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Vulnerability of the developing brain to thyroid abnormalities: environmental insults to the thyroid system.Neurologic development follows orderly patterns that can be severely disturbed when thyroid hormones are deficient or excessive. Should this occur at appropriate development periods, irreversible neurologic damage can result. The nature of the deficits depends upon the specific development period and the severity of the thyroid disturbance. PCBs and dioxins are structurally similar to the thyroid hormones. Their binding characteristics are similar to those of thyroid hormones and all three groups bind to the cytosolic Ah receptor, the thyroid hormone receptor and the serum thyroid hormone binding protein transthyretin. Depending upon the dose of toxin and the congener used, the toxins either decrease or mimic the biological action of the thyroid hormones. Either effect, if occurring during brain development, can have disastrous consequences. Children and animals exposed to PCBs or dioxins in utero and/or as infants can exhibit varying degrees of behavioral disorders. These disorders resemble those seen in children exposed to thyroid hormone deficiencies in utero and/or in infancy. The mechanism of developmental neurotoxicity of PCBs and dioxins is not known but data suggest it could be partially or entirely mediated by alterations in availability and action of thyroid hormones during neurological development. It is possible that transient exposure of the mother to doses of toxins presently considered nontoxic to the mother could have an impact upon fetal or perinatal neurological development. If the toxins act via their effect on thyroid hormone action, it is possible that doses of toxins that would normally not alter fetal development, could become deleterious if superimposed on a pre-existing maternal/or fetal thyroid disorder.