Common biocide may disrupt thyroid system

Tests on tadpoles have shown that the biocide triclosan can disrupt thyroid hormones.1

Triclosan is made by Ciba Specialty Chemicals and is added to many personal care products, including soaps, toothpastes and deodorants.

Up to 90 tonnes end up in UK sewers each year, but most is removed during sewage treatment. However, it has been found in streams, sewage sludge and human breast milk (ENDS Report 358, pp 10-11 ).

Triclosan, or its derivative methyl-triclosan, is thought to bioaccumulate in fish at sub-lethal levels. At higher levels it is toxic to several aquatic species.

Previous studies have suggested that, unlike some chemicals, it does not affect the sex hormone oestrogen. But scientists in Canada were concerned about its similarity to triiodothyronine (T3), a thyroid hormone that helps control the metabolic rate of cells. T3 plays an important role in human development and frog metamorphosis.

The researchers investigated the effect that exposure to triclosan had on gene expression and metamorphosis in the North American bullfrog.

Metamorphosis occurred early in tadpoles exposed to T3, but if triclosan was also added at levels as low as 0.15µg/l, hind leg development accelerated further. The activation of some associated genes was also affected.

The researchers, concluded the findings indicated triclosan alters the action of T3 rather than mimicking it.

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