Endocrine Disruptors and Critical Windows: Development and Disruption of the Thyroid Hormone Pathway in Early Life

Chemical signaling processes and regulatory mechanisms are necessary for an organism’s development, function, and ability to respond to the environment. This article explores one such system: the thyroid hormone signaling pathway. The article illustrates important developmental milestones of the hypothalamus–pituitary–thyroid (HPT) axis in the prenatal and postnatal life stages. Discussion also focuses on the variability of thyroid hormone concentrations, which are often used as a proxy for HPT function, across individuals and lifespan, especially in early development. Several thyroid disrupting chemicals and their mechanisms of action are reviewed. Ongoing research suggests that subtle fluctuations in thyroid hormone concentrations from environmental stressors at sensitive windows of thyroid development are likely to trigger adverse health outcomes later in life. We conclude that further research is necessary in order to fully elucidate the definition of abnormal thyroid hormone concentrations, degree of hormone change required to elicit a response, mechanisms of action, and specific developmental windows of vulnerability.