Exposure to phthalates in childhood could be affecting the thyroid function of girls as young as three, according to new research from Columbia University, USA.
The new study, carried out by a team from the Mailman School of Public Health, looks at the link between thyroid function in children and exposure to phthalates -- a class of chemicals thought to disrupt the endocrine system -- over a period of time.
Commonly found in consumer products, from household building materials to shampoos, phthalates are also found in products for children such as plastic toys.
The research team measured levels of five phthalates and two thyroid hormones from 229 women during pregnancy and 229 children at age 3.
The team found that in the girls only, lower levels of the active thyroid hormone free thyroxin (FT4) was associated with metabolites -- molecules that are a byproduct of reactions that occur within cells -- of the phthalates mono-n-butyl phthalate (MnBP), mono isobutyl phthalate (MiBP), monobenzyl phthalate (MBzP), and monoethyl phthalate (MEP).
Levels of the hormone FT4 lower when the thyroid -- which produces hormones that affect various bodily functions including growth and development, heart rate and body temperature -- is struggling.
However, the team were surprised to find that prenatal exposure to a metabolite of Di (2- ethylhexyl) phthalate (DEHP) was associated with not lower but higher levels of FT4, suggesting that phthalates affect thyroid function differently depending on age of exposure.
"The thyroid acts as the master controller of brain development," explained senior author Pam Factor-Litvak, "Thyroid hormones set the schedule, and if the timing is out of synch, there may be later consequences in the brain."
"The thyroid disruptions we see in this study, although they fall within the normal range, could explain some of the cognitive problems we see in children exposed to phthalates and we are currently investigating that. As we know from lead, even small exposures can make a big difference."
As women are more susceptible to thyroid disturbances than men, this could potentially also make them more susceptible to thyroid-disrupting chemicals, even in early childhood, possibly explaining why a link between phthalate exposure and a depressed thyroid was found in girls and not boys.
Although the team found little evidence to suggest that exposure to phthalates in the womb affected thyroid function in children at age 3, they did add that maternal thyroid function could have been affected, however this was not measured in the study.
Previous studies from Mailman School have also found associations between prenatal exposure to phthalates and risk for lower IQ at age 7, childhood asthma, and mental and motor development problems in preschool children, with Factor-Litvak advising that, "Parents with young children should avoid using products containing phthalates such as shampoos, nail polish, and vinyl flooring."
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