The purpose of this study was to determine the effects of
perchlorate on thyroid function in mosquitofish. Adult mosquitofish were exposed to 0, 0.1, 1, 10, 100, and 1000 mg/L
sodium perchlorate for 2, 10, and 30 d. Whole body
thyroxin (T4) content and histological assessment of thyroid follicles (e.g., follicular epithelial height,
hyperplasia,
hypertrophy, and
colloid depletion) were used to gauge alterations in thyroid function. Follicular epithelial cell height,
hyperplasia, and
hypertrophy increased with increasing
perchlorate concentration, especially in fish exposed for 30 d, and these effects were statistically significantly different from control at concentrations as low as 0.1 mg/L (nominal concentration). The percent occurrence of follicles with depleted
colloid decreased with increasing
perchlorate concentration, which is contrary to what is expected with thyroid inhibition. There also was a decrease in whole body T4 concentration in fish exposed to
perchlorate for 30 d, but clear dose-response relationships were less evident for whole body T4 than for histopathological endpoints. In conclusion, thyroid histopathology provides a sensitive
biomarker for thyroid endocrine disruption at environmentally relevant concentrations of
sodium perchlorate, and whole body T4 is a less sensitive
indicator of
perchlorate exposure than is histopathology.