Collision of basic and applied approaches to risk assessment of thyroid toxicants

Thyroid hormone (TH) is essential for normal brain development; therefore, any environmental chemical that interferes sufficiently with thyroid function, TH metabolism, or TH action may exert adverse effects on brain development. Important known differences in aspects of thyroid endocrinology between the fetus, infant, and adult allow us to identify age-dependent vulnerabilities to thyroid toxicants with some confidence. These differences include the size of the hormone pool stored in the thyroid gland at different ages as well as the age-dependent sensitivity to mild TH insufficiency. Several recent studies that describe risk assessments of the environmental contaminant, ammonium perchlorate, provide good examples of conclusions based on the selective consideration of these known aspects of the thyroid system. Specifically, authors who consider age-dependent differences in thyroid endocrinology suggest that safe levels of perchlorate should be set at relatively low levels (low parts per billion). In contrast, authors who do not consider these known age-dependent differences in thyroid endocrinology recommend safe levels of perchlorate at high (hundreds) parts per billion to parts per million. Emerging evidence indicates that a variety of high production volume chemicals can directly interact with the TH receptor. As testing paradigms are designed by regulatory agencies, these age-dependent differences in thyroid endocrinology must be considered.