Dioxin effects on neonatal and infant thyroid function: routes of perinatal exposure, mechanisms of action and evidence from epidemiology studies

Objectives: Animal experiments suggest that thyroid function alterations in newborns and infants may represent one of the most sensitive markers of toxicity from 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Dioxin can be transferred from the mother to the offspring either in utero or through lactation. It has been suggested that thyroid-hormone alterations produced by dioxin in utero or shortly after birth may underlie long-term effects, such as cognitive-ability and neurodevelopment impairment. In the present review article, we appraise available evidence on the effects of perinatal exposure to dioxin on fetal and infant thyroid function. Methods: We summarized the routes of perinatal dioxin exposure and research results on possible mechanisms of dioxin toxic effects on thyroid function. We performed a systematic review of epidemiology studies conducted on mother–child pairs exposed to background environmental levels to investigate dioxin effects on neonatal and infant thyroid function. Results: Toxicological and mechanistic data indicate that dioxin may impair thyroid function in exposed newborns and infants. Investigations on background-exposed children have not consistently demonstrated an association between perinatal TCDD exposure and thyroid function, although some of the studies suggest that sub-clinical hypothyroidism may be induced by perinatal dioxin exposure within 3 months from birth. Between studies inconsistencies may be related to lab method differences, mixed exposures, and small sample size of the populations evaluated. Conclusion: Epidemiology studies have as yet failed to demonstrate an association between perinatal TCDD exposure and thyroid function alterations in human subjects, although suggestive evidence from animal and in-vitro experimental data is available.