Current and potential rodent screens and tests for thyroid toxicants.

Area of Science:

• Environmental Toxicology
• Endocrinology
• Reproductive and Developmental Toxicology

Background:

• Thyroid toxicants can disrupt multiple physiological pathways, including hormone levels, iodide uptake, and hormone action.
• Existing Organization for Economic Cooperation and Development (OECD), Japanese, and U.S. Environmental Protection Agency (EPA) assays use endpoints like thyroid gland weight, histopathology, and hormone levels.
• Understanding thyroid toxicant effects is crucial for human health risk assessment.

Purpose of the Study:

• To review current rodent screens and tests for detecting thyroid toxicants.
• To identify key disruption points in the thyroid system.
• To discuss the integration of novel endpoints into existing assays for enhanced detection of thyroid disruption.

Main Methods:

• Review of existing in vivo rodent assays and their endpoints (e.g., OECD, EPA protocols).
• Identification of potential disruption points for thyroid toxicants.
• Discussion of novel endpoints under development, including molecular and neurodevelopmental assessments.

Main Results:

• Current assays primarily measure thyroid gland weight, histopathology, serum thyroid hormones, and thyroid-stimulating hormone (TSH).
• Numerous potential disruption points exist, including effects on thyroperoxidase, iodide uptake, deiodinases, and hormone action.
• New endpoints, such as gene expression and neurodevelopmental assessments, show promise but require further validation.

Conclusions:

• In vivo rodent assays are currently the most appropriate for assessing thyroid toxicants.
• Adding strategic endpoints can improve the detection of thyroid system disruption.
• Further research and development are needed for novel endpoints to be incorporated into regulatory guidelines.