Exposure to chemicals used in the manufacture of non-stick surfaces has been linked to an increased rate of thyroid disease.

But experts warn more research is needed to confirm whether the chemicals are the cause of the increased rate.

Perfluorooctanoic acid (PFOA) and perfluorooctanoic sulfonate (PFOS) are used in the manufacture of non-stick coating and water-repellent textiles.

But the use of these chemicals has been phased out by most countries after research linked PFOA to increased incidence of thyroid problems in animals.

A new study, which appears in the journal Environmental Health Perspectives, is the first to observe a link in humans.

Led by Professor Tamara Galloway, of the University of Exeter, the researchers looked at blood samples from 3966 adults aged 20 years and older from a US Centers for Disease Control and Prevention survey.

The survey found people with higher concentrations of PFOA in their blood have higher rates of thyroid disease.

The researchers found that individuals whose PFOA concentrations in the highest 25 per cent were more than twice as likely to report thyroid disease than those in the lowest 50 per cent.

They also found that 16 per cent of women in the top quarter had the disease compared with 8 per cent in the bottom quarter.

"We have pretty strong statistical evidence of an association between blood levels in people and the reporting of having thyroid disease," says co-author Dr David Melzer.

But he adds, "as in any study of this sort, we can't be sure that it is causal - there could be other explanations."

Previous research has shown that low levels of PFOA appear in most people's blood, but how it gets into the bloodstream is not clear.

Researchers believe it could be through contaminated water and food.

People may also be exposed to the chemical from consumer goods such as textiles, footwear, furniture, and carpets, which can contaminate indoor air and dust.

More research needed

Professor Chris Winder, a toxicologist at the University of New South Wales, says the study highlights the need for more research into the effects of plastics, particularly those which were once considered safe.

"Thyroid disease is fairly common and a small increase may not be significant," he says.

"However, it points to another health problem of plastics that requires close study, and possible reconsideration of the use of this chemical in plastics manufacture."

But the Professor Brian Priestly, the director of the Australian Centre for Human Health Risk Assessment at Monash University in Melbourne, says the link indentified in the study should not be interpreted as PFOA causing thyroid disease.

"It was not possible to determine the nature of the thyroid disease, which can include either increased or decreased thyroid function associated with quite different mechanisms," he said.

"While the findings merit further study, including further exploration of biologically plausible mechanisms, they do not definitively prove that PFOA or PFOS exposures cause thyroid disease in humans."

The study comes one week after a Taiwanese study, published in the American Journal of Gastroenterology, found people with higher levels of PFOA in their blood had increased liver enzyme levels, which could lead to diseases such as hepatitis and cirrhosis.