A "ubiquitous" chemical widely used in the manufacture of non-stick frying pans is linked to thyroid disease, researchers say.

A study involving almost 4,000 people in the United States found people that those with the highest levels of man-made perfluorooctanoic acid - also known as PFOA - in their blood were twice as likely to be diagnosed with thyroid disease or taking medication compared with those with the lowest levels.

PFOA is widely used in industry because it excels at repelling heat, water, grease and stains, and traces of the chemical are found in water, air and soil throughout the world.

But the researchers from the University of Exeter, who carried out the latest study, stressed that further research was needed to establish whether the chemical was a direct cause of illness in humans.

Previous studies in animals have suggested that high exposure to the chemical can affect the action of the thyroid gland, located in the neck, which is responsible for controlling the body's rate of metabolism through the release of hormones.

Tamara Galloway, Professor of Ecotoxicology at Exeter, said that it was likely that the chemical could be ingested or inhaled from the air or as a component of household dust, and might stay in the bloodstream for about five days.

Her team analysed an American database of 3,966 adults aged 20 and older, because no figures for blood levels of PFOA is available in Britain. Of these, 163 women and 46 men reported having current thyroid disease and were taking medication.

But a total of 16 per cent of women with the highest blood levels of the chemical (above 5.7ng/ml) reported having thyroid disease or were taking related medication, compared to 8 per cent of those with the lowest levels. the trend was similar in men, but not said to be statistically significant because of the low numbers of individuals involved.

"Our results highlight a real need for further research into the human health effects of low-level exposures to environmental chemicals like PFOA that are ubiquitous in the environment and in people's homes. We need to know what they are doing," Professor Galloway said.

But Diane Benford, Head of Toxicology of Food, Food Standards Agency, commented: "This type of study investigates associations and cannot establish causality. It is important to look at the consistency of the data with the results of other studies, and with what is known about the toxicity of PFOA and PFOS, which differ.

"Studies of workers with higher exposure to these compounds have not shown consistent evidence of increased risk of thyroid disease, which would be expected if effects are occurring in the general population. More research is needed to establish whether this association is causal."

Ashley Grossman, Professor of Neuroendocrinology, at Queen Mary, University of London, said that it was also unknown how PFOA might affect the thyroid gland.

"Thyroid disease is often caused by the body's own immune system attacking the thyroid gland so perhaps this chemical is having some effect on the immune system, rather than directly on the thyroid," he said.

"We'd need to do a lot more research to verify this link and to understand how the two are linked. In the meantime, it's important to remember that thyroid disease can be successfully treated."