Effects of environmental synthetic chemicals on thyroid function

Thyroid hormone and γ-aminobutyric acid (GABA) interactions in neuroendocrine systems

Thyroid hormones (THs) have critical roles in brain development and normal brain function in vertebrates. Clinical evidence suggests that some human nervous disorders involving GABA(gamma-aminobutyric acid)-ergic systems are related to thyroid dysfunction (i.e. hyperthyroidism or hypothyroidism). There is experimental evidence from in vivo and in vitro studies on rats and mice indicating that THs have effects on multiple components of the GABA system. These include effects on enzyme activities responsible for synthesis and degradation of GABA, levels of glutamate and GABA, GABA release and reuptake, and GABA(A) receptor expression and function. In developing brain, hypothyroidism generally decreases enzyme activities and GABA levels whereas in adult brain, hypothyroidism generally increases enzyme activities and GABA levels. Hyperthyroidism does not always have the opposite effect. In vitro studies on adult brain have shown that THs enhance GABA release and inhibit GABA-reuptake by rapid, extranuclear actions, suggesting that presence of THs in the synapse could prolong the action of GABA after release. There are conflicting results on effects of long term changes in TH levels on GABA reuptake. Increasing and decreasing circulating TH levels experimentally in vivo alter density of GABA(A) receptor-binding sites for GABA and benzodiazepines in brain, but results vary from study to study, which may reflect important regional differences in the brain. There is substantial evidence that THs also have an extranuclear effect to inhibit GABA-stimulated Cl(-) currents by a non-competitive mechanism in vitro. The thyroid gland exhibits GABA transport mechanisms as well as enzyme activities for GABA synthesis and degradation, all of which are sensitive to thyroidal state. In rats and humans, GABA inhibits thyroid stimulating hormone (TSH) release from the pituitary, possibly by action directly on the pituitary or on hypothalamic thyrotropin-releasing hormone neurons. In mice, GABA inhibits TSH-stimulated TH release from the thyroid gland. Taken together, these studies provide strong support for the hypothesis that there is reciprocal regulation of the thyroid and GABA systems in vertebrates.

Development of metamorphosis assay using Silurana tropicalis for the detection of thyroid system-disrupting chemicals

The West African clawed frog (Silurana tropicalis), which resembles the South African clawed frog (Xenopus laevis), but is somewhat smaller, has a diploid genome and a shorter generation time. Therefore, S. tropicalis has the potential for use as a new model in ecotoxicology. We demonstrated a S. tropicalis metamorphosis assay based on Xenopus Metamorphosis Assay (XEMA) using 1 microg/L thyroxine (T4) and 75 mg/L propylthiouracil (PTU). Tadpoles at developmental stages 48-50 were exposed to chemicals for 28 days and total body length, developmental stage, and hind limb length were recorded every 7 days. Significant differences in developmental stage and total body length were found for both T4 and PTU after 7-day exposure, which were similar to the results of the XEMA ring-test using the same chemicals. Moreover, in the present study, we measured hind limb length as a new endpoint of thyroid axis. Significant differences in the hind limb length were encountered in both T4 and PTU treatments after 7 days of exposure. These results suggest that S. tropicalis can be used in a XEMA-like protocol to detect agonist and antagonist effects of chemicals on the thyroid system. Hind limb length is also a suitable endpoint in such protocols. A new test protocol detecting both thyroid disruption and reproductive effects of chemicals using S. tropicalis should be established in the near future.

Estrogenic activity of dicofol with the human estrogen receptor: Isomer- and enantiomer-specific implications

Dicofol is a non-systemic acaricide/miticide currently registered in the US and Canada for use on a wide variety of crops. This agrochemical has been identified as a potential candidate substance for the United Nations Economic Commission for Europe (UN-ECE) Persistent Organic Pollutant (POP) Protocol and implicated as a potential "endocrine disrupting compound". The technical product is usually synthesized from technical DDT and consists of approximately 80% and 20% of p,p'- and o,p'-dicofol isomers. The o,p'-substituted isomer of dicofol is chiral and may have enantiomer-specific activity; however, the stereospecific activity of o,p'-dicofol has not been reported. In this study, we examined the isomer- and enantiomer-specific endocrine disruption potential of dicofol using yeast-based steroid hormone receptor gene transcription assay designed with the human estrogen receptor (hER). Estrogenic activity of (+)-17-beta estradiol (positive control), p,p'-dicofol, racemic o,p'-dicofol [(+/-)-o,p'-dicofol] and the individual o,p'-dicofol enantiomers was measured via quantification of beta-galactosidase. The (+/-)-o,p'- and p,p'-dicofol were weak estrogen mimics (EC(50): 4.2 x 10(-6) and 1.6 x 10(-6)M, respectively) relative to estradiol (3.7 x 10(-10)M). For o,p'-dicofol, the beta-galactosidase induction by (-)-o,p'-dicofol (EC(50): 5.1 x 10(-7)M) was greater than the racemic mixture. However, the (+)-o,p'-dicofol enantiomer was found to have negligible estrogenic activity. These data indicate that dicofol is a weak hER agonist due to activity of the achiral p,p'-isomer and (-)-o,p'-substituted enantiomer and emphasizes the influence of chemical structure and configuration on biological responses to exposure from chiral compounds.

In vitro and in vivo analysis of the thyroid system-disrupting activities of brominated phenolic and phenol compounds in Xenopus laevis

We investigated the effects of the brominated phenolic and phenol compounds, some of which are brominated flame retardants, on the binding of (125)I-3,3',5-L-triiodothyronine ((125)I-T(3)) to purified Xenopus laevis transthyretin (xTTR) and to the ligand-binding domain of X. laevis thyroid hormone receptor beta (xTR LBD), on the induction of a T(3)-responsive reporter gene in a recombinant X. laevis cell line (XL58-TRE-Luc) and on T(3)-induced or spontaneous metamorphosis in X. laevis tadpoles. Of the brominated phenolic and phenol compounds tested, 3,3',5-tribromobisphenol A and 3,3'-dibromobisphenol A were the most potent competitors of (125)I-T(3) binding to xTTR and the xTR LBD, respectively. Structures with a bromine in either ortho positions with respect to the hydroxy group competed more efficiently with T(3) binding to xTTR and the xTR LBD. 3,3',5-Tribromobisphenol A and 3,3',5,5'-tetrabromobisphenol A, at 0.1-1.0 microM, exerted both T(3) agonist and antagonist activities in the T(3)-responsive reporter gene assay. Sera obtained from fetal bovine and bullfrog tadpoles weakened the T(3) agonist and antagonist activities of 3,3',5-tribromobisphenol A, but not the T(3) antagonist activity of o-t-butylphenol, for which xTTR has no significant affinity. The T(3) agonist and antagonist activities of 0.5 microM 3,3',5-tribromobisphenol A were confirmed in the in vivo, short-term gene expression assay in premetamorphic X. laevis tadpoles using endogenous, T(3)-responsive genes as molecular markers. Our results suggest that 3,3',5-tribromobisphenol A affects T(3) binding to xTTR and xTR and that it interferes with the intracellular T(3) signaling pathway.

Bromate: Concern for developmental neurotoxicity?

The use of ozonation in the purification of drinking water can lead to the formation of bromate. The current regulatory challenges for bromate contamination of drinking water include the need to assess potential human health risks. One health risk of concern is developmental neurotoxicity. Currently, the need for a developmental neurotoxicity study for bromate, based on the weight of evidence, is uncertain. Bromate induces neurotoxicity in adults at high acute exposures and produces hearing loss and structural damage in the cochlea in humans and rodents. However, there is a wide margin of exposure in these studies compared to environmental levels of bromate in water supplies. Data on the effects of bromate on thyroid hormone levels is not consistent and thyroid endocrine disruption is not likely a causative factor in thyroid tumor formation. There is no evidence that bromate caused central nervous system malformations, brain weight changes in developmental studies, nor are there any known structure-activity relationships to other known neurotoxicants. A prudent approach to reduce the uncertainty in the need for a developmental neurotoxicity study of exposure to bromate in drinking water should include determinations of whether bromate ototoxicity occurs with extended duration, low concentration exposures. These studies would provide invaluable data for the weight-of-evidence approach used to determine the necessity of a developmental neurotoxicity study of bromate.

Evaluation of Histological and Molecular Endpoints for Enhanced Detection of Thyroid System Disruption in Xenopus laevis Tadpoles

Amphibian metamorphosis represents a promising model for the identification of thyroid system-disrupting chemicals due to the pivotal role played by thyroid hormones for the initiation and regulation of metamorphosis. An important aspect of bioassay development is the identification and evaluation of sensitive and diagnostic endpoints. In this study, several morphological, histological, and molecular endpoints were evaluated for their utility to detect alterations in thyroid system function after exposure of stage 51 Xenopus laevis tadpoles to various concentrations (1.0, 2.5, 10, 25, and 50 mg/l) of the anti-thyroidal compound ethylenethiourea (ETU). Analysis of developmental stages on exposure day 20 and monitoring of time to fore limb emergence (FLE) revealed retardation and complete arrest of tadpole development at 25 mg/l and 50 mg/l ETU, respectively. Development was not affected by 1.0, 2.5, and 10 mg/l ETU. Histological alterations in the thyroid gland were observed in FLE-displaying tadpoles after exposure to 2.5, 10, and 25 mg/l ETU, as well as in developmentally arrested tadpoles exposed to 50 mg/l ETU. Prevalence and severity of histological changes increased in a concentration-dependent manner. Semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) showed increased mRNA expression of the alpha- and beta-subunits of thyroid-stimulating hormone (TSHalpha, TSHbeta) in pituitary tissue of tadpoles exposed to 25 and 50 mg/l ETU. Results demonstrate the successful detection of anti-thyroidal effects of ETU in Xenopus laevis tadpoles using various endpoints and highlight the particular sensitivity of thyroid gland histology to detect thyroid system disruption in tadpoles.

Zebrafish as a model for developmental neurotoxicity testing

BACKGROUND

To establish zebrafish as a developmental toxicity model, we used 7 well-characterized compounds to examine several parameters of neurotoxicity during development.

METHODS

Embryos were exposed by semistatic immersion from 6 hrs postfertilization (hpf). Teratogenicity was assessed using a modified method previously developed by Phylonix. Dying cells in the brain were assessed by acridine orange staining (these cells are likely to be apoptotic). Motor neurons were assessed by antiacetylated tubulin staining and catecholaminergic neurons were visualized by antityrosine hydroxylase staining.

RESULTS

Atrazine, dichlorodiphenyltrichloroethane (DDT), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were primarily teratogenic and not specifically neurotoxic. 2,4-dichlorophenoxyacetic acid (2,4-D), dieldrin, and nonylphenol showed specific neurotoxicity; dieldrin and nonylphenol were specifically toxic to catecholaminergic neurons. Malathion, although not teratogenic, showed some nonspecific toxicity.

CONCLUSIONS

Teratogenicity measured in 96-hpf zebrafish is predictive of mammalian teratogenicity and is useful in determining whether a compound causes specific neurotoxicity or general developmental toxicity. Induction of apoptosis or necrosis is an indicator of neurotoxicity. An effect on motor neurons in the caudal third of the embryo correlates with expected defects in motility. Overall, our results showed a strong correlation with mammalian data and suggest that zebrafish is a predictive animal model for neurotoxicity screening.

A case for revisiting the safety of pesticides: a closer look at neurodevelopment

The quality and quantity of the data about the risk posed to humans by individual pesticides vary considerably. Unlike obvious birth defects, most developmental effects cannot be seen at birth or even later in life. Instead, brain and nervous system disturbances are expressed in terms of how an individual behaves and functions, which can vary considerably from birth through adulthood. In this article I challenge the protective value of current pesticide risk assessment strategies in light of the vast numbers of pesticides on the market and the vast number of possible target tissues and end points that often differ depending upon timing of exposure. Using the insecticide chlorpyrifos as a model, I reinforce the need for a new approach to determine the safety of all pesticide classes. Because of the uncertainty that will continue to exist about the safety of pesticides, it is apparent that a new regulatory approach to protect human health is needed.

Impact of environmental chemicals on the thyroid hormone function in pituitary rat GH3 cells

Endocrine disrupting chemicals (EDCs) are widespread in the environment and suspected to interfere with the function of thyroid hormones (THs). We investigated the TH disrupting activity of different classes of EDCs including plasticizers (bisphenol A, bisphenol A dimethacrylate), alkylphenols (4-n-nonylphenol, 4-octylphenol), pesticides (prochloraz, iprodion, chlorpyrifos), PCB metabolites (OH-PCB 106, OH-PCB 121, OH-PCB 69) and brominated flame-retardants (tetrabromobisphenol A). The ED potential of a chemical was determined by its effect on the cell proliferation of TH-dependent rat pituitary GH3 cell line. All tested chemicals significantly interfered with the cell proliferation alone or upon co-treatment with T3. The growth of GH3 cells was stimulated by all tested chemicals, but 4-n-nonylphenol, 4-octylphenol, prochloraz and iprodion elicited an inhibitory effect on cell growth. In conclusion, these EDCs have the potential to exert TH disruption increasing the risk or a negative impact on fetal brain development, resulting in cognitive dysfunctions.

Environmental signals: synthetic humic substances act as xeno-estrogen and affect the thyroid system of Xenopus laevis

According to outdated paradigms humic substances (HS) are considered to be refractory or inert that do not directly interact with aquatic organisms. However, they are taken up and induce biotransformation activities and may act as hormone-like substances. In the present study, we tested whether HS can interfere with endocrine regulation in the amphibian Xenopus laevis. In order to exclude contamination with phyto-hormones, which may occur in environmental isolates, the artificial HS1500 was applied. The in vivo results showed that HS1500 causes significant estrogenic effects on X. laevis during its larval development and results of semi-quantitative RT-PCR revealed a marked increase of the estrogenic biomarker estrogen receptor mRNA (ER-mRNA). Furthermore, preliminary RT-PCR results showed that the thyroid-stimulating hormone (TSHbeta-mRNA) is enhanced after exposure to HS1500, indicating a weak adverse effect on T3/T4 availability. Hence, HS may have estrogenic and anti-thyroidal effects on aquatic animals, and therefore may influence the structure of aquatic communities and they may be considered environmental signaling chemicals.

Modulation of iodide uptake by dialkyl phthalate plasticisers in FRTL-5 rat thyroid follicular cells

Plasticisers imparting flexibility to plastics are man-made chemicals abundantly present in the environment. Effects of six different dialkyl phthalates were studied in vitro in the rat thyroid cell line FRTL-5 on their ability to modulate basal iodide uptake mediated by the sodium/iodide symporter (NIS). The present study shows that diisodecyl phthalate (DIDP), dioctyl phthalate (DOP), diisononyl phthalate (DINP) and bis (2-ethylhexyl) phthalate (DEHP) significantly enhance iodide uptake when concentrations in the magnitude between 10(-4) M and 10(-3) M were applied. In this range, these phthalates do not assess toxicity on the cells. Specific inhibiton of NIS demonstrated that enhancement of iodide uptake is due to NIS. In contrast, benzyl butyl phthalate (BBP) also augments iodide uptake at 1mM but this concentration has just exceeded the toxicity threshold and dibutyl phthalate (DBP), the most toxic compound did not modulate iodide uptake at any concentration applied. As we can deduce from our results, plasticisers are capable of significantly modulating NIS mediated iodide uptake activity.

The promoter of the human sodium/iodide symporter responds to certain phthalate plasticisers

The diesters of benzene-1,2-dicarboxylic (phthalic) acid, the phthalates, are used to make plastics flexible and can comprise 40% of the weight of plastic. Human exposure to phthalates can occur via ingestion, inhalation and dermal routes, as well as through parenteral exposure from medical devices containing phthalates. Since earlier morphological studies showed that some phthalates induced thyroid hyperactivity, we thought it important to investigate possible effects of six major phthalates on the transcriptional activity of sodium/iodide symporter (NIS). Di-isodecyl phthalate (DIDP), benzyl butyl phthalate (BBP) and di-octyl phthalate (DOP) increased the activity of the human NIS promoter construct 2.5-, 2.6- and 2.4-fold, respectively. Likewise, these phthalates also enhanced the rat NIS endogenous mRNA expression ca. 2-fold. No effect was observed for bis-(2-ethylhexyl) phthalate (DEHP) and di-isononyl phthalate (DINP), whereas dibutyl phthalate (DBP) appeared to down-regulate hNIS promoter. Although the demonstrated stimulation of NIS gene transcription by DIDP, BBP and DOP is not very strong, this finding is of great importance as humans are routinely exposed for long periods to phthalate plasticisers, the accumulation of which may contribute to thyroid hyperfunction.

Environmental chemicals as thyroid hormone analogues: new studies indicate that thyroid hormone receptors are targets of industrial chemicals?

Thyroid hormone (TH) is essential for normal brain development, but the specific actions of TH differ across developmental time and brain region. These actions of TH are mediated largely by a combination of thyroid hormone receptor (TR) isoforms that exhibit specific temporal and spatial patterns of expression during animal and human brain development. In addition, TR action is influenced by different co-factors, proteins that directly link the TR protein to functional changes in gene expression. Several recent studies now show that TRs may be unintended targets of chemicals manufactured for industrial purposes, and to which humans and wildlife are routinely exposed. Polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and bisphenol-A (BPA), and specific halogenated derivatives and metabolites of these compounds, have been shown to bind to TRs and perhaps have selective effects on TR functions. A number of common chemicals including polybrominated biphenyls (PBBs) and phthalates may also exert such effects. Considering the importance of TH in brain development, it will be important to pursue the possibilities that these chemicals - or interactions among chemical classes - are affecting children's health by influencing TH signaling in the developing brain.

Detection of thyroid system-disrupting chemicals using in vitro and in vivo screening assays in Xenopus laevis

We developed a thyroid hormone (TH) inducible primary screening assay for the identification and assessment of man-made chemicals that interfere with the TH-signalling pathway within target cells. The assay was developed in a Xenopus laevis cell line that was transduced with a self-inactivating (SIN) lentivirus vector (LV) containing a luciferase gene. The luciferase activation in this cell line was TH-specific: 3,3',5-L-triiodothyronine (T(3)) > 3,3'5-L-triiodothyroacetic acid (Triac) > 3,3',5-D-triiodothyronine (D-T(3)), > L-thyroxine (T(4)) > 3,3',5'-L-triiodothyronine (rT(3)). The application of the ligand-dependent luciferase assay for screening for thyroid system-disrupting chemicals revealed that three phthalates (dicyclohexyl phthalate, n-butylbenzyl phthalate, and di-n-butyl phthalate), two herbicides (ioxynil and pentachlorophenol) and a miticide (dicofol) had 3,3',5-L-triiodothyronine- T(3)- antagonist activity at concentrations ranging from 10(-6) to 10(-5) M. These chemicals also inhibited the expression of the endogenous primary T(3)-response TH nuclear receptor beta (TRbeta) gene. The inhibitory characteristics of these chemicals were similar for both assays performed, although the assay for T(3)-dependent activation of TRbeta gene was more sensitive than the luciferase assay. These results indicate that the luciferase assay was a rapid method with a small intra-assay variation for the primary screening of thyroid system-disrupting chemicals. Of the six chemicals, only n-butylbenzyl phthalate and pentachlorophenol exhibited T(3)-antagonist activity in an in vivo metamorphosis-based assay. It should be noted that chemicals elicited thyroid system-disrupting activity in the luciferase assay did not always interfere with the thyroid system in vivo.

Analysis of thyroid hormone receptor betaA mRNA expression in Xenopus laevis tadpoles as a means to detect agonism and antagonism of thyroid hormone action

Amphibian metamorphosis represents a unique biological model to study thyroid hormone (TH) action in vivo. In this study, we examined the utility of thyroid hormone receptors alpha (TRalpha) and betaA (TRbetaA) mRNA expression patterns in Xenopus laevis tadpoles as molecular markers indicating modulation of TH action. During spontaneous metamorphosis, only moderate changes were evident for TRalpha gene expression whereas a marked up-regulation of TRbetaA mRNA occurred in hind limbs (prometamorphosis), head (late prometamorphosis), and tail tissue (metamorphic climax). Treatment of premetamorphic tadpoles with 1 nM 3,5,3'-triiodothyronine (T3) caused a rapid induction of TRbetaA mRNA in head and tail tissue within 6 to 12 h which was maintained for at least 72 h after initiation of T3 treatment. Developmental stage had a strong influence on the responsiveness of tadpole tissues to induce TRbetaA mRNA during 24 h treatment with thyroxine (0, 1, 5, 10 nM T4) or T3 (0, 1, 5, 10 nM). Premetamorphic tadpoles were highly sensitive in their response to T4 and T3 treatments, whereas sensitivity to TH was decreased in early prometamorphic tadpoles and strongly diminished in late prometamorphic tadpoles. To examine the utility of TRbetaA gene expression analysis for detection of agonistic and antagonistic effects on T3 action, mRNA expression was assessed in premetamorphic tadpoles after 48 h of treatment with the synthetic agonist GC-1 (0, 10, 50, 250 nM), the synthetic antagonist NH-3 (0, 40, 200, 1000 nM), and binary combinations of NH-3 (0, 40, 200, 1000 nM) and T3 (1 nM). All tested concentrations of GC-1 as well as the highest concentration of NH-3 caused an up-regulation of TRbetaA expression. Co-treatment with NH-3 and T3 revealed strong antagonistic effects by NH-3 on T3-induced TRbetaA mRNA up-regulation. Results of this study suggest that TRbetaA mRNA expression analysis could serve as a sensitive molecular testing approach to study effects of environmental compounds on the thyroid system in X. laevis tadpoles.

Progress towards Development of an Amphibian-Based Thyroid Screening Assay Using Xenopus laevis. Organismal and Thyroidal Responses to the Model Compounds 6-Propylthiouracil, Methimazole, and Thyroxine

In response to the initial Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC) recommendations, research was conducted on the development of a Xenopus laevis based tail resorption assay for evaluating thyroid axis disruption. This research highlighted key limitations associated with relying on tail resorption as a measure of anti/thyroid activity. The most critical limitation being that tail tissues of tadpoles at metamorphic climax are insensitive to perturbation by thyroid axis agonists/antagonists. To improve upon the initial proposal, we have conducted experiments comparing the sensitivity of pre-metamorphic (stage 51) and pro-metamorphic (stage 54) larvae to the model thyroid axis disruptors methimazole (control, 6.25, 12.5, 25, 50, 100 mg/l), 6-propylthiouracil (PTU) (control, 1.25, 2.5, 5, 10, and 20 mg/l), and thyroxine (T4) (0.25, 0.5, 1, 2, 4 microg/l). Exposures were conducted using two different experimental designs. For experimental design 1, tadpoles were exposed to methimazole or PTU starting at either NF stage 51 or NF 54 for 14 days. For experimental design 2, tadpoles were exposed to PTU or T4 starting at NF stage 51 or NF 54 for 14 and 21 days, respectively. Methimazole and PTU, which are thyroid hormone synthesis inhibitors, both caused a concentration dependent delay in larval development. As determined from this endpoint, there were only minor differences in sensitivity observed among the two stages examined. Further, both compounds caused concentration dependent changes in thyroid gland morphology. These changes were characterized as reduced colloid, glandular hypertrophy, and cellular hyperplasia and hypertrophy. Treatment failed to negatively affect growth, even in tadpoles that experienced significant metamorphic inhibition. T4 treatment resulted in a concentration dependent increase in developmental rate, as would be expected. Similar to studies with methimazole, there were no differences in sensitivity among the two developmental stages examined. These results indicate that tadpoles in the early stages of metamorphosis are sensitive to thyroid axis disruption and that development of a short-term, diagnostic amphibian-based thyroid screening assay shows considerable promise.

Plasma sex steroid and thyroid hormones profile in male water frogs of the Rana esculenta complex from agricultural and pristine areas

Some chemical compounds used in intensive agriculture have been found to induce estrogenic effects; therefore a histological analysis of the testes and an evaluation of plasma levels of sex steroid, thyroid hormones, and vitellogenin were carried out in adult male water frogs of two coexisting taxa (Rana lessonae and the hemiclonal hybrid Rana esculenta) sampled in agricultural and pristine areas. Differences in seasonal profiles of hormones were found in water frogs living in the agricultural area where the presence of endocrine disrupting compounds was suspected on the basis of a previous study. In R. esculenta, sampled in the pristine area, high androgen levels were found in May; the opposite trend was found for R. esculenta sampled in agricultural areas in which the highest androgen levels were found in September, significantly lower compared with those found in R. esculenta sampled in the pristine area. Low androgen levels were also recorded in R. lessonae males sampled both in pristine and agricultural areas, while the highest levels were found in September. Regarding the trend of estradiol-17beta, an increase of this hormone was found in July both in esculenta and lessonae sampled in the agricultural area, and in the same month an estradiol-17beta peak, even though lower, was also found both in esculenta and lessonae males captured in the pristine area; detectable vitellogenin was found neither in males captured in the agricultural area, nor in those sampled in the pristine one. Moreover, while no significant changes of thyroid hormones were found either in the esculenta or lessonae males sampled in the pristine area, increased T3 and T4 titers were found in July in both esculenta and lessonae captured in the agricultural area. Morphological differences of the testes in males of parental species captured in the agricultural area were also observed. These findings indicate alterations in endocrine and reproductive function in frogs in the agricultural area, that could suggest the presence of endocrine disrupting compounds.

Seasonal variation in plasma thyroxine concentrations in juvenile alligators (Alligators mississippiensis) from three Florida Lakes

Circulating concentrations of thyroxine (T(4)) vary seasonally in many vertebrates. This study examined the seasonal variation in plasma concentrations of T(4) in juvenile American alligators (Alligator mississippiensis) from three populations in central Florida, USA. One site, Lake Woodruff National Wildlife Refuge, is considered a reference site whereas the other two lakes, Lake Apopka and Orange Lake, are significantly impacted by human activity. Juvenile American alligators ranging from 75-150 cm in total length were hand-captured at night from November 2000-April 2002. Plasma thyroxine concentrations were analyzed using a radioimmunoassay (RIA) previously validated for alligator plasma. Juvenile American alligators display seasonal variation in circulating T(4) concentrations. Plasma T(4) concentrations decrease from August/September to November and then begin a slow rise until April, at which point they plateau. Sex of juveniles influenced plasma concentrations of T(4) in some months but did not appear to alter the pattern in seasonal variation. The pattern we observed in plasma T(4) concentrations is not directly related to an environmental factor such as ambient temperature but is similar to that seen in plasma sex steroid concentrations during the reproductive cycle of adult alligators. Although the pattern and plasma concentration of T(4) exhibits significant variation among the three lakes studied, the pattern in seasonal variation appears similar. Comparing the seasonal pattern in plasma T(4) with plasma concentrations of sex steroids (testosterone and estradiol-17beta) or corticosterone could provide important information on the peripubescent life stage of the American alligator.

Metamorphic inhibition of Xenopus laevis by sodium perchlorate: effects on development and thyroid histology

The perchlorate anion inhibits thyroid hormone (TH) synthesis via inhibition of the sodium-iodide symporter. It is, therefore, a good model chemical to aid in the development of a bioassay to screen chemicals for affects on thyroid function. Xenopus laevis larvae were exposed to sodium perchlorate during metamorphosis, a period of TH-dependent development, in two experiments. In the first experiment, stage 51 and 54 larvae were exposed for 14 d to 16, 63, 250, 1,000, and 4,000 microg perchlorate/ L. In the second experiment, stage 51 larvae were exposed throughout metamorphosis to 8, 16, 32, 63, and 125 microg perchlorate/L. Metamorphic development and thyroid histology were the primary endpoints examined. Metamorphosis was retarded significantly in the first study at concentrations of 250 microg/L and higher, but histological effects were observed at 16 microg/L. In the second study, metamorphosis was delayed by 125 microg/L and thyroid size was increased significantly at 63 microg/L. These studies demonstrate that inhibition of metamorphosis readily can be detected using an abbreviated protocol. However, thyroid gland effects occur at concentrations below those required to elicit developmental delay, demonstrating the sensitivity of this endpoint and suggesting that thyroidal compensation is sufficient to promote normal development until perchlorate reaches critical concentrations.

Effects of pesticides on the peripheral and central nervous system in tobacco farmers in Malaysia: studies on peripheral nerve conduction, brain-evoked potentials and computerized posturography

We examined the effects of pesticides on the central and peripheral nervous system in the setting of a tobacco farm at a developing country. Maximal motor and sensory nerve conduction velocities (MCV and SCV, respectively) in the median, sural and tibial nerves, postural sway, and brain-evoked potentials (auditory event-related and visual-evoked potentials) were measured in 80 male tobacco farmers and age- and sex-matched 40 controls in Kelantan, Malaysia. Median SCV (finger-wrist) in farmers using Delsen (mancozeb, dithiocarbamate fungicide), who showed significant decrease of serum cholinesterase activities, were significantly lower compared with the controls. Sural SCV in farmers using Fastac (alpha-cypermethrin, pyrethroid insecticide) and median MCV (elbow-wrist) in farmers using Tamex (butralin, dinitroaniline herbicide) were significantly slowed compared with their respective controls. In Delsen (mancozeb, dithiocarbamate) users, the power of postural sway of 0-1 Hz was significantly larger than that in the controls both in the anterior-posterior direction with eyes open and in the right-left direction with eyes closed. The former type of sway was also significantly increased in Tamaron (methamidophos, organophosphorus insecticide) users. In conclusion, nerve conduction velocities and postural sway seem to be sensitive indicators of the effects of pesticides on the central and peripheral nervous system.

Dimethoate effects on thyroid function in suckling rats

The aim of our work is to study dimethoate effects on thyroid function given in drinking water (40 mg/kg body weight, equivalent to 0.2 g/L) to mothers from day zero until the 10th day after delivery. Pups and their mothers were sacrificed on day ten after parturition. Compared to a control group, dimethoate-treated pups showed a 48% decrease in body weight which could be attributed to a defect in thyroid hormones. Indeed, after treatment by dimethoate, plasma rates of free T4 and T3 decreased by 56% and 40% in the young and by 27% and 15% in dams respectively. We can attribute the reduction in plasma thyroxine and triiodothyronine rates to a decrease in thyroid iodine levels (-75%) in the young and (-24%) in their mothers. The decrease in production of thyroid hormones after dimethoate treatment affect thyroid stimulating hormone (TSH) levels. In fact, plasma TSH levels were multiplied in dimethoate-treated group by factors of 2.31 in dams and 1.96 in their offspring. These biochemical modifications confirmed the histological thyroid aspects of pups and dams. In fact, in dimethoate-treated rats, some thyroid follicles of pups presented vesicular cavities without colloid; others contained colloid. However in dams, thyroid follicles presented cubical epithelial cells which surrounded empty vesicular cavities.

Thyroid function in nestling tree swallows and eastern bluebirds exposed to non-persistent pesticides and p, p'-DDE in apple orchards of southern Ontario, Canada

This study examined the associated effects of pesticides and persistent residues of p,p'-DDE on thyroid function in 16-day-old tree swallows (Tachycineta bicolor) and 12-day-old eastern bluebirds (Sialia sialis). Apple orchards sprayed with pesticides in current use and reference sites were chosen for study in southern Ontario, Canada, during 2000-2001. We assessed thyroid hormone concentrations (plasma and muscle thyroxine [T4], and triiodothyronine [T3]) and the response to a challenge with thyroid stimulating hormone (TSH) as well as parameters of thyroid histology. Individual nests were exposed to as many as seven individual pesticide applications and up to five pesticide mixtures. Concentrations of p,p'-DDE in eggs from both species were highest in orchard sites, and ranged from 0.05 to 5.44 mg/kg wet weight for tree swallows, and from 0.17 to 95.4 mg/kg for eastern bluebirds. In 2000, tree swallows from sprayed orchards had elevated plasma T4 concentrations, and thyroid glands with collapsed follicles, hypertrophic epithelia, and higher follicular epithelial cells relative to chicks from reference sites. Plasma T4 concentration was positively correlated with the total number of pesticide mixtures applied during egg incubation through chick rearing. In 2001, basal plasma T3 concentration in tree swallow chicks did not differ between orchard and reference sites, though there were differences among orchards; T3 was positively correlated with total number of individual sprays applied during egg incubation through chick rearing. Tree swallows challenged with TSH showed no significant difference in thyroid hormone concentration or follicular epithelial cell height between exposure groups; however, a significantly greater proportion of chicks from reference sites were found to have thyroids with focal or diffuse hypertrophic epithelia relative to orchard chicks. In 2001, bluebirds from sprayed orchards had significantly higher T3 concentrations in response to TSH challenge; this response was positively correlated with the total number of pesticides applied during egg incubation through chick rearing.

Disruption of Thyroid Hormone Homeostasis at Weaning of Holtzman Rats by Lactational but Not In Utero Exposure to 2,3,7,8-Tetrachlorodibenzo-p-Dioxin

The purpose of this study is to clarify whether lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is entirely responsible for the perturbation in thyroid hormone homeostasis during the neonatal period. Pregnant Holtzman rats were given a single oral dose of 1.0 mug TCDD/kg body weight on gestational day 15. Half of the litters were cross-fostered with the half of the dams treated with vehicle on postnatal day (PND) 1 to make four groups of rats, control (C/C), prenatal TCDD exposure only (T/C), postnatal TCDD exposure only (C/T), and both prenatal and postnatal TCDD exposure (T/T). On PND 21, the C/T and T/T groups, but not the T/C and C/C groups, showed a significant decrease in serum total thyroxin (TT4) and free thyroxin (FT4) concentrations in both sexes and a significant increase in serum thyroid-stimulating hormone (TSH) levels, particularly male pups. These two groups of male and female pups had significantly higher concentrations of TCDD in the liver, with marked induction of cytochrome P450 (CYP) 1A1 mRNA and intense immunostaining of CYP1A1 in the liver. UDP glycosyltransferase 1 family, polypeptide A6 (UGT1A6) and UGT1A7 mRNAs were induced in their livers, with marked immunostaining of UGT1A6. The transfer of TCDD from dams to the pups was confirmed by the detection of TCDD in mother's milk remaining in the stomachs of lactationally exposed pups on PND 1. The present results demonstrate that lactational, but not in utero, exposure to TCDD was responsible for the disruption of thyroid hormone homeostasis.

T-Screen as a tool to identify thyroid hormone receptor active compounds

The T-Screen represents an in vitro bioassay based on thyroid hormone dependent cell proliferation of a rat pituitary tumour cell line (GH3) in serum-free medium. It can be used to study interference of compounds with thyroid hormone at the cellular level, thus bridging the gap between limitations of assays using either isolated molecules (enzymes, transport proteins) or complex in vivo experiments with all the complex feedback mechanisms present. Compounds are tested both in the absence and presence of thyroid hormone (EC(50) concentration of T(3)) to test for both agonistic and antagonistic potency. Thyroid hormones (3,3'-5-triiodothyronine: T(3) and 3,3',5,5'-tetraiodothyroxine: T(4)) and compounds resembling the structure of thyroid hormones (3,3'-5-triiodothyroacetic acid: Triac; 3,3',5,5'-tetraiodothyroacetic acid: Tetrac) induced cell growth, with the rank order Triac > T(3) > Tetrac > T(4) (relative potency = 1.35 > 1 > 0.29 > 0.07), which is identical to published affinities of these compounds for nuclear thyroid hormone receptors. Exposure to 5,5'-diphenylhydantoin (DPH) in the presence of 0.25nM T(3) resulted in up to 60% decreased cell growth at 200μM DPH. No effect of DPH on basal metabolic activity of GH3 cells was observed at this concentration. Fentinchloride (IC(50) = 21nM) decreased cell growth induced by 0.25nM T(3), whereas parallel exposure to these concentrations in the absence of T(3) did not alter basal metabolic activities of GH3 cells. Apolar sediment extracts from the Dommel (34%) and Terneuzen (14%) decreased cell growth in the presence of 0.25nM T(3), whereas the extract from Hoogeveen increased cell growth (26%) and the extract from North Sea Channel had no effect. The T-Screen proved to be a fast and functional assay for assessing thyroid hormone receptor active potencies of pure chemicals or environmental mixtures.

Developmental disruption of thyroid hormone: correlations with hearing dysfunction in rats

A wide variety of environmental contaminants adversely affect thyroid hormone (TH) homeostasis. Hypothyroidism and/or hypothyroxinemia during the early postnatal period in the rat leads to permanent structural damage and loss of function in the cochlea. A major uncertainty in assessing the risks of developmental exposure to thyroid-disrupting chemicals (TDCs) is the lack of a clear characterization of the dose-response relationship, especially in the lower region, between disruption of hormones and adverse consequences. The current work correlated early postnatal hypothyroxinemia with hearing loss in the adult rat. Linear regression was performed on the log transform for total serum thyroxine (T4) concentrations on postnatal day 14 or 21 versus dB(SPL) of hearing loss in adult animals developmentally exposed to TDCs. Regression analyses revealed a highly significant correlation between T4 concentration and hearing loss. In the rat, a 50-60% decrease in circulating T4 was needed to significantly impact hearing function. This correlation suggests that T4 serum concentrations at 14 or 21 days of postnatal age may be a good predictive biomarker in rodents of the adverse consequence of developmental exposure to TDCs.

Altered thyroxin and retinoid metabolic response to 2,3,7,8-tetrachlorodibenzo-p-dioxin in aryl hydrocarbon receptor-null mice

To determine whether the disruption of thyroid hormone and retinoid homeostasis that occurs after exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can be mediated by the arylhydrocarbon receptor (AhR), pregnant AhR-heterozygous (AhR+/-) mice were administered a single oral dose of 10 microg kg(-1) TCDD at gestation day 12.5. Serum and liver were collected on postnatal day 21 from vehicle-treated control or TCDD-treated AhR+/- and AhR-null (AhR-/-) mouse pups. Whereas TCDD exposure resulted in a marked reduction of total thyroxin (TT4) and free T4 (FT4) levels in the serum of AhR+/- mice, TCDD had no effects on AhR-/- mice. Gene expression of UDP-glucuronosyltransferase (UGT)1A6, cytochrome P450 (CYP)1A1, and CYP1A2 in the liver was induced markedly by TCDD in AhR+/- but not AhR-/- mice. Induction of CYP1A1 in response to TCDD was confirmed by immunohistochemical evidence in that CYP1A1 protein was conspicuously localized in the cytoplasm of hepatocytes in the centrilobular region. Levels of retinyl palmitate were greatly reduced in the liver of TCDD-exposed AhR+/- mice, but not in vehicle-treated AhR+/- mice. No effects of TCDD on retinoid levels in the liver were found in AhR-/- mice. We conclude that disruption of thyroid hormone and retinoid homeostasis is mediated entirely via AhR. Induction of UGT1A6 is thought to be responsible at least partly for reduced serum thyroid hormone levels in TCDD-exposed mice.

Thyroid function in relation to burden of PCBs, p,p'-DDE, HCB, mirex and lead among Akwesasne Mohawk youth: a preliminary study

The effects of endocrine disrupting environmental contaminants such as polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB), p,p'-dichlorophenyldichloroethylene (p,p'-DDE), mirex, and lead, are examined in a sample of youth of the Akwesasne Mohawk Nation who are 10-16.9 years of age (n=115). The Akwesasne community is located adjacent to hazardous waste sites where PCBs and other toxicants have contaminated the local ecology. This study examines the relationship between measures of thyroid function and endocrine disrupting environmental toxicants. Both with and without statistical adjustment for other toxicants, as well as age, sex and lipid levels, several indicators of PCB burden are significantly, negatively related to levels of free thyroxine and total thyroxine, and positively to thyrotropin. Lead level was positively related to triidothyronine. Future testing of non-linear relationships is warranted. Despite the linear associations with thyrotropin, free and total thyroxine, PCBs were not related to increased occurrence of these hormones outside the clinically normal range for the testing laboratory.

Pesticides and children

Prevention and control of damage to health, crops, and property by insects, fungi, and noxious weeds are the major goals of pesticide applications. As with use of any biologically active agent, pesticides have unwanted side-effects. In this review, we will examine the thesis that adverse pesticide effects are more likely to occur in children who are at special developmental and behavioral risk. Children's exposures to pesticides in the rural and urban settings and differences in their exposure patterns are discussed. The relative frequency of pesticide poisoning in children is examined. In this connection, most reported acute pesticide poisonings occur in children younger than age 5. The possible epidemiological relationships between parental pesticide use or exposure and the risk of adverse reproductive outcomes and childhood cancer are discussed. The level of consensus among these studies is examined. Current concerns regarding neurobehavioral toxicity and endocrine disruption in juxtaposition to the relative paucity of toxicant mechanism-based studies of children are explored.

Role of thyroid hormones in human and laboratory animal reproductive health

The highly conserved nature of the thyroid gland and the thyroid system among mammalian species suggests it is critical to species survival. Studies show the thyroid system plays a critical role in the development of several organ systems, including the reproductive tract. Despite its highly conserved nature, the thyroid system can have widely different effects on reproduction and reproductive tract development in different species. The present review focuses on assessing the role of thyroid hormones in human reproduction and reproductive tract development and comparing it to the role of thyroid hormones in laboratory animal reproduction and reproductive tract development. The review also assesses the effects of thyroid dysfunction on reproductive tract development and function in humans and laboratory animals. Consideration of such information is important in designing, conducting, and interpreting studies to assess the potential effects of thyroid toxicants on reproduction and development.

Unravelling the genetic complexity of autoimmune thyroid disease: HLA, CTLA-4 and beyond

The autoimmune thyroid diseases (AITDs) including Graves' disease (GD) and autoimmune hypothyroidism (AIH) are the commonest of the autoimmune conditions affecting 2-5% of the western population. Twin studies have clearly demonstrated that AITDs are caused by a combination of both environmental and genetic factors. Association of the HLA class II region with AITD has been documented for over 20 years now, but the primary aetiological variant in this region remains unknown. More recently the CTLA-4 gene region has been identified as the second locus conferring susceptibility to AITD. In contrast to HLA, a polymorphism of the CTLA-4 gene, which encodes an important negative regulator of the immune system, has been identified as a candidate for a primary determinant for AITD. A large number of candidate gene and genome wide linkage studies have been involved in the search for the elusive 'third' locus. The thyroglobulin (Tg) gene in humans maps to chromosome 8q, which has been linked in family studies to AITD. A number of association studies in humans and the mouse model for AITD are beginning to implicate the Tg gene although convincing evidence for a primary causative role is still needed. The establishment of large DNA disease resources along with more detailed genetic maps and the development of faster, more effective, high throughput genotyping and sequencing methods, provides some sense of optimism that novel loci will be identified in the near future and the complex aetiology of AITD will be further unraveled.

Neurodevelopment and endocrine disruption

In this article I explore the possibility that contaminants contribute to the increasing prevalence of attention deficit hyperactivity disorder, autism, and associated neurodevelopmental and behavioral problems in developed countries. I discuss the exquisite sensitivity of the embryo and fetus to thyroid disturbance and provide evidence of human in utero exposure to contaminants that can interfere with the thyroid. Because it may never be possible to link prenatal exposure to a specific chemical with neurodevelopmental damage in humans, I also present alternate models where associations have been made between exposure to specific chemicals or chemical classes and developmental difficulties in laboratory animals, wildlife, and humans.

Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats

OBJECTIVE

To determine whether the increasing prevalence of feline hyperthyroidism is the result of aging of the cat population and whether consumption of canned foods at various times throughout life is associated with increased risk of hyperthyroidism.

DESIGN

Retrospective and case-control studies.

STUDY POPULATION

Medical records of 169,576 cats, including 3,570 cats with hyperthyroidism, evaluated at 9 veterinary school hospitals during a 20-year period, and 109 cats with hyperthyroidism (cases) and 173 cats without hyperthyroidism (controls).

PROCEDURE

Age-adjusted hospital prevalence of hyperthyroidism was calculated by use of Veterinary Medical Database records. On the basis of owners' questionnaire responses, logistic regression was used to evaluate associations between consumption of canned food and development of hyperthyroidism.

RESULTS

Age-specific hospital prevalence of feline hyperthyroidism increased significantly from 1978 to 1997. Overall, consumption of pop-top canned (vs dry) food at various times throughout life and each additional year of age were associated with greater risk of developing hyperthyroidism. In female cats, increased risk was associated with consumption of food packaged in pop-top cans or in combinations of pop-top and non-pop-top cans. In male cats, increased risk was associated with consumption of food packaged in pop-top cans and age.

CONCLUSIONS AND CLINICAL RELEVANCE

These findings suggest that the increasing prevalence of feline hyperthyroidism is not solely the result of aging of the cat population and that canned foods may play a role.

Organophosphorous and organochlorine pesticides affect human placental phosphoinositides metabolism and PI-4 kinase activity

The objective of this work was to describe the effect of organophosphorous and organochlorine pesticides on phosphoinositides metabolism in human placenta. Pesticides concentration (10 microM) was used for in vitro incubations of cell-free homogenates labelled with (32)P orthophosphate. Heptachlor (HC) and dichloro-diphenyl-trichloroethane (o-p' DDT) increased phosphatidyl-inositol, phosphatidylinositolphosphate, and phosphatidyl-inositolbiphosphate phosphorylation while azinphosmethyl (AM) increased phosphatidylinositolbiphosphate labeling. Decreased (32)P incorporation in phosphatidylinositol was found with phosmet (PM), AM, and chlorpyriphos (CHL). The effects of these xenobiotics on PI4-kinase activity using different subcellular fractions were also examined. Both type of pesticides affected the postmembrane supernatant enzyme activity. A biphasic effect on membrane and nuclear PI4-kinase activity was seen with HC. The strongest effect found was seen with o-p' DDT in nuclear kinase activity while substantial changes were also observed in membrane. These data demonstrate the sensitivity of human placental PI4-kinase to pesticides currently found in human tissues and suggest deleterious consequences in different processes regulated by 4-phosphoinositides.

Endocrine disruption and hypospadias

The complexity of human biology makes it impossible to know for certain if endocrine disruption accounts for human penile deformities. Toxicologists point out that an overall assessment of risk must include other factors in addition to exposure including absorption, metabolism, excretion, bioaccumulation and other chemical interactions (Harrison et al., 1997). Many skeptics observe lack of analytic ability to document contaminant levels during critical windows of exposure (Safe, 2000). Further, the environmental estrogens studied (DDT, PCB and bis-phenol A) are quite weak compared to the well studied potent estrogen DES which did not cause penile deformities (Joffe, 2001). While environmental estrogens may be unlikely in contributing to penile deformities, the antiandrogens (phthalates, vinclozolin and DDE) are more plausible is this regard, as maleness is critically dependent upon androgen action. Observers note that, in general, the environmental concentrations of persistent organochlorine compounds have been decreasing over the past two decades. Some feel that our current levels of exposure are too low and the potency of the anti-androgens too weak to account for any significant developmental genital effect (Williams et al., 2001). Caution and restraint are always reasonable in matters of data intrepretation. Past researchers were reassured that pthtalate esters were quite safe when they first were assessed for possible harmful effects on male fertility. Unfortunately it took different models, analyzing transgenerational effects, before it became crystal clear that these compounds can dramatically affect male genital development following experimental maternal exposure at dosages and concentrations currently present in most women. We can not now be so reassured that our male development is unaffected by any of the over 65,000 manmade organochlorine compounds on the planet. Multiple observations from diverse disciplines provide credible evidence that proliferation of xenobiotic chemicals can cause potentially disastrous unintended consequences for the male gender, and upon reflection, our species.

Neonatal exposure to propylthiouracil induces a shift in lymphoid cell sub-populations in the developing postnatal male rat spleen and thymus

Evidence of the connections between the immune system and the thyroid axis is increasingly strong; however, much of the data are focused on immune effects of altered thyroid status in adults or rodents with congenital defects of the pituitary/thyroid axis. The object of the present study was to determine the effects of PTU-induced hypothyroidism on the developing immune system of the rat by focussing on both the spleen and thymus gland. Male Sprague-Dawley rat pups were exposed to PTU through maternal milk by giving the mothers 0.02% PTU in their drinking water starting on the pups' day of birth until day 24 (d24), shortly before weaning on d28. Animals were sampled on days 14, 22, 30, and 91. The mean body weight was decreased in the PTU-treated animals on days 14, 22, and 30. The mean spleen and thymic weights and cellularity were all decreased in the PTU-treated animals on d22 and d30. PTU exposure increased the proportion of NK cells in the spleen on days 14, 22, and 30. The proportion of T-cells was increased on days 22 and 30 with a particular increase in the CD4+ T-cells, resulting in an increase in the ratio of helper T-cells to suppressor/cytotoxic T-cells at d22. PTU also decreased the proportion of splenic B-cells at days 14, 22, and 30 which could explain the increased proportion of both NK and T-cells during these sampling periods. PTU treatment decreased the lytic ability of NK cells at d22, but no functional differences were observed at days 14, 30, 91, despite the increased proportion of NK cells in PTU-exposed animals at days 14, 22, and 30. PTU exposure also increased the proportion of CD4+CD8- cells in the thymus on d22 and caused an increase in both the CD4+CD8- and CD4-CD8+ populations on d30. These data suggest that the effects of temporary, PTU-induced hypothyroidism on the cell populations in the spleen partially result from transient changes in thymic T-cell development, including a shift towards increased CD4+CD8- cells. The data also suggest that temporary hypothyroidism early in development decreases B-cell development in a transient fashion. Temporary hypothyroidism induced from birth to the latter stages of the weaning period induced transitory effects on the spleen, thymus, and immune cell sub-populations--all of which recovered to normal values when the animals matured.

The effect of endocrine disrupting chemicals on thyroid hormone binding to Japanese quail transthyretin and thyroid hormone receptor

We investigated the effect of endocrine disrupting chemicals (EDCs), including medical, industrial, and agricultural chemicals, on 3,3',5-L-[125I]triiodothyronine ([125I]T3) binding to purified Japanese quail transthyretin (qTTR), a major thyroid hormone-binding protein in plasma, and to the ligand-binding domain of thyroid hormone receptor beta (qTR LBD). Scatchard plots of T3 binding to qTTR and qTR LBD revealed two classes of binding sites, with Kd values of 6.9 and 185 nM, and a single class of binding sites, with Kd value of 0.31 nM, respectively. Among the test chemicals, diethylstilbestrol was the most powerful inhibitor of [125I]T3 binding to qTTR (IC50 < 0.4 nM). Diethylstilbestrol, ioxynil (IC50 =1.1+/-0.5 nM) and pentachlorophenol (IC50 = 6.3+/-3.8 nM) displaced [125I]T3 from qTTR more effectively than unlabeled T3 (IC50 = 9.7+/-0.9 nM) did. Although malathion, 4-nonylphenol, bisphenol A and n-butylbenzyl phthalate were effective inhibitors of [125I]T3 binding to qTTR, their potency was two orders of magnitude less than that of T3. All test chemicals except for diethylstilbestrol had either a weak or no effect on [125I]T3 binding to qTR LBD. These results show that several EDCs tested in this study target qTTR rather than qTR LBD.

In vitro assay of thyroid disruptors affecting TSH-stimulated adenylate cyclase activity

Several natural or synthetic chemicals have been indicated as potential thyroid disruptors. The development of in vitro assays has been recommended to comprehensively assess the potential thyroid disrupting activity of a substance or a complex mixture. In this study, 12 substances suspected for acting as thyroid disruptors were tested for their ability to inhibit TSH-stimulated cAMP production in vitro. Those substances producing an inhibition were further studied to establish the level at which they interfere with this step of thyroid cell function. Using Chinese hamster ovary cells (CHO) transfected with the recombinant human TSH receptor, a dose-dependent inhibition of TSH-stimulated adenylate cyclase activity was produced by 1,1-bis-(4-chlorphenyl)-2,2,2-trichloroethan (DDT), Aroclor 1254 and Melissa Officinalis. All three substances also inhibited the cAMP production stimulated by TSH receptor antibody. Melissa Officinalis produced a significant inhibition of TSH binding to its receptor and of antibody binding to TSH, while no significant changes were produced by Aroclor 1254 or DDT in these assays. These data suggest that principles contained in Melissa Officinalis may block the binding of TSH to its receptor by acting both on the hormone and the receptor itself, while DDT and Aroclor 1254 affect cAMP production mainly at post-receptor step. In conclusion, we have developed a set of in vitro assays that allow investigation into the effect of thyroid disruptors on the TSH-mediated activation of the cAMP cascade. These assays may be useful to identify the mechanism of action of thyroid disruptors, coming beside and supporting animal studies or epidemiological surveys.

Effects of a mixture of polychlorinated biphenyls (Aroclor 1254) on the transcriptional activity of thyroid hormone receptor

Polychlorinated biphenyls (PCBs) are environmental contaminants which may affect thyroid function. PCBs may reduce serum thyroid hormone (TH) concentrations by either displacing T4 from TH transport proteins or increasing its hepatic metabolism. The reduced serum T4 causes neurological and growth defects in animals exposed to PCBs during the perinatal period, which can partially be reverted by T4 administration. In addition to a hypothyroid-like syndrome, a direct action of PCBs on TH-sensitive genes has been postulated. In the present study the effects of Aroclor 1254 (ARO), a mixture of PCBs, on transcription of TH-dependent genes were investigated. A reporter plasmid containing the TH-responsive element (TRE) of malic enzyme (ME) gene was used in transient transfections to assess the responsiveness to ARO. ARO (10 microM) reduced the CAT activity by about 50% and competed with T3 to reduce the induction of transcription. Cotransfection of TH receptor (TR) and a wild type TRE was required to reveal ARO inhibitiry effect, which was abolished by a mock reaction not containing TR or by a mutated TRE. ARO reduced the 125I-T3 binding to TR by 30%, but did not affect the interaction of TR with a 32P-labeled TRE in gel shift assay. ARO is likely to produce a conformational change in in vitro translated TR, leading to its increased proteolysis by trypsin. These results demonstrate that ARO interacts with TR, thereby affecting the transcription of TH-sensitive genes, and provide a molecular basis to further explain the complex effects of PCBs on TH disruption.

Male reproductive hormones and thyroid function in pesticide applicators in the Red River Valley of Minnesota

In the present effort, 144 pesticide applicators and 49 urban control subjects who reported no chronic disease were studied. Applicators provided records of the season's pesticides used by product, volumes, dates, and methods of application. Blood specimens for examination of hormone levels were obtained in summer and fall. In the herbicide-only applicator group, significant increases in testosterone levels in fall compared to summer and also elevated levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in the fall were noted. With respect to fungicide use, in an earlier cross-sectional epidemiologic study, data demonstrated that historic fungicide use was associated with a significant alteration of the sex ratio of children borne to applicators. As before, among current study subjects it was noted that historic fungicide use was associated with increased numbers of girls being born. Lower mean total testosterone concentrations by quartile were also correlated with increased numbers of live-born female infants. A downward summer to fall seasonal shift in thyroid-stimulating hormone (TSH) concentrations occurred among applicators but not among controls. Farmers who had aerial application of fungicides to their land in the current season showed a significant shift in TSH values (from 1.75 to 1.11 mU/L). Subclinical hypothyroidism was noted in 5/144 applicators (TSH values >4.5 mU/L), but not in urban control subjects. Based on current and past studies, it was concluded that, in addition to pesticide exposure, individual susceptibility and perhaps economic factors may play a supporting role in the reported results.

Possible effects of polychlorinated biphenyls and organochlorinated pesticides on the thyroid after long-term exposure to heavy environmental pollution

The purpose of this work was to study the effects of high environmental exposure to polychlorinated biphenyls (PCBs) and other organochlorines on the thyroid. Thyroid volume, hypoechogenicity and nodules (by ultrasound), presence of antithyroid peroxidase (anti-TPO) antibodies, and abnormal thyroid-stimulating hormone (TSH) levels in serum (by radioimmunoassay) were examined in 101 adults from the PCB-polluted area in 360 controls. Serum levels of PCBs, hexachlorobenzene, gamma-hexachlorocyclohexane (HCH), p,p'-DDT(1,1,1-trichloro-2,2'-bis(p-chlorophenyl)ethane), and p,p'-DDE(1,1-dichloro-2,2'-bis(p-chlorophenyl)ethene) were measured by high-resolution gas chromatography. Very high levels of PCBs were found in the polluted area (7300 +/- 871 ng/g lipids) compared with controls (2045 +/- 147 ng/g). Positive correlations (P < 0.001) were found between the levels of all organochlorines and their total except for hexachlorocyclohexane (HCH). In the polluted area, the highest thyroid volumes (18.7 +/- 2.32 mL; mean +/- SE) were clustered among 23 subjects (17 males and six females) with PCB levels above 10,000 ng/g (range 10,000-58,667 ng/g). In the remaining 438 subjects the thyroid volume was 14.2 +/- 0.29 mL. These data suggest that there might be a threshold serum PCB level of approximately 10,000 ng/g that may influence the thyroid volume. A two-way ANOVA showed that all thyroid volumes in the polluted area were significantly higher (P < 0.001) than in the control area. In males from the polluted area, the frequencies of thyroid hypoechogenicity, thyroid nodules, positive anti-TPO, and abnormal TSH level were higher than in males from the control area, whereas such differences were not observed in females. Increased thyroid volume and indicators of potential thyroid dysfunction were associated with long-term environmental exposure to PCBs. These effects on the thyroid were confined to subjects with PCB levels above 10,000 ng/g of lipid (thyroid volume) and to males from the polluted area (thyroid hypoechogenicity, thyroid nodules, positive anti-TPO, and abnormal TSH).

Competitive interactions of chlorinated phenol compounds with 3,3',5-triiodothyronine binding to transthyretin: detection of possible thyroid-disrupting chemicals in environmental waste water

Chlorinated phenol compounds, such as the chlorinated derivatives of bisphenol A, have been detected in effluents from paper manufacturing plants. We investigated the effects of bisphenol A, nonylphenol, and their seven chlorinated derivatives on 3,3',5-[(125)I]triiodothyronine ([(125)I]T(3)) binding to purified chicken and bullfrog transthyretin (cTTR and bTTR) and to the ligand-binding domains of chicken and bullfrog thyroid hormone receptor beta (cTR LBD and bTR LBD). The concentrations at which the chlorinated derivatives displaced [(125)I]T(3) from TTR were 10-10(3) times less than those of their parent molecules. 2,6-Dichloro-4-nonylphenol and 3,3',5-trichlorobisphenol A were the most potent competitors of T(3) binding to cTTR and to bTTR, respectively. The interactions of the chlorinated derivatives with the cTR and the bTR LBDs were weaker than those of the chlorinated derivatives with cTTR and bTTR. Chlorinated derivatives with a greater degree of chlorination were more efficient competitors of T(3) binding to TTR and TR. A structure-activity relationship between the phenol compounds and TTR (TTR assay) and TR (TR assay) was established. Structures with chlorine in either ortho position or both ortho positions, with respect to the hydroxy group, were more efficient competitors. Chemicals that interacted with bTTR, cTTR, and Japanese quail TTR were detected in water downstream of effluents from paper manufacturing plants using the TTR assay. As some of the chlorinated bisphenols and nonylphenols were potent competitors of T(3) binding to TTRs, the TTR assay could be applied as primary screening for possible thyroid-disrupting chemicals in environmental waste water.

Challenges confronting risk analysis of potential thyroid toxicants

Screening and testing for potential thyroid toxicants using endpoints of thyroid function, including circulating levels of thyroid hormones and thyrotropin, will not capture toxicants that directly interfere with thyroid hormone action at the receptor. The goals of the present review are to provide a critique of the literature focused on thyroid hormone and brain development as it relates to testing and evaluating thyroid toxicants, and to propose possible solutions to this perceived dilemma.