Environmental thyroid toxicants and child endocrine health.

Humans are continuously exposed to many man-made chemicals, which are environmentally persistent and often hormone-like active. Substantial in vitro and in vivo evidence indicate that polyhalogenated aromatic pollutants, such as dioxins,furans,polychlorinated biphenyls and polybrominated diphenylethers, can adversely affect thyroid function mainly resulting in hypothyroidism. Although most studies on human background-exposure have as yet failed consistently to associate thyroid function with environmental toxicants, current views point towards subtle or transient impairment of thyroid secretion. Small hormonal changes chemically induced, though within normal reference ranges, may have negative consequences for the developing individual. In particular, the fetus and the neonate/infant may be vulnerable to subtle changes of thyroid function as their turnover of the thyroid hormonal store is very rapid and they may become depleted more rapidly than adults. This critical developmental phase may be vulnerable to even subtle toxicant effects on the thyroid system. Moreover, data inconsistencies may be related to sample size limitations and methodological issues, including mixed toxicant congener exposure that has precluded conclusions about chemical congeners per se. More studies are crucial to fill in the research gaps regarding permanent endocrine and neurological outcome in next generations exposed to background thyroid toxicants.