Studies on the goitrogenic mechanism of action of N,N,N',N'-tetramethylthiourea

A high throughput screening assay for human Thyroperoxidase inhibitors

Rapid, human relevant assays are needed to assess potential hazards of the many chemicals in commerce. An assay of thyroid peroxidase (TPO) inhibition, using the substrate Amplex Ultra Red, was recently adapted for human TPO (AUR-hTPO). We tested a large number (788) of chemicals through this AUR-hTPO assay and compared performance with published results from an assay using enzyme from rat thyroid microsomes (AUR-rTPO). Coded chemicals, from the US EPA ToxCast Inventory, were tested in a tiered approach: 1) Initial screening at a single concentration; 2) Potency estimation for active chemicals with multiple concentrations; 3) Screening active chemicals for the non-specific activity. The assay gave consistent results for positive chemical methimazole and several positive and negative reference chemicals. hTPO inhibition was observed for 190 chemicals reported as positive in rTPO. Of these, 158 showed no confounding activity (interference due to fluorescence or non-specific protein inhibition). Comparison of all result with rTPO data and with evidence of TPO inhibition found in the literature suggest that the current assay has a higher rate of false negative but a much lower rate of false positive compared with the rTPO screen. These findings underscore the effectiveness of the AUR assay, using hTPO enzyme from engineered cell lines, to identify moderate to strong inhibitors but some improvements may be needed to detect weak TPO inhibitors.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny-part III: how is substance-mediated thyroid hormone imbalance in pregnant/lactating rats or their progeny related to neurodevelopmental effects?

This review investigated which patterns of thyroid- and brain-related effects are seen in rats upon gestational/lactational exposure to 14 substances causing thyroid hormone imbalance by four different modes-of-action (inhibition of thyroid peroxidase, sodium-iodide symporter and deiodinase activities, enhancement of thyroid hormone clearance) or to dietary iodine deficiency. Brain-related parameters included motor activity, cognitive function, acoustic startle response, hearing function, periventricular heterotopia, electrophysiology and brain gene expression. Specific modes-of-action were not related to specific patterns of brain-related effects. Based upon the rat data reviewed, maternal serum thyroid hormone levels do not show a causal relationship with statistically significant neurodevelopmental effects. Offspring serum thyroxine together with offspring serum triiodothyronine and thyroid stimulating hormone appear relevant to predict the likelihood for neurodevelopmental effects. Based upon the collated database, thresholds of ≥60%/≥50% offspring serum thyroxine reduction and ≥20% and statistically significant offspring serum triiodothyronine reduction indicate an increased likelihood for statistically significant neurodevelopmental effects; accuracies: 83% and 67% when excluding electrophysiology (and gene expression). Measurements of brain thyroid hormone levels are likely relevant, too. The extent of substance-mediated thyroid hormone imbalance appears more important than substance mode-of-action to predict neurodevelopmental impairment in rats. Pertinent research needs were identified, e.g. to determine whether the phenomenological offspring thyroid hormone thresholds are relevant for regulatory toxicity testing. The insight from this review shall be used to suggest a tiered testing strategy to determine whether gestational/lactational substance exposure may elicit thyroid hormone imbalance and potentially also neurodevelopmental effects.

Analytical and toxicological aspects of dithiocarbamates: an overview of the last 10 years

Compilation studies related to toxicological aspects and also biological monitoring and analysis methods for specific fungicides and, mainly, those that belong to the class of the dithiocarbamates (DTCs) have not been carried out at least in the last ten years. DTCs - dimethyldithiocarbamates, ethylenebisditiocarbamates, propylenebisditiocarbamates - are organosulfur compounds that form complexes due to the presence of different chemical elements, which bind strongly and inhibit enzymes that are essential to the functioning of the organism, causing a serious proven adverse effect on biological systems, such as alteration of thyroid hormones, teratogenesis and neurotoxicity. It is still evident, as shown by world data, that the growing consumption of fungicides has increasingly exposed the population in general and, in particular, workers who deal with these substances. There is a scarcity of studies in the literature discussing the toxicological and analytical aspects that are important for understanding the real effects of DTCs and monitoring human exposure to them. Therefore, the aim of this work was to expose, in a comprehensive way and through a narrative review, the negligence of research related to the fungicides of the DTCs class, their metabolites, as well as the toxicological and analytical aspects involved. The review is divided into two parts: (1) Toxicological aspects, including toxicokinetics, toxicodynamics and toxidromes; and (2) Analytical Toxicology, which comprises biomarkers, sample preparation and identification/quantification methods.

Exposure to antithyroid drugs and ethylenethiourea and risk of thyroid cancer: a systematic review of the epidemiologic evidence

INTRODUCTION

The thyroid peroxidase inhibiting compounds methimazole, methylthiouracil, propylthiouracil, thiouracil (i.e. 'antithyroid' drugs) and ethylenethiourea have been associated to thyroid tumours in rodents. According to a systematic review by the International Agency for Research on Cancer (IARC) published in 2000, evidence for the human carcinogenicity was inadequate.

METHODS

We performed an up-to-date systematic review of human epidemiological studies on the association between such compounds and thyroid cancer incidence or mortality.

RESULTS

The literature research (1999-March 2020) identified four relevant articles. Considering also reports from the previous IARC review, this systematic review considered seven reports (five distinct studies) on antithyroid drugs and two on ethylenethiourea. As for antithyroid drugs, three reports based on different follow-ups gave results from a cohort of patients treated for hyperthyroidism in 1946-1964. In the earlier report, thyroid cancer incidence was higher in patients primarily treated with antithyroid drugs (3.2/1000) than in those originally treated with thyroidectomy (0.34/1000) or radioactive iodine (0.88/1000), which can be explained by the higher frequency of subsequent thyroidectomy, and hence the higher chance of cancer detection, in that group (30 vs. 0.5 and 1.2%). The two subsequent reports found no deaths from thyroid cancer among patients treated exclusively with antithyroid drugs through 1990 and 2014. A nested case-control study found an odds ratio (OR) of thyroid cancer of 2.79 [95% confidence interval (CI), 0.78-10.02, from a 2-year lag analysis] for ≥3 vs. no propylthiouracil prescriptions. The increased risk can be attributed to advanced diagnosis of an underlying cancer, as suggested by the stronger association observed in a no-lag analysis (OR, 8.03). In a historical cohort of newly diagnosed hyperthyroid patients, the hazard ratio for treatment with radioactive iodine vs. thionamides only was 0.45 (95% CI, 0.21-0.99), possibly due to the closer surveillance of patients receiving thionamides only. Two case-control studies did not find any association with the use of antithyroid drugs. As for ethylenethiourea, no thyroid cancer cases were found in a historical cohort of 1929 workers occupationally exposed in a 15-year period and no association with proxies of mancozeb exposure (a fungicide whose main metabolite is ethylenethiourea) was detected in a cohort of >236 000 farmers.

CONCLUSION

There is no evidence for a relevant role of either antithyroid drugs or ethylenethiourea on thyroid cancer.

Adversity Considerations for Thyroid Follicular Cell Hypertrophy and Hyperplasia in Nonclinical Toxicity Studies: Results From the 6th ESTP International Expert Workshop

The European Society of Toxicologic Pathology organized an expert workshop in May 2018 to address adversity considerations related to thyroid follicular cell hypertrophy and/or hyperplasia (FCHH), which is a common finding in nonclinical toxicity studies that can have important implications for risk assessment of pharmaceuticals, food additives, and environmental chemicals. The broad goal of the workshop was to facilitate better alignment in toxicologic pathology and regulatory sciences on how to determine adversity of FCHH. Key objectives were to describe common mechanisms leading to thyroid FCHH and potential functional consequences; provide working criteria to assess adversity of FCHH in context of associated findings; and describe additional methods and experimental data that may influence adversity determinations. The workshop panel was comprised of representatives from the European Union, Japan, and the United States. Participants shared case examples illustrating issues related to adversity assessments of thyroid changes. Provided here are summary discussions, key case presentations, and panel recommendations. This information should increase consistency in the interpretation of adverse changes in the thyroid based on pathology findings in nonclinical toxicity studies, help integrate new types of biomarker data into the review process, and facilitate a more systematic approach to communicating adversity determinations in toxicology reports.

Toxicogenomic fin(ger)prints for thyroid disruption AOP refinement and biomarker identification in zebrafish embryos

Endocrine disruption (ED) can trigger far-reaching effects on environmental populations, justifying a refusal of market approval for chemicals with ED properties. For the hazard assessment of ED effects on the thyroid system, regulatory decisions mostly rely on amphibian studies. Here, we used transcriptomics and proteomics for identifying molecular signatures of interference with thyroid hormone signaling preceding physiological effects in zebrafish embryos. For this, we analyzed the thyroid hormone 3,3',5-triiodothyronine (T3) and the thyroid peroxidase inhibitor 6-propyl-2-thiouracil (6-PTU) as model substances for increased and repressed thyroid hormone signaling in a modified zebrafish embryo toxicity test. We identified consistent gene expression fingerprints for both modes-of-action (MoA) at sublethal test concentrations. T3 and 6-PTU both significantly target the expression of genes involved in muscle contraction and functioning in an opposing fashion, allowing for a mechanistic refinement of key event relationships in thyroid-related adverse outcome pathways in fish. Furthermore, our fingerprints identify biomarker candidates for thyroid disruption hazard screening approaches. Perspectively, our findings will promote the AOP-based development of in vitro assays for thyroidal ED assessment, which in the long term will contribute to a reduction of regulatory animal tests.

In vitro assays for characterization of distinct multiple catalytic activities of thyroid peroxidase using LC-MS/MS

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Multiple reactions catalyzed by thyroid peroxidase (TPO) were monitored by a battery of unique in vitro assays.

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Monoiodination and diiodination of L-Tyr to MIT and DIT was examined in a single assay.

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MIT to DIT and T3 to T4 monoiodination reactions were monitored separately.

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DIT to T4 conversion assay was used to study coupling of iodotyrosine phenolic rings.

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Distinct Km, Vmax, Kcat and Kcat/ Km values for each of the TPO catalysed reaction are presented.

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Differential response of 5 known inhibitors with specific TPO reactions was studied.

A diverse set of environmental contaminants have raised a concern about their potential adverse effects on endocrine signaling. Robust and widely accepted battery of in vitro assays is available to assess the disruption of androgenic and estrogenic pathways. However, such definitive systems to investigate effects on the disruption of thyroid pathways by the xenobiotics are not yet well established. One of the major “Molecular Initiating Events” (MIEs) in thyroid disruption involves targeting of thyroid peroxidase (TPO), a key enzyme involved in thyroid hormone synthesis. TPO catalyzes mono- and diiodination of L-Tyrosine (L-Tyr) to generate 3-Iodo-l-tyrosine (MIT) and 3,5-Diiodo-l-tyrosine (DIT), respectively, followed by the coupling of iodinated tyrosine rings to generate thyroid hormones, 3,3’5-Triiodo-l-thyronine (T3) and Levothyroxine (T4).

We sought to develop a robust, sensitive, and rapid in vitro assay systems to evaluate the effects of test chemicals on the multiple catalytic activities of thyroid peroxidase. Simple in vitro assays were designed to study TPO mediated distinct reactions using a single LC-MS/MS method.

Herein, we describe a battery of assays to investigate the iodination of L-Tyr to MIT and DIT, MIT to DIT as well as, T3 to T4 catalyzed by rat thyroid TPO. Importantly, two sequential reactions involving mono- and diiodination of L-Tyr could be analyzed in a single assay. The assay that monitors in vitro conversion of DIT to T4 was developed to study the coupling of tyrosine rings. Enzyme kinetics studies revealed distinct characteristics of multiple reactions catalyzed by TPO. Further, the known TPO inhibitors were used to assess their potency towards individual TPO substrates and reactions. The resultant half maximum inhibitory concentration (IC₅₀) values highlighted differential targeting of TPO catalyzed reactions by the same inhibitor. Overall results underscore the need to develop more nuanced approaches that account for distinct multiple catalytic activities of TPO.

An assay for screening xenobiotics for inhibition of rat thyroid gland peroxidase activity

1. A number of chemicals have been shown to produce disruption of the thyroid gland, resulting in reduced thyroid hormone synthesis, by a mechanism involving inhibition of thyroid peroxidase (TPO) activity (EC 1.11.1.8).2. An assay was developed for rat thyroid gland microsomal TPO activity, employing L-tyrosine as the physiological substrate, with analysis of the formation of the 3-iodo-L-tyrosine (3MIT) metabolite by ultra-performance liquid chromatography-mass spectrometry-mass spectrometry.3. Formation of 3MIT was linear with respect to both rat thyroid gland microsomal protein concentration and incubation time, whereas only small quantities of 3,5-diodo-L-tyrosine were formed.4. Studies were performed with nine known TPO inhibitors. The most potent inhibitors were 3-amino-1,2,4-triazole, ethylene thiourea, methimazole and 6-propyl-2-thiouracil which had IC₅₀ values (i.e. concentration to produce a 50% inhibition of enzyme activity) of 0.059, 0.791, 1.07 and 1.96 μM, respectively, whereas the least potent inhibitor was sodium perchlorate which had an IC₅₀ value of 13,800 µM.5. For five inhibitors, where literature data were available, the observed IC₅₀ values obtained in this study employing rat thyroid gland microsomes and L-tyrosine as substrate were similar to those previously reported using the spectrophotometric guaiacol oxidation assay.

An AOP-based alternative testing strategy to predict the impact of thyroid hormone disruption on swim bladder inflation in zebrafish

The adverse outcome pathway (AOP) framework can be used to help support the development of alternative testing strategies aimed at predicting adverse outcomes caused by triggering specific toxicity pathways. In this paper, we present a case-study demonstrating the selection of alternative in chemico assays targeting the molecular initiating events of established AOPs, and evaluate use of the resulting data to predict higher level biological endpoints. Based on two AOPs linking inhibition of the deiodinase (DIO) enzymes to impaired posterior swim bladder inflation in fish, we used in chemico enzyme inhibition assays to measure the molecular initiating events for an array of 51 chemicals. Zebrafish embryos were then exposed to 14 compounds with different measured inhibition potentials. Effects on posterior swim bladder inflation, predicted based on the information captured by the AOPs, were evaluated. By linking the two datasets and setting thresholds, we were able to demonstrate that the in chemico dataset can be used to predict biological effects on posterior chamber inflation, with only two outliers out of the 14 tested compounds. Our results show how information organized using the AOP framework can be employed to develop or select alternative assays, and successfully forecast downstream key events along the AOP. In general, such in chemico assays could serve as a first-tier high-throughput system to screen and prioritize chemicals for subsequent acute and chronic fish testing, potentially reducing the need for long-term and costly toxicity tests requiring large numbers of animals.

A simple short term method to study thyroid disruption using a fetal rat thyroid culture

INTRODUCTION

Thyroid modulation activity has not been investigated for many chemical substances. Due to ethical, practical and financial reasons, in vivo evaluation of a large number of compounds is not feasible. It has been proposed that an in vitro mechanism-based strategy could be more adequate for the identification of thyroid hormone disrupting chemicals. Here we describe a simple and mostly inexpensive, short term culture assay to study thyroid disruption.

METHODS

Fetal thyroids collected from gestation day 20.5 were cultured up to 24h in Hank's saline solution, at 37°C with oxygenation at 0 and 12h. Viability of the cultured explants was evaluated by the MTT assay. Positive (thyroid stimulating hormone, TSH) and negative (6-propyl-2-thiouracil, PTU) modulation of cultured thyroids was assessed with morphometrical analysis of H & E stained gland sections. Thyroxine expression was evaluated by immunohistochemistry.

RESULTS

Viability was shown to increase with time of culture with higher metabolic activity being achieved at 24h as compared to shorter periods of incubation. Follicular epithelial cells exhibited a statistically significant dependence on thyrotropin concentration, although more evident in the inner than in the outer portion of the glands. As expected, TSH induced expression of thyroxin while PTU inhibited it.

DISCUSSION

GD20.5 fetal thyroids may be cultured up to 24h under relatively simple laboratory conditions during which viability and function of the gland are preserved showing that it is possible to reproduce in vivo response under in vitro conditions. This culture could be a suitable short term assay to study mechanism of thyroid disruption.

Iodine nutritional status and thyroid effects of exposure to ethylenebisdithiocarbamates

INTRODUCTION

Italy is still characterized by a mild iodine deficiency and is among the most intensive users of chemical products for agriculture in Europe. The aim of this study was i) to evaluate thyroid effects of exposure to mancozeb, a fungicide widely used in agriculture, in a sample of Italian grapevine workers, and ii) to verify whether the iodine intake may modulate the risk of thyroid disruption due to the mancozeb metabolite ethylenthiourea (ETU).

METHODS

One hundred seventy-seven occupationally exposed male workers (29 from Chianti, a mild iodine deficient area, and 148 from Bolzano an iodine sufficient province) and 74 non-occupationally exposed male controls (34 from Chianti and 40 from Bolzano) were enrolled in the study. Serum biomarkers of thyroid function, as well as urinary iodine and ETU concentrations were assessed. Moreover all the recruited subjects underwent clinical examination and thyroid ultrasound.

RESULTS

Multivariate comparisons showed lower mean serum levels of FT4 in Chianti-workers as compared to Bolzano-workers. Moreover, an increased urinary iodine excretion (>250µg/L) was more frequently found among more exposed workers (ETU>20µg/L) than among less exposed ones and this effect was more pronounced in Chianti- than in Bolzano-workers. Chianti-workers also showed a significantly higher frequency of very low thyroid volume (≤6.0ml) as compared to controls.

CONCLUSIONS

These findings showed a mild thyroid disrupting effect due to occupational exposure to mancozeb, more pronounced in workers residing in an area characterized by a mild to moderate iodine deficiency as compared to workers residing in an area covered by a long-lasting iodine prophylaxis program.

Food components and contaminants as (anti)androgenic molecules

Androgens, the main male sex steroids, are the critical factors responsible for the development of the male phenotype during embryogenesis and for the achievement of sexual maturation and puberty. In adulthood, androgens remain essential for the maintenance of male reproductive function and behavior. Androgens, acting through the androgen receptor (AR), regulate male sexual differentiation during development, sperm production beginning from puberty, and maintenance of prostate homeostasis. Several substances present in the environment, now classified as endocrine disruptors (EDCs), strongly interfere with androgen actions in reproductive and non-reproductive tissues. EDCs are a heterogeneous group of xenobiotics which include synthetic chemicals used as industrial solvents/lubricants, plasticizers, additives, agrochemicals, pharmaceutical agents, and polyphenols of plant origin. These compounds are even present in the food as components (polyphenols) or food/water contaminants (pesticides, plasticizers used as food packaging) rendering the diet as the main route of exposure to EDCs for humans. Although huge amount of literature reports the (anti)estrogenic effects of different EDCs, relatively scarce information is available on the (anti)androgenic effects of EDCs. Here, the effects and mechanism of action of phytochemicals and pesticides and plasticizers as possible modulators of AR activities will be reviewed taking into account that insight derived from principles of endocrinology are required to estimate EDC consequences on endocrine deregulation and disease.

Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole part I: Fathead minnow

In the present study, a hypothesized adverse outcome pathway linking inhibition of thyroid peroxidase (TPO) activity to impaired swim bladder inflation was investigated in two experiments in which fathead minnows (Pimephales promelas) were exposed to 2-mercaptobenzothiazole (MBT). Continuous exposure to 1mg MBT/L for up to 22 days had no effect on inflation of the posterior chamber of the swim bladder, which typically inflates around 6 days post fertilization (dpf), a period during which maternally-derived thyroid hormone is presumed to be present. In contrast, inflation of the anterior swim bladder, which occurs around 14dpf, was impacted. Specifically, at 14dpf, approximately 50% of fish exposed to 1mg MBT/L did not have an inflated anterior swim bladder. In fish exposed to MBT through 21 or 22dpf, the anterior swim bladder was able to inflate, but the ratio of the anterior/posterior chamber length was significantly reduced compared to controls. Both abundance of thyroid peroxidase mRNA and thyroid follicle histology suggest that fathead minnows mounted a compensatory response to the presumed inhibition of TPO activity by MBT. Time-course characterization showed that fish exposed to MBT for at least 4 days prior to normal anterior swim bladder inflation had significant reductions in anterior swim bladder size, relative to the posterior chamber, compared to controls. These results, along with similar results observed in zebrafish (see part II, this issue) are consistent with the hypothesis that thyroid hormone signaling plays a significant role in mediating anterior swim bladder inflation and development in cyprinids, and that role can be disrupted by exposure to thyroid hormone synthesis inhibitors. Nonetheless, possible thyroid-independent actions of MBT on anterior swim bladder inflation cannot be ruled out based on the present results. Overall, although anterior swim bladder inflation has not been directly linked to survival as posterior swim bladder inflation has, potential links to adverse ecological outcomes are plausible given involvement of the anterior chamber in sound production and detection.

Effect of triclosan, triclocarban, 2,2',4,4'-tetrabromodiphenyl ether, and bisphenol A on the iodide uptake, thyroid peroxidase activity, and expression of genes involved in thyroid hormone synthesis

Triclosan, triclocarban, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), and bisphenol A (BPA) have been reported to disturb thyroid hormone (TH) homeostasis. We have examined the effects of these chemicals on sodium/iodide symporter (NIS)-mediated iodide uptake and the expression of genes involved in TH synthesis in rat thyroid follicular FRTL-5 cells, and on the activity of thyroid peroxidase (TPO) using rat thyroid microsomes. All four chemicals inhibited NIS-mediated iodide uptake in a concentration-dependent manner. A decrease in the iodide uptake was also observed in the absence of sodium iodide. Kinetic studies showed that all four chemicals were non-competitive inhibitors of NIS, with the order of Ki values being triclosan<triclocarban<BDE-47<BPA. The transcriptional expression of three genes involved in TH synthesis, Slc5a5, Tpo, and Tgo, and three thyroid transcription factor genes, Pax8, Foxe1, and Nkx2-1, was examined using quantitative real-time PCR. No significant changes in the expression of any genes were observed with triclosan or triclocarban. BDE-47 decreased the level of Tpo, while BPA altered the expression of all six genes. Triclosan and triclocarban inhibited the activity of TPO at 166 and >300 μM, respectively. Neither BDE-47 nor BPA affected TPO activity. In conclusion, triclosan, triclocarban, BDE-47, and BPA inhibited iodide uptake, but had differential effects on the expression of TH synthesis-related genes and the activity of TPO.

Evaluation of the ToxRTool's ability to rate the reliability of toxicological data for human health hazard assessments

Regulatory agencies often utilize results from peer reviewed publications for hazard assessments. A problem in doing so is the lack of well-accepted tools to objectively, efficiently and systematically assess the quality of published toxicological studies. Herein, we evaluated the publicly available software-based ToxRTool (Toxicological data Reliability assessment Tool) for use in human health hazard assessments. The ToxRTool was developed by the European Commission's Joint Research Center in 2009. It builds on Klimisch categories, a rating system established in 1997, by providing additional criteria and guidance for assessing the reliability of toxicological studies. It also transparently documents the study-selection process. Eight scientists used the ToxRTool to rate the same 20 journal articles on thyroid toxicants. Results were then compared using the Finn coefficient and "AC1" to determine inter-rater consistency. Ratings were most consistent for high-quality journal articles, but less consistent as study quality decreased. Primary reasons for inconsistencies were that some criteria were subjective and some were not clearly described. It was concluded, however, that the ToxRTool has potential and, with refinement, could provide a more objective approach for screening published toxicology studies for use in health risk evaluations, although the ToxRTool ratings are primarily based on study reporting quality.

Dansyl-carbazole AIEE material for selective recognition of thiourea derivatives

New dansyl-carbazole based compound 3 is designed and synthesized, which exhibits aggregation induced emission enhancement (AIEE) behaviour in H₂O : THF (60 : 40, v/v). Aggregates of compound 3 in H₂O : THF (60 : 40, v/v) selectively recognize thiourea motifs over urea motifs.

Combined repeated-dose and reproductive/developmental toxicity screening test of 3-amino-5-mercapto-1,2,4-triazole in rats

The substance 3-amino-5-mercapto-1,2,4-triazole (AMT, CAS No. 16691-43-3) was daily administered by gavage to Crl:CD (SD)IGS rats at doses of 0 (control), 10, 50, and 250 mg/kg bw/day. Males (12/group) were treated for a total of 42 days beginning 14 days before mating. Females (12/group) were treated beginning 14 days before mating to day 4 of lactation throughout the mating and gestation periods. No deaths occurred in males but three females died on day 23 of gestation at 250 mg/kg/day. Only temporary decreases in body weight and food intake were found in both sexes at 250 mg/kg/day. There were no considerable changes in general appearance, the functional battery tests, biochemical analysis or urinalysis. Anemia was observed in both sexes at 250 mg/kg/day. The relative weight of thyroid glands was significantly increased in both sexes at 250 mg/kg/day and hypertrophy of thyroid follicular cells was observed in 50 and 250 mg/kg/day males and 250 mg/kg/day females. As this effect on thyroid glands was considered to be the major toxicity, the possible mechanism was discussed comparing with the toxicity of structural similar analogs. Other histopathological changes in males were hypertrophy of centrilobular hepatocytes at 250 mg/kg/day, and anterior pituitary glands at 50 mg/kg/day and more. Vacuolization in renal tubular epithelium of females was observed at 50 and 250 mg/kg/day. For reproduction, the gestation period was prolonged and the delivery index was decreased at 250 mg/kg/day. The number of pups born and the birth index were also reduced. It was thus concluded that the NOAEL for repeated-dose toxicity was 10 mg/kg/day based on the thyrotoxicity and renal toxicity, and that the NOAEL for reproductive/developmental toxicity was 50 mg/kg/day based on the reduced number of offspring, etc.

New parameter that supports speculation on the possible mechanism of hypothyroidism induced by chemical substances in repeated-dose toxicity studies

Hypothyroidism induced by xenobiotic treatment was analyzed for possible underlying mechanism(s) on the basis of different responses of the thyroid gland and the liver, using a newly-created database of repeated-dose toxicity of 500 chemicals. Two mechanisms are proposed: direct inhibition of thyroid hormone biosynthesis in the thyroid gland, and stimulated degradation of thyroid hormone by induction of hepatic drug-metabolizing enzymes. In the database there were 10 chemicals inducing hypertrophy/hyperplasia of follicular cells in the thyroid gland and having data on thyroid glands. On the basis of the chemical structure and information available in the literature, we judged three chemicals to be typical thioamide derivatives that act directly on the thyroid gland, and the others as non-thioamide derivatives that were unlikely to have any direct action on the thyroid gland. All these chemicals were classified into two groups using the ratios of relative weight increase rate of thyroid gland versus that of the liver. These values were at least 1.7, but 3.2 or more in the most of the cases for thioamide derivatives, and 1.2 or less for non-thioamide derivatives. This background analysis suggests the feasibility of parameter-supported speculation on the possible underlying mechanism when new repeated-dose toxicity data on hypothyroidism becomes available.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.