Date of Graduation


Document Type


Degree Type



Davis College of Agriculture, Natural Resources and Design


Wildlife and Fisheries Resources

Committee Chair

Patricia Mazik

Committee Co-Chair

Vicki Blazer

Committee Member

Amy Welsh

Committee Member

AB Billings


A high prevalence of endocrine disrupting chemicals (EDCs) in Great Lakes Areas of Concern (AOCs) has raised concerns as to the health and fitness of fish and wildlife. It is not well understood to what extent existing contaminants, many with continuing inflows into the environment, may impact fish populations. This study provides an initial characterization of thyroid endocrine-related effects in two indigenous fish species sampled from Great Lakes AOCs. Biomonitoring was conducted on a pelagic, top predator species, smallmouth bass (Micropterus dolomieu) and benthic, omnivorous brown bullhead (Ameiurus nebulosus) at 7 sites in spring and fall of 2012. Thyroidal endpoints, such as plasma hormone levels (plasma thyroid stimulating hormone—TSH, thyroxine—T4, and triiodothyronine—T3), thyroid histology (thyroid epithelial cell height and colloid depletion), and expression levels of thyroid-related genes (Thyroid receptor alpha—THRα, Thyroid receptor beta—THRβ, deiodinase type I—DIO1, and deiodinase type II—DIO2) were measured in both species, and relationships between the endpoints were evaluated to see if associations exist between thyroid endpoints at multiple biological levels. Histological evaluation of the thyroid tissue indicated hyperstimulation (as indicated by increased thyroid epithelial cell height and partially depleted colloid) in smallmouth bass and brown bullhead sampled in the spring. Despite observed histologic alterations, changes in thyroid gland histology did not coincide with changes in concentrations of circulating thyroid hormones. However, gene transcript abundance of THRα was negatively correlated with TSH and T3 while levels of DIO2 were positively correlated with TSH and T3 in smallmouth bass, suggesting these genes are sensitive and stable indicators for thyroid status. The results demonstrate the importance of using a multi-tiered approach to evaluate the potential risks of EDCs on the teleost thyroid system, as well as the importance of choosing sensitive species and accounting for seasonality.