High levels of water fluoridation have been linked to a 30pc increase in rates of underactive thyroid in England.
The findings have prompted the researchers to call for a rethink of public health policy to fluoridate the water supply in a bid to protect dental health.
They compared the rate of underactive thyroid –hypothyroidism – in two build up areas – one in the West Midlands with fluoridated drinking water and the other in Greater Manchester which was not.
They found higher than expected rates of hypothyroidism in areas with fluoride levels above 0.7mg per litre.
High rates of the condition were at least 30pc more likely in areas with fluoride levels in excess of 0.3mg per litre.
In the West Midlands the rate of hypothyroidism was nearly twice that of Greater Manchester..
The researchers stress they were not able to take account of other sources of fluoride in dental products, food and drink.
They point out, however, that their findings echo those of previous research and while they were only able to look at diagnosed hypothyroidism there might also be other cases of impaired thyroid function that had not yet been diagnosed and treated.
“Consideration needs to be given to reducing fluoride exposure, and public dental health interventions should stop [those] reliant on ingested fluoride and switch to topical fluoride-based and non-fluoride-based interventions,” they conclude.
The study is published online in the Journal of Epidemiology and Community health.