MCNABB, F.M.A.*; MAHER, R.D.; JONES, J.E.; JANG, D.A.; Virginia Tech, Blacksburg: Ammonium Perchlorate Effects on Developing and Adult Thyroid Function in Bobwhite Quail and Mallard Ducks

We investigated the effects of ammonium perchlorate (AP), an environmental contaminant resulting from its use as an oxidizer in solid rocket fuels, on thyroid function in two avian species. In vertebrates, perchlorate ion alters thyroid function by competitive inhibition of iodide uptake into the thyroid gland (TG). This results in low circulating thyroid hormones (THs) and negative feedback activation of the hypothalamic-pituitary-thyroid (HPT) axis. Despite historical awareness that perchlorate inhibits thyroid function, there is essentially no information about AP effects in birds. We have used dose-response studies to evaluate toxicity and organismal thyroid status (based on plasma THs), HPT axis activation (based on TG weight) and overall TG function (based on TG-TH content). Studies to date, in Bobwhites (embryos, chicks and adults) and in Mallards (embryos and ducklings), show that AP effects on thyroid function decrease with age at exposure and increase with exposure time and dose. TG-TH content is by far the most sensitive indicator of alterations in thyroid function with AP exposure. TG weight and plasma TH concentrations have similar ability to reveal thyroid changes and are less sensitive indicators than TG-TH content. The capability of the TG to store large amounts of THs plays an important role in the sensitivity of these indicators for revealing alterations in thyroid function and in the differences in AP effects with age at exposure. Thus, TG-TH stores can supply THs to the circulation and can be maintained for some time after decreases in or cessation of TH synthesis. During development, embryos and chicks, with smaller TG-TH stores, are considerably more sensitive to AP than are adult quail. Supported by SERDP contract #CU1242.