Persistent Organic Pollutants May Affect Child Growth and Development through Placental Thyroid Hormones

Exposure to persistent organic pollutants (POPs) is associated with thyroid hormone (TH) levels in placenta, which could explain how exposure to these chemicals affects child growth and development, according to a study recently published in Endocrinology.

Researchers led by Zhong-Min Li, MSc, and Meri De Angelis, PhD, both of Helmholtz Zentrum München-German Research Center for Environmental Health, Molecular EXposomics in Neuherberg, Germany, point out that POPs have been widely used in commerce and industry over the past few decades. These chemicals are transferred across the placenta during pregnancy and disruption of TH levels after POP exposure has been observed in vitro and in animal studies.

“The potential thyroid-disrupting effects of POPs in background exposed populations are of interest because THs act at extremely low serum concentrations (free concentration range, 8 to 30 pg/mL), and POPs can mimic or inhibit the response of the hormones at low doses (e.g., the low dose cutoff of BDE 99 is 0.3 mg/kg/d),” the authors write. “The purpose of this study was to investigate the possible association of background exposures of POPs [specifically, polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs), organotin chemicals (OTCs), and organochlorine pesticides (OCPs)] with placental levels of THs (T₄, T₃, and rT₃) in a Danish population that gave birth to boys with and without cryptorchidism.”

The researchers collected 58 placenta samples from mothers of boys born with and without cryptorchidism (28 and 30, respectively) and measured the concentrations a total of 82 POPs — PBDEs, PCBs, PCDD/Fs, OTCs, and OCPs – as well as the concentrations of T₄, T₃, and rT₃. “No correlation between THs and the odds of cryptorchidism was found. Several POPs were significantly associated with THs: (1) T₄ was inversely associated with BDEs 99, 100, ΣPBDE, and 2378-TeCDD, and positively associated with 1234678-HpCDF; (2) T₃ was positively associated with 2378-TeCDF and 12378-PeCDF; and (3) rT₃ was positively associated with PCB 81, 12378-PeCDF, and 234678-HxCDF, and inversely associated with tributyltin, ΣOTC, and methoxychlor,” the authors write.

The researchers conclude that their results suggest background exposure to POPs is associated with TH levels in placenta. “Our results highlight the challenges of assessing effects on thyroid function, especially during pregnancy, due to the complexity of contaminant mixtures and the sensitivity of the thyroid system of the pregnant woman and the fetus,” the authors write.

However, they also write that the results should be interpreted with caution because of the small sample size and the fact they only analyzed samples from mothers of boys, so they could not determine any sex-dependent effect. “The findings should be confirmed with more placenta samples from boys and girls,” the authors write, “also including the deiodination enzyme activities and the hydroxylated metabolites of PBDEs and PCBs.”