WEDNESDAY, Feb. 25, 2015 (HealthDay News) -- 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."