| Polychlorinated biphenyls (PCBs) are a class of man-made chemicals which were produced from about 1929 through the 1970's for use in a wide variety of industrial applications. Due to past open and uncontrolled uses, careless disposal practices, leakage and accidents, PCBs were released in the environment in which they persist. To most humans, food represents the most important source of exposure. Health effects that have been associated with exposure to PCBs in humans and/or animals include hepatic, endocrine, dermal and ocular effects, as well as immunological alterations, neurological/neurodevelopmcntal changes, reproductive toxicity, and cancer. PCBs are structurally similar to dioxins and a limited number of PCB congeners can did the same spectrum of biochemical effects. Although clearly less toxic, non-dioxin-like PCBs are suspected of endocrine disruption, i.e. interfering with endocrine systems, and adversely affecting neurodevelopment.; We investigated the possible effects of PCBs on the thyroid system and neurodevelopment of both chicken embryos and the human fetus. Our animal experiments showed that the dioxin-like PCB 77 but not the ortho-substituted PCBs 153 & 180 severely reduced availability of the thyroid hormones 3,3',5-triiodothyronine and thyroxine around the time of hatching. This effect was observed both in the periphery and the central nervous system of the chicken embryos. Moreover, PCB 77 treatment delayed the hatching process and decreased overall hatching success, events which may be related to the hypothyroid status. PCBs 153 and 180 affected cerebral neuron migration and thyroid hormone dependent gene expression.; As part of a large scale Flemish biomonitoring program (Centre of Expertise for Environment and Health), we also examined the possible relationships between concentrations of environmental pollutants and thyroid hormone levels in humans. Analysis of the umbilical cord blood of nearly 200 neonates revealed a statistically significant, negative association of concentrations of organochiorine compounds (PCBs, dioxin-like compounds, and pesticides) with levels of free triiodothyronine and free thyroxine, but not thyroid-stimulating hormone. Similar results were previously observed in animal experiments but, to our knowledge, these results are the first demonstrating such clear and consistent associations between exposure to PCBs/dioxins and levels of thyroid hormones in a large cohort of human neonates. |
