The Effects of Thyroid Hormone Level and Action in Developing Brain: Are These Targets for the Actions of Polychlorinated Biphenyls and Dioxins?

Abstract

Alterations in thyroid hormone level or responsivity to thyroid hormone have significant neurologic sequelae throughout the life cycle. During fetal and early neonatal periods, disorders of thyroid hormone may lead to the development of motor and cognitive disorders. During childhood and adult life, thyroid hormone is required for neuronal maintenance as well as normal metabolic function. Those with an underlying disorder of thyroid hormone homeostasis or mitochondrial function may be at greater risk for developing cognitive, motor, or metabolic dysfunction upon exposure to substances which alter thyroid hormone economy. Polychlorinated biphenyls (PCBs) and dioxins have been argued to interfere with thyroid hormone action and thus may affect the developing and mature brain. Animal models provide useful tools for studying the effects of thyroid hormone disorders and the effects of environmental endocrine disruptors. The congenitally hypothyroid, hyt/hyt, mouse exhibits abnormalities in both the cognitive and motor systems. In this mouse and other animal models of thyroid hormone disorders, delayed somatic and reflexive development are noted, as are permanent deficits in hearing and locomotor and adaptive motor behavior. This animal's behavioral abnormalities are predicated on anatomic abnormalities in the nervous system. In turn, these abnormalities are correlated with differences in neuronal structural proteins. In normal mice, the expression of mRNAs coding for these proteins occurs temporally with the onset of autonomous thyroid hormone production. The hyt/hyt mouse has a mutation in the thyroid stimulating hormone receptor (TSHr) gene which renders it incapable of transducing the TSH signal in the thyrocyte to produce thyroid hormone. Some behavioral and possibly some biochemical abnormalities in mice exposed to PCBs are similar to those seen in the hyt/hyt mouse. In addition to direct effects on brain development and neuronal maintenance, thyroid hormone is necessary for maintaining metabolic functioning through its influence on mitochondria. Because the brain is particularly sensitive to inadequate energy generation, disorders of thyroid hormone economy also indirectly impair brain functioning. Alterations in thyroid hormone level result in differing expression of mitochondrial genes. Mutations in these mitochondrial genes lead to well-recognized syndromes of encephalomyopathy, myopathy, and multisystem disorder. Hence, PCBs and dioxins, by possibly altering the thyroid hormone milieu, may alter the functioning of mitochondria in the generation of adenosine triphosphate (ATP). The use of animal models of thyroid hormone deficiency for behavioral, anatomic, histologic, and molecular comparison will help elucidate the mechanisms of action of these putative endocrine-disrupting compounds. The study of thyroid hormone disorders provides a template for relating thyroid hormone mediated effects on the brain to these compounds.

Interfollicular Fibrosis in the Thyroid of the Harbour Porpoise: An Endocrine Disruption?

“…Few environmental and laboratory researchers have focused on the disruption of the thyroid system, despite the fact that many wildlife species in the world suffer unusual thyroid gland development and ratios of circulating thyroid hormones (THs) (Colborn 2002). Thyroid hormones are important regulators of intermediary metabolism, growth, neural development, and aspects of reproduction (Gregory and Cyr 2002;Sher et al 1998). In cetaceans and other mammals, these hormones are also believed to play an important role in controlling heat loss (McNabb 1992).…”

Effects of in Utero Exposure to 2,2',4,4',5,5'-hexachlorobiphenyl on Postnatal Development and Thyroid Function in Rat Offspring

“…Further studies of prenatal PCB exposure will establish whether these chemical substances can contribute to gender-related or gender-specific toxicity. Although possible molecular mechanisms explaining PCB-induced thyroid hormone depression have been reviewed 26) , the impact and effects of each PCB in causing thyroid hormone defects have not been clarified. PCB 153, a non-coplanar congener, shows little or no binding to the aryl-hydrocarbon receptor and appears to act independently of this receptor to disrupt thyroidal homeostasis.…”

Expressing urine from a gel disposable diaper for biomonitoring using phthalates as an example

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