Impact of Thyroid Hormones Disrupting Potential of Environmental Polychlorinated Biphenyls and Heavy Metals on Brain Functions: A Review Article

Abstract

The normal functions of the brain depend upon thyroid hormones including tetraiodothyronine and tri-iodothyronine. The environmental polychlorinated biphenyls produce neurological deficiencies by disturbing the normal cascade of thyroid hormones in the brain. These chemicals mimic the structure of thyroid hormones. Like thyroid hormones, these toxic compounds also contain a biphenyl ring and halogen atoms in their basic structure. These toxic environmental chemicals mostly act at receptor levels and have the ability to attach to thyroid hormones binding proteins. The disturbance created by these environmental toxicants results in serious brain abnormalities. Furthermore, heavy metals like lead, mercury, and cadmium can also disturb the normal levels of thyroid hormones in plasma. The thyroid disrupting potential of these heavy metals creates a negative impact on the cognitive performance of the brain. The heavy metals act either by inhibiting the carrier and enzymatic (Deiodinases) proteins of thyroid hormones or by altering the morphological features of the thyroid gland. The developing fetus is more prone to develop serious abnormal brain functions due to the exposure of these toxicants because early brain development is dependent upon thyroid hormones coming from mother. The main objective of this study is to explore the indirect effect of polychlorinated biphenyls and heavy metals on brain functions. In our country, many cases of brain abnormalities arise due to these environmental toxicants but the reality is ignored due to lack of proper information about this fact.