Issue 21 – Thyroid


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In this month’s issue:

  • Spot the difference: can the body distinguish between thyroid hormones and chemical imitators?
  • American Medical Association commits to protecting public from hormone disruptors
  • Study raises fresh concerns about feminising effects of phthalate exposure
  • The best of November’s news and studies about environmental health

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Spot the difference: can the body distinguish between thyroid hormones and chemical imitators?

Structure of thyroxine-like chemicals. Note the two aromatic rings and the positions of the halogen atoms. Click to enlarge.

By John Newby, PhD.

Deep in our cells, more and more man-made molecules are competing for the attentions of hormone receptors, the parts a cell uses to receive chemical signals from the body, activating and deactivating processes in response.

Some receptors are less choosy than others about which molecules they bind to, with the oestrogen receptor especially promiscuous, apparently able to bind to any molecule which even vaguely resembles oestrogen.

The promiscuity, or looseness, of the oestrogen receptor (ER) means not only can plant phytoestrogens interfere in sex hormone signalling [J Pharm Sci, 96(8), Turner et al.] but so can man-made molecules such as the increasingly controversial plastic hardener BPA, phthalates, dioxins and PCBs.

The ER has been widely studied in this aspect but more recently the thyroid receptor (TR) and its associated hormones, thyroxine (T4) and triiodothyronine (T3), have come under the scrutiny of scientists.

The thyroid hormones control many physiological and developmental processes critical for normal growth and regulation of metabolism and maturation of the foetus and child.

While interference in these processes would likely have many implications for development and health, several diseases associated with the thyroid such as hypothyroidism, hyperthyroidism and thyroid cancers are all increasing. [Cancer, 115(16), Chen et al., Clin Endocrinol 68(2), Leese et al.]

The key question as to whether or not chemical exposures are playing a role in the increase in thyroid disease is whether or not the thyroid receptor is loose enough to bind to man-made molecules which share structural similarities with T3 and T4 molecules.

This is in fact the case, with experiments showing the following three chemicals, among others, are able to bind to the TR and initiate a response.

Polychlorinated bipenhyls (PCBs): Known neurotoxins which have been shown to bind to the TR. PCBs have been shown to alter thyroid associated gene expression in the foetal and neonatal brain. [Risk Anal 24, Crofton]. PCBs are ubiquitous in the environment despite being banned in the 1970s.

Polybrominated diphenyl ethers (PBDEs): Used as flame retardants in everyday items such as furniture and computers and shown to bind to the thyroid receptor, with mice showing behavioural changes such as hyperactivity [NeuroToxicology 24, Branchi et al.]. Another study has shown young children have up to 3.2 times more PBDEs in their blood compared to their mothers [EWG 24].

Triclosan: An antibacterial and antifungal agent commonly found in toothpastes and handwash, and ubiquitous in human populations [Environ Health Perspect 116(3), Calafat et al.] Exposure to low levels of triclosan has been shown to disrupt thyroid hormone-associated gene expression in vivo [Aquatic Toxicol 80(3), Veldhoena et al].

The fact that these contaminants bind to the TR and may disrupt normal thyroid function does not mean they are responsible for the increased thyroid diseases we are observing. However, similarity of molecular structure to other molecules is the reason pharmaceuticals can cause unwanted side-effects, by binding to receptors or proteins besides the intended target.

The looseness of thyroid hormone receptors could therefore be a valuable indicator of how vulnerable our health might be to a range of chemicals with similar molecular structures to T3 and T4 hormones.

Further reading: We have posted here a reading list of scientific studies relating to the issues discussed in this article.

American Medical Association commits to Protecting Public from Hormone disruptors

A resolution to work with government to decrease public exposure to endocrine-disrupting chemicals (EDCs) has been adopted by the American Medical Association (AMA).

Introduced by the US Endocrine Society, the resolution is based on the Society’s peer-reviewed Scientific Statement on EDCs, published in June this year.

Endocrine-disrupting chemicals are substances in the environment that interfere with hormone biosynthesis, metabolism or action resulting in adverse developmental, reproductive, neurological and immune effects in both humans and wildlife.

The resolution states that the AMA will work with the US Federal Government to centralise regulatory oversight of EDCs and base policy on data covering both high- and low-dose exposures

Robert Vigersky MD, president of The Endocrine Society, said: “The Endocrine Society is concerned that the public may be placed at risk because critical information about the potential health effects of endocrine-disrupting chemicals is being overlooked in the development of federal guidelines and regulations.”

The resolution also commits the AMA to helping government develop policy with input from a much broader range of scientific experts including endocrinologists and specialists in environmental medicine.

Adoption of this resolution means that it is now AMA policy and is wholly supported by the House of Medicine.

Study raises fresh concerns about feminising effects of phthalate exposure

New research has found that boys exposed to higher doses of phthalates in the womb are less likely to engage in gender-typical play activities.

The research, headed by Dr Shanna Swan, professor of Obstetrics and Gynaecology at the University of Rochester, New York, adds to evidence that prenatal phthalate exposure permanently alters the development of male children.

Swan tested mothers for phthalates during the 28th week of pregnancy. When the children were aged between four and seven, the parents were asked about their play habits and personalities.

Swan found that higher concentrations of two common phthalates, DEHP and DBP, were strongly linked with more feminine play in boys but had no effect on the play of girls.

Previous studies by the same researcher have shown that phthalate exposure can reduce ano-genital distance in boys and increase incidence rates of undescended testicles.

While there has been controversy about the usefulness of ano-genital distance as an indicator of impaired development, a recent study appears to have validated its use as a marker of feminisation.

Girls have also been found to be affected by prenatal exposure to phthalates. A study published in 2009 found that girls exposed to phthalates before being born scored poorly in a standard behaviour test.

Further reading

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Recent news and science

Is the plastic in our oceans a cause for concern in terms of human health: Plastic decomposing in our oceans may be a source of widely dispersed dissolved toxic contaminants.

Sexual dysfunction in workers exposed to high-levels of BPA: BPA hardens plastic but may be associated with the opposite effect in men.

Fewer males born in Canadian industrial area: Boys are becoming a rare species on an Indian resevation near the town of Aamjiwnaang in South Ontario’s “chemical Valley”.

Nanoparticles in household items: more evidence they can damage DNA: UK researchers find nanoparticles can indirectly damage DNA; they don’t need to penetrate cells.

Gloves off as new toxicology takes on old: The authors vigorously contest the assumption that high-dose testing is helpful for predicting effects of low-dose exposure to chemicals.

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