Last year Public Health England released a report saying fluoride was a 'safe and effective' way of improving dental health. But new research from the University of Kent suggests that there is a spike in the number of cases of underactive thyroid in high fluoride areas such as the West Midlands and the North East of England. In England, around 10 per cent of the population (6 million) live in areas with a naturally or artificially fluoridated water supply of 1 mg fluoride per litre of drinking water. The researchers compared areas to records from 7935 general practices covering around 95 per cent of the English population in 2012-2013. Rates of high underactive thyroid were at least 30 per cent more likely in practices located in areas with fluoride levels in excess of 0.3 mg/l. Fluoride is a naturally occurring mineral found in water in varying amounts, depending on the region and it is also found in certain foods and drinks, including tea and fish. It helps combat tooth decay by making enamel more resistant to bacteria. But previous studies have found that it inhibits the production of iodine, which is essential for a healthy thyroid. The thyroid gland, which is found in the neck, regulates the metabolism as well as many other systems in the body. An underactive thyroid can lead to depression, weight gain, fatigue and aching muscles and affects 15 times more women than men, around 15 in 1,000 women.