Professor Stephen Peckham, of the University's Centre for Health Service Studies (CHSS), says a switch to other approaches to protecting tooth health should be considered.
In research published in the Journal of Epidemiology & Community Health, Professor Peckham and the research team identified that water fluoridation above a certain level is linked to 30% higher than expected rate of underactive thyroid (hypothyroidism) in England.
The researchers looked at the 2012 levels of fluoride in the drinking water supply, using data provided by the Drinking Water Inspectorate for individual postcodes. They looked at the national prevalence of underactive thyroid diagnosed by family doctors in England in 2012-13 and recorded in their national quality and incentive scheme (QOF) returns. Complete data were provided for 7935 general practices out of a total of 8020.
The researchers also carried out a secondary analysis, comparing two built up areas, one of which (West Midlands) was supplied with fluoridated drinking water, and the other of which (Greater Manchester) was not.
After taking account of influential factors, such as gender and age, both of which are linked to increased risk of hypothyroidism, they found an association between rates of the condition and levels of fluoride in the drinking water. In areas with fluoride levels above 0.7mg/l, they found higher than expected rates of hypothyroidism than in areas with levels below this dilution.
High rates of hypothyroidism were at least 30% more likely in practices located in areas with fluoride levels in excess of 0.3mg/l. And practices in the West Midlands were nearly twice as likely to report high rates of hypothyroidism as those in Greater Manchester.
In England, around 10% of the population (6 million) live in areas with a naturally or artificially fluoridated water supply of 1mg fluoride per litre of drinking water.
Professor Peckham said that research was 'observational', so no definitive conclusions should be drawn about cause and effect. He also emphasised that the researchers were not able to take account of other sources of fluoride, often found in dental products and food and drink.
But he pointed out that the findings echo those of previous research and that while they were only able to look at diagnosed hypothyroidism, there might also be other cases of impaired thyroid function that have not yet been diagnosed - and treated.
Professor Peckham, who is Director of CHSS and Professor of Health Policy, concluded that consideration needs to be given to reducing fluoride exposure. Public dental health interventions should 'stop those reliant on ingested fluoride and switch to topical fluoride-based and non-fluoride-based interventions', he said.
The paper, titled Are fluoride levels in drinking water associated with hypothyroidism prevalence in England? A large observational study of GP practice data and fluoride levels in drinking water, is published in the latest issue of the Journal of Epidemiology & Community Health, which is the official journal of the Society of Social Medicine.