Thyroid-disrupting chemicals: Interpreting upstream biomarkers of adverse outcomes

Abstract

Background: There is increasing evidence in humans and in experimental animals for a relationship between exposure to specific environmental chemicals and perturbations in levels of critically important thyroid hormones (THs). Identification and proper interpretation of these relationships are required for accurate assessment of risk to public health. Objectives: We review the role of TH in nervous system developments and specific outcomes in adults, the impact of xenobiotics on thyroid signaling, the relationship between adverse outcomes of thyroid disruption and upstream causal biomarkers, and the societal implications of perturbations in thyroid signaling by xenobiotic chemicals. Data sources: We drew on an extensive body of epidemiologic, toxicologic, and mechanistic studies Data synthesis: THs are critical for normal nervous system development, and decreased maternal TH levels are associated with adverse neuropsychological developments in children and poorer blood lipid profiles, both risk factors for cardiological disease and death. These effects of thyroid suppression are observed even within the "normal" range for the population. Environmental chemicals may affect thyroid homeostasis by a number of mechanismss, and multiple chemicals have been identified that interfere with thyroid function by each of the identified mechanisms. Conclusions: Individuals are potentially vulnerable to adverse effects as a consequence of exposure to thyroid-disrupting chemicals. Any degree of thyroid disruption that affects TH levels on a population basis should be considered a biomarker of adverse outcomes, which may have important societal outcomes.