Fears over fluoridated water have been denied by city health chiefs after a study of Birmingham residents suggested higher levels of thyroid conditions were leading to weight gain and depression.

Researchers investigated GP surgeries in the West Midlands which has the highest levels of water fluoridation in the country.

The team from the Centre for Health Services Studies at the University of Kent found doctors in the region had recorded 30 per cent higher than expected rates of underactive thyroid.

But Birmingham health bosses insisted there were no risks, emphasising this was just ‘one report’ against extensive research over the years – and said the city’s excellent teeth was a key benefit.

Researchers said GPs in the West Midlands were nearly twice as likely to report high hypothyroidism prevalence as Greater Manchester, where it is not added to drinking water.

The report said: “The findings of the study raise particular concerns about the validity of community fluoridation as a safe public health measure.”

Hypothyroidism symptoms can include tiredness, weight gain, mental slowing, and depression.

Nationally, research suggested up to 15,000 people could be suffering needlessly from thyroid problems which can cause depression, weight gain, fatigue and aching muscles.

Lead author Professor Stephen Peckham, from the Centre for Health Service Studies, said: “I think it is concerning for people living in those areas. The difference between the West Midlands, which fluoridates, and Manchester, which doesn’t was particularly striking. There were nearly double the number of cases in Manchester.

“Underactive thyroid is a particularly nasty thing to have and it can lead to other long-term health problems. I do think councils need to think again about putting fluoride in the water. There are far safer ways to improve dental health.”

The thyroid gland, which is found in the neck, regulates the metabolism as well as many other systems in the body. 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.

Birmingham director of public health, Dr Adrian Phillips, said the public should not be alarmed by this single report.

He said: “Extensive research over many years tells us that water fluoridation is a safe and effective public health measure and reports from Public Health England have found no association with reduced thyroid function.

“Birmingham’s water supplies have been fluoridated for 50 years and consequently the dental health of our children is noticeably better than in non-fluoridated cities.”

Dr Phillips highlighted a 2014 survey of children’s oral health across English local authorities which found that five-year-olds in fluoridated Birmingham have, on average, 34 per cent fewer teeth affected by decay than those in non-fluoridated Manchester;

Public Health England said previous evidence overwhelmingly showed that fluoride in water was safe .

Dr Sandra White, director of dental public health at the body, said: “The totality of evidence, accumulated over decades of research, tells us that water fluoridation is a safe and effective public health measure, and shows no association with reduced thyroid function.”

Other experts warned the study may have been skewed by population bias.

Prof David Coggon, professor of occupational and environmental medicine, University of Southampton, said: “It is quite possible that the observed association is a consequence of other ways in which the areas with higher fluoride differ from the rest of the country.

“There are substantially more rigorous epidemiological methods by which the research team could have tested their idea.”