Thesis Title

Metabolic and Thyroidal Effects of Vinclozolin on Cotton Rats (Sigmodon Hispidus)

Date of Graduation

Fall 1999


Master of Science in Biology



Committee Chair

Thomas Tomasi

Subject Categories



The effects that man-made chemicals in the environment can have on the hormones of all species encountering them have yet to be completely understood. The possibility that some compounds (fungicides, insecticides, nematocides, etc.) used to protect produce grown throughout the world can have endocrine mimicking/disrupting effects has been postulated and studied intensively over the last decade. Thyroid hormone concentrations (free and total), thyroxine turnover capabilities, and metabolism (resting, cold-induced, and norepinephrine-induced) were measured in wild cotton rats (Sigmodon hispidus) while being treated with a commonly used fungicide, vinclozolin. Treatments included two dosages (2.8 mg/m² and 281 mg/m²) and three treatment periods (pregnancy, lactating, and post-lactating) for a total of seven treatment groups (with the control). Animals treated with the high dose while post-lactational were found to have consistently lower thyroid hormone levels (not always significant) and higher thyroxine turnover rates. In addition to the hormone findings, the high-dose post-lactating group had the lowest weight gain throughout the study, about half that of the control group. However, the endpoint of the thyroid system measured in this study (metabolism) was not adversely effected with the altered thyroid hormone parameters. Given the U.S. E.P.A. standards of "detrimental" effects, it can be concluded from this study that there are no observed "detrimental" effects on the metabolic capability of the rats. The animals appear to be compensating for decreased thyroid hormone levels by increasing their thyroid hormone turnover rates. The decrease in the daily weight gain may be the result of the thyroid hormone dysfunctions and may ultimately prove to be "detrimental" to this species in the wild.


© John Henry Ashcraft