid even at levels more than four times greater than that used in fluoridation."
And fluoride in the water supply protects dental health, Hewlett added. "Even with the widespread availability of fluoride toothpaste, studies show that community water fluoridation prevents at least 25% of tooth decay in children and adults throughout the lifespan," he said.
The findings are published online Feb. 23 in the.
According to the U.S. National Institutes of Health, the thyroid is a butterfly-shaped gland near the base of the neck that produces hormones. Thyroid hormones control the rate of many body activities, including how fast calories are burned and how fast the heart beats. If the thyroid gland isn't active enough, it does not make enough thyroid hormone to meet the body's needs.