Ammonium perchlorate effects on thyroid function and growth in bobwhite quail chicks

Abstract

Bobwhite quail chicks were used to investigate ammonium perchlorate (AP; NH4ClO4) effects on thyroid function and growth. Beginning at 3 to 4 d posthatch, we evaluated organismal thyroid status (circulating hormones), activation of the hypothalamic-pituitary-thyroid axis (thyroid wt) and thyroidal hormone content over a wide range of AP concentrations (50 microg/L - 4,000 mg/L) in drinking water, for relatively short (2-week) and longer (8-week) exposures. Thyroidal thyroxine (T4) content, the most sensitive index of decreased thyroid function, decreased markedly in response to increasing perchlorate exposure. Thyroid weight and plasma T4 were less sensitive indicators and similar in their ability to detect thyroid changes. Growth measurements (body wt and skeletal growth) were very insensitive indices. Because thyroids contain large hormone stores, with low exposures or short time periods, these stores can be used to maintain circulating hormones, at least temporarily. Most depletion of thyroidal T4 occurred during the first two weeks of AP exposure. Subsequent decreases were at a slower rate presumably because thyrotropin stimulation of the thyroids at least partially compensated for some of the perchlorate effect. Additional studies of the interactions between AP concentration and exposure time are needed for understanding the complex nature of thyroid responses to perchlorate.

Environmental Thyroid Disruptors and Human Endocrine Health

“…In addition, TDs present in small amounts in the environment may not cause overt changes of TH levels but may nonetheless alter hormonal homeostasis (Boas et al, 2006). A wellestablished example is perchlorate, which in small amounts does not alter circulating TH levels but diminished T4 content in the thyroid gland (Isanhart et al, 2005;McNabb et al, 2004a;McNabb et al, 2004b). These data agreed with in vitro studies which proposed an perchlorate-induced inhibition of sodium-iodide symporter (Tonacchera et al, 2004).…”

Environmental chemicals and thyroid function

“…Intra-thyroidal T4 content, gene transcription activity and cellular growth appear to be more sensitive endpoints when assessing the significance of endocrine disruption from various chemicals. A well established example is perchlorate, which in small amounts does not alter plasma hormone levels, but diminishes thyroid gland T4 content (111)(112)(113), supporting the observation from in vitro studies of an inhibition of sodium -iodide symporter (NIS) (114). Thus, endocrine-disrupting chemicals present in small amounts in the environment may not cause overt changes of hormone levels in animals and humans, but may nonetheless alter the hormonal homeostasis.…”

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Birds and Its Role in Bird Development and Reproduction

“…Studies of perchlorate exposure (perchlorate competitively inhibits thyroidal iodide uptake) in bobwhite quail have been used to compare the sensitivity of circulating thyroid hormone concentrations, thyroid gland weights and thyroid gland hormone content. These studies showed circulating hormones to be the most variable measurement and to be the least sensitive indicator of altered thyroid function (McNabb et al, 2004a(McNabb et al, , 2004b. At the lower and mid range concentrations of perchlorate, both high and low hormone concentrations were observed in different individuals and from experiment to experiment.…”

“…Thus, in those cases where increased thyroid gland function cannot compensate, a cyclic pattern of decreases in circulating hormones and release of stored hormones can occur with resulting depletion of thyroid gland hormone stores. This type of cyclic pattern of responses occurs in response to iodine deficiency in humans (Delange and Ermans, 1996) and appears to be the case in both quail (McNabb et al, 2004a) and rats (York et al, 2001) exposed to perchlorate, which interferes with thyroidal iodide uptake. Such cyclic patterns of HPT axis response appear to be a key reason why measurements of plasma thyroid hormones often are highly variable in studies of endocrine disruption by chemicals (see "Circulating T 4 and T 3 ").…”