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Developmental disruption of thyroid hormone: correlations with hearing dysfunction in rats
1Neurotoxicology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. EPA, Research Triangle Park, NC 27711, USA. crofton.kevin@epa.gov
Summary
Environmental contaminants disrupt thyroid hormone (TH) homeostasis. Early postnatal low thyroxine (T4) levels in rats correlate with adult hearing loss, indicating T4 as a predictive biomarker for thyroid-disrupting chemical (TDC) exposure risks.
Area of Science:
- Endocrinology
- Toxicology
- Neuroscience
Background:
- Environmental contaminants can disrupt thyroid hormone (TH) homeostasis.
- Postnatal hypothyroidism or hypothyroxinemia in rats causes permanent cochlear damage and hearing loss.
- Assessing risks of developmental exposure to thyroid-disrupting chemicals (TDCs) lacks clear dose-response data.
Purpose of the Study:
- To correlate early postnatal hypothyroxinemia with adult hearing loss in rats.
- To establish a dose-response relationship between TH disruption and adverse outcomes from TDC exposure.
- To identify potential biomarkers for TDC developmental toxicity.
Main Methods:
- Linear regression analysis was used.
- Log-transformed total serum thyroxine (T4) concentrations on postnatal days 14 or 21 were correlated with hearing loss (dB SPL) in adult rats.
- Animals were developmentally exposed to TDCs.
Main Results:
- A highly significant correlation was found between T4 concentration and hearing loss.
- A 50-60% decrease in circulating T4 was required to significantly affect hearing function in rats.
- Early postnatal T4 levels demonstrated predictive value for hearing impairment.
Conclusions:
- Serum T4 concentrations at postnatal days 14 or 21 are a reliable predictive biomarker in rodents.
- This biomarker can indicate adverse consequences of developmental TDC exposure.
- Understanding TH disruption is crucial for assessing environmental chemical risks.