Water fluoridation above a certain level is linked to 30% higher than expected rates of underactive thyroid in England, a study has found.

Researchers said that GPs' surgeries in the West Midlands, which has the biggest water fluoridation scheme in the UK, are nearly twice as likely to report high hypothyroidism prevalence as Greater Manchester, where it is not added to drinking water.

"The findings of the study raise particular concerns about the validity of community fluoridation as a safe public health measure," the report from the Centre for Health Services Studies at the University of Kent said.

Although fluoride occurs naturally in water, around 10% of people in England receive water that is fluoridated at a target level of one part per million (1ppm) as it reduces the risk of tooth decay.

Researchers said they found that high hypothyroidism prevalence was found to be at least 30% more likely in practices in areas with fluoride levels of more than 0.3ppm.

The study, which is published online in the Journal of Epidemiology & Community Health, said the effects of fluoride on the thyroid have long been observed, but there have been no population studies that have examined this.

"The finding of this cross-sectional study has important implications for public health policy in the UK and in other countries where fluoride is added to drinking water or in other forms such as fluoridated milk and salt," it added.

"Consideration needs to be given to reducing fluoride exposure, and public dental health interventions should stop interventions reliant on ingested fluoride and switch to topical fluoride-based and non-fluoride-based interventions."

Researchers conducted the study by analysing the prevalence of underactive thyroid diagnosed by GPs in 2012 to 2013 in one model and also by comparing data from West Midlands and Greater Manchester in another. They said they took into account the fact that hypothyroidism is more common in women and increases with age.