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ToxSci Advance Access originally published online on August 13, 2004
Toxicological Sciences 2004 82(1):106-113; doi:10.1093/toxsci/kfh247
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Toxicological Sciences vol. 82 no. 1 © Society of Toxicology 2004; all rights reserved.

Does Thyroid Function in Developing Birds Adapt to Sustained Ammonium Perchlorate Exposure?

F. M. Anne McNabb*,1, Darryl A. Jang*,2 and Calvert T. Larsen{dagger}

* Department of Biology, Virginia Tech, Blacksburg Virginia 24061-0406; {dagger} Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Virginia Tech, Blacksburg VA 24061-0443

Received May 27, 2004; accepted August 4, 2004

The effects of a wide range of ammonium perchlorate (AP) concentrations in drinking water on thyroid function in bobwhite quail chicks was investigated at 2, 4, and 8 weeks of exposure. We measured plasma thyroid hormones (THs) to evaluate organismal thyroid status, thyroid weights to evaluate hypothalamic-pituitary-thyroid (HPT) axis activation, and thyroidal TH content to assess TH stores. At the highest AP exposures plasma THs were decreased, thyroid glands were hypertrophied, and thyroidal TH content was decreased. As in our previous studies, thyroidal thyroxine (T4) content was the most sensitive indicator of decreased thyroid function; plasma T4 and thyroid weight were much less sensitive. The lack of sensitivity of these variables appears to result from cyclic patterns of thyroid responses involving the HPT axis and intermittent release of stored THs from the thyroid gland. With sustained AP exposure (8 weeks), at the lowest range of AP concentrations used, chicks showed adaptation in thyroid function that fully compensated for the initial (2 week) effects of AP. At the intermediate AP concentrations there was partial compensation for the initial AP effects. At the highest AP exposures used, thyroid function was very low throughout the study, with no indication of compensatory responses. The capability of chicks to increase some aspects of their thyroid function adaptively in response to some levels of sustained AP exposure is contrary to the common generalization that developing animals are most vulnerable to environmental contaminants.

Key Words: perchlorate; thyroid; quail chicks; exposure time.


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