A U.K. study suggests people with elevated levels of a group of chemicals used in the manufacture of "non-stick" surfaces had an increased rate of thyroid disease.

But experts warn that more research is needed to confirm whether the chemicals are the cause.

Perfluorooctanoic acid (PFOA) and perfluorooctanoic sulfonate (PFOS) were used in the manufacture of non-stick coating and water-repellent textiles. Use of these chemicals has been phased out by most countries after research linked PFOA to increased incidence of thyroid problems in animals.

'We have pretty strong statistical evidence of an association between blood levels in people and the reporting of having thyroid disease.'— David Melzer

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

Led by Prof. Tamara Galloway of the University of Exeter, the researchers looked at blood samples from 3,966 adults aged 20 years and older from the U.S. Centers for Disease Control and Prevention's National Health and Nutrition Examination Survey (NHANES).

They found people with higher concentrations of PFOA in their blood had higher rates of thyroid disease.

The researchers also found that individuals whose PFOA concentrations in the highest quartile were more than twice as likely to report thyroid disease than those in the lowest two quartiles. They also found 16 per cent of women in the top quartile had the disease, compared to eight per cent in the bottom quartile.

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

Cause/effect unclear

Prof. 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 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 director of the Australian Centre for Human Health Risk Assessment at Monash University in Melbourne, Prof. Brian Priestly, 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," says Priestly.

"While the findings merit further study, including further exploration of biologically plausible mechanisms, they do not definitively prove that PFOA/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.