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Experts have investigated the evidence for a link between PFOA (perfluorooctanoic acid) and PFOS (perfluoroctane sulphonate), byproducts formed in the manufacture of non-stick surfaces, flame-resistant and waterproof items, and thyroid disease.
See also details of our briefing, at which the findings were announced.
Prof David Coggon, Professor of Occupational and Environmental Medicine, University of Southampton, said:
“As the authors acknowledge, the study has important methodological limitations. Moreover, the findings seem inconsistent with those from other investigations in people with higher levels of exposure.
1) The design is cross-sectional, and levels of PFOA at the time of the survey may not reliably reflect exposures prior to the development of the thyroid disease that participants reported.
2) The health outcome that was studied – report of a doctor diagnosis of thyroid disease – is relatively non-specific. There are various thyroid diseases, which differ in their nature and causation, but in this study they were all lumped together. Moreover, they may not have been reliably reported.
3) There was no consistent exposure-response relation with increasing levels of PFOA.
“Also, the authors report that other studies of people with higher exposures to PFOA and apparently more reliable assessment of thyroid function, have failed to show clear associations.”
Prof Ashley Grossman, Professor of Neuroendocrinology, Queen Mary, University of London, said:
“In this study, researchers looked a large number of people and found an association between a commonly used chemical and thyroid disease. However, studies like this cannot tell us that the two things are definitely linked. What’s more, the paper doesn’t tell us what sort of thyroid disease people were suffering with.
“We also don’t know whether this chemical is directly affecting the thyroid. 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.
“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.”
Prof Ieuan Hughes, Prof of Paediatrics and Head of Department of Paediatrics, University of Cambridge and Chair of the Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment, said:
“I am surprised that the paper is generating so much interest. Yes, these compounds are ubiquitous and can be detected readily in humans. The problem with this study is the evidence for a direct link with thyroid effects from PFOS is at best tenuous. There has been some clever statistical analysis to suggest that self-report of a ‘thyroid problem’ is associated with higher levels of exposure (but based on just one serum sample) to these chemicals AND, corrected for a number of possible confounders. One must also recognise that thyroid disease is very common and those with a so-called ‘thyroid problem’ must be even more common. The authors recognise the blindingly obvious…that more work needs to be done to prove any association. That would at least mean data collected to analyse exposure in relation to actual thyroid data, i.e. thyroid hormone levels: TSH, FT4, FT3, thyroglobulin, TBG (these chemicals appear to preferentially bind to proteins). If nuclear receptors are invoked, then the TRs would need to be studied.”
Dr Diane Benford, Head of Toxicology of Food, Food Standards Agency, said:
“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. A very large number of analyses have been conducted with the NHANEs samples, and statistically it would be expected that some will show associations due to chance alone. More research is needed to establish whether this association is causal.”
Prof Alan Boobis, Toxicologist, Division of Investigative Science, Imperial College London, said:
“It is not possible to establish causation only from association in a cross-sectional epidemiological study. The considerations outlined by Bradford Hill would be of value here, looking at factors such as exposure gradient, biological plausibility, experimental evidence, etc. If these do not exist, it strongly suggests that the case is not proven. At best, the study would be hypothesis generating, for further more directed studies. PFOA and PFOS have somewhat different toxicological profiles, so I am not sure it is sensible to combine them. There are data from occupational cohorts to these compounds. I do not recollect any evidence for effects on thyroid in humans. The experimental studies suggest that any effect on thyroid is indirect and seen only with PFOS. Other more direct, effects should be apparent at lower exposures, if indeed the thyroid is affected by these compounds.”