Effects of polychlorinated biphenyls on thyroid hormones and liver type I monodeiodinase in the chick embryo

Polychlorinated biphenyls (PCBs) are widespread environmental contaminants which can biomagnify to higher tropic level organisms including birds. Circulating thyroid hormones (TH) and growth are decreased by PCB exposure. The first set of studies investigated the effects of PCBs on an enzyme responsible for TH homeostasis, hepatic type I monodeiodinase (MDI) in chicken embryos. Fertile chicken eggs were injected with Aroclor 1242, Aroclor 1254, 2,2',6, 6'-tetrachlorobiphenyl (TCB), 3,3',4,4'-TCB, or 3,3',5,5'-TCB on Day 0 and studies were terminated on Incubation Day 21. Hepatic MDI activity was reduced in embryos treated with the Aroclor mixtures. No effects on MDI activities were observed after PCB isomer treatment. Liver weights from embryos treated with Aroclor 1242 were decreased. In the second study, chick embryos were exposed to these same PCBs in order to evaluate their effect on circulating THs and growth. Treatment with PCBs had no effect on body weight. Femur length were decreased with Arcolor 1242 treatment. A decrease in plasma concentration of thyroxine was observed after treatment with Aroclor 1242 and Aroclor 1254. Based on these findings, it is evident that PCBs alter the thyroid axis. Bird circulating TH levels, which are generally reported, may not be a good biomarker for low-dose exposure to PCBs. However, the reduction in MDI activity was more sensitive to PCB mixture exposure and may be a useful biomarker.