MCNABB, F.M.A.; Virginia Tech: Evaluating Pollutant Effects on Thyroid Function: Comparing Variables Across Species

Avian thyroid function was assessed at three levels to determine the sensitivity of these variables as indicators of thyroid disruption by pollutant chemicals. We used bobwhite quail and mallard ducks exposed to ammonium perchlorate (AP) in the laboratory and herring gulls from Great Lakes sites exposed to PCBs in the field. The stages studied were embryos and ducklings for mallards, embryos, chicks and adults for quail and herring gulls. The variables measured were plasma thyroid hormones (THs) as indicators of organismal thyroid status, thyroid gland (TG) weight as an indicator of hypothalamic-pituitary-thyroid (HPT) axis activation and TG-TH content as an indicator of TH stores. AP competitively inhibits iodide uptake into the TG, so TH synthesis is compromised, circulating THs decrease, and the HPT axis is activated. PCBs alter thyroid status by increasing TH turnover. If the HPT axis cannot compensate for the altered turnover, hypothyroidism results. The same pattern of sensitivity of variables for revealing thyroid disruption was shown by studies in all three species and all ages studied. The order of thyroid variables from most to least sensitive is: TG-TH content > TG weight ≥ plasma T4. Among endocrine glands, the TG is unique in maintaining extracellular stores of THs, so circulating THs (and organismal thyroid status) can be maintained for some time despite decreases in or cessation of TH synthesis, or increased TH losses from the body. Our results suggest that plasma TH measurement, the most frequently used measure of avian thyroid function, is the least sensitive variable while TG-TH content is the most sensitive variable for assessing thyroid disruption. Some comparisons with mammalian thyroid disruption studies will be made. Supported by SERDP #CU1242 and USEPA #R-827400-01-0.