Thyroid disrupting chemicals and developmental neurotoxicity - New tools and approaches to evaluate hormone action

Mild Gestational Hypothyroidism in Mice Has Transient Developmental Effects and Long-Term Consequences on Neuroendocrine Systems

Background: Thyroid hormones (TH) play a key role in fetal brain development. While severe thyroid dysfunction, has been shown to cause neurodevelopmental and reproductive disorders, the rising levels of TH-disruptors in the environment in the past few decades have increased the need to assess effects of subclinical (mild) TH insufficiency during gestation. Since embryos do not produce their own TH before mid-gestation, early development processes rely on maternal production. Notably, the reproductive network governed by gonadotropin-releasing hormone (GnRH) neurons develops during this critical period. Methods: The risk of mild maternal hypothyroidism on the development of GnRH neurons and long-term effect on neuroendocrine function in the offspring was investigated using a mouse model of gestational hypothyroidism induced by methimazole (MMI) treatment. Results: MMI treatment during gestation led to reduced litter size, consistent with increased miscarriages due to hypothyroidism. E12/13 embryos, collected from MMI-treated dams, had a decreased number of GnRH neurons, but the migration of the remaining GnRH neurons was normal. Cell proliferation was reduced in the vomeronasal organ (VNO), correlating with the reduced number of GnRH neurons detected in this region. Using a GnRH cell line confirmed attenuated proliferation in the absence of T3. Pups born from hypothyroid mothers had normal postweaning growth and estrus cycles, yet adult offspring had significantly more cells expressing estrogen receptor alpha in the arcuate nucleus. Notably, by adulthood, GnRH cell number and distribution was comparable with nontreated controls indicating that compensatory mechanisms occurred after E13. Conclusion: Overall, our work shows that mild TH disruption during gestation transiently affects proliferation of the pool of GnRH neurons within the VNO and has a long-term impact on neuroendocrine systems.

Risks to human and animal health from the presence of bromide in food and feed

The European Commission mandated EFSA to assess the toxicity of bromide, the existing maximum residue levels (MRLs), and possible transfer from feed into food of animal origin. The critical effects of bromide in experimental animals are on the thyroid and central nervous system. Changes in thyroid hormone homeostasis could result in neurodevelopmental toxicity, among other adverse effects. Changes in thyroid hormone concentrations and neurophysiological parameters have also been observed in experimental human studies, but the evidence was limited. Dose-response modelling of decreased blood thyroxine concentrations in rats resulted in a reference point of 40 mg/kg body weight (bw) per day. The Scientific Committee established a tolerable daily intake (TDI) of 0.4 mg/kg bw per day and an acute reference dose (ARfD) of 0.4 mg/kg bw per day to protect against adverse neurodevelopmental effects. The TDI value is supported by the results of experimental human studies with a NOAEL of 4 mg/kg bw per day and 10-fold interindividual variability. The TDI and ARfD are considered as conservative with 90% certainty. Insufficient evidence related to the toxicological effects of bromide was available for animals, with the exception of dogs. Therefore, the reference point of 40 mg/kg bw per day was extrapolated to maximum safe concentrations of bromide in complete feed for other animal species. Bromide can transfer from feed to food of animal origin, but, from the limited data, it was not possible to quantify the transfer rate. Monitoring data exceeded the current MRLs for some food commodities, generally with a low frequency. A conservative safety screening of the MRLs indicated that the TDI and ARfD are exceeded for some EU diets. Dietary exposure assessment for animals was not feasible due to insufficient data. The Scientific Committee recommends data be generated to allow robust dietary exposure assessments in the future, and data that support the risk assessment.

Pyriproxyfen, villain or good guy? A brief review

Pyriproxyfen (PPF) acts as a juvenile growth regulator, interfering with normal metamorphosis and blocking the development of insects into adulthood. Although the World Health Organization (WHO) considers the use of PPF at a concentration of 0.01 mg/L as unlikely to pose health risks, recent studies have unveiled potential risks associated with PPF exposure to non-target organisms. Exposure to PPF disrupts insect development primarily by mimicking juvenile hormones; therefore, concerns linger over its impact on unintended species. Studies have highlighted the adverse effects of PPF on aquatic invertebrates, fish, and amphibians and revealed mortality and developmental abnormalities in non-target mosquito species exposed to PPF-treated water. Moreover, PPF may act as an endocrine disruptor, interfering with hormonal pathways crucial for growth, reproduction, and behavior in exposed organisms. Amphibians, for instance, display altered reproductive physiology and developmental abnormalities due to disruptions in endocrine signaling pathways caused by PPF. The ecological ramifications of PPF extend beyond direct toxicity to non-target species. Indirect effects include shifts in food web dynamics and ecosystem functioning. Reductions in insect populations, induced by PPF, can disrupt food availability for higher trophic levels, potentially destabilizing community structure and ecosystem equilibrium. Given mounting evidence of unintended consequences, robust risk assessment and regulatory oversight are imperative. Accurate classification of PPF by regulatory bodies is essential to balancing its role in disease control and pest management benefits with the need to safeguard non-target species and maintain ecosystem health. Future research must prioritize comprehensive assessments of PPF's ecological impact across various habitats and taxa to inform evidence-based policymaking.

Optimal testing time for cerebral heterotopia formation in the rat comparative thyroid assay, a downstream indicator for perinatal thyroid hormone insufficiency

In a past study, we proposed a modified Comparative Thyroid Assay (CTA) with additional examinations of brain thyroid hormone (TH) concentrations and brain histopathology but with smaller group sizes. The results showed that the modified CTA in Sprague Dawley rats detected 10 ppm 6-propylthiouracil (6-PTU)-induced significant suppressions of serum/brain TH concentrations in offspring. To confirm the reliability of qualitative brain histopathology and identify the optimal testing time for heterotopia (a cluster of ectopic neurons) in the modified CTA, brain histopathology together with serum/brain TH concentrations were assessed in GD20 fetuses and PND2, 4, 21, and 28 pups using a similar study protocol but with a smaller number of animals (N=3-6/group/time). Significant hypothyroidism was observed and brain histopathology revealed cerebral heterotopia formation in PND21 and PND28 pups, with likely precursor findings in PND2 and PND4 pups but not in GD20 fetuses. This study confirmed that the optimal testing time for cerebral heterotopia in rat CTA was PND21 and thereafter. These findings suggest that cerebral heterotopia assessment at appropriate times may be a useful alternative to the original CTA design.

The pollutant perfluorohexane sulfonate (PFHxS) reduces serum thyroxine but does not alter thyroid action in the postnatal rat brain

Known as "forever chemicals", per- and polyfluoroalkyl substances (PFAS) are synthetic compounds used in consumer goods but pose significant public health concerns, including disruption of the thyroid system. As thyroid hormones (THs) are required for normal brain development, PFAS may also be developmental neurotoxicants. However, this is not well understood. Here we examine the endocrine and neurodevelopmental consequences of perfluorohexane sulfonate (PFHxS) exposure in pregnant, lactating, and developing rats, and compare its effects to an anti-thyroid pharmaceutical (propylthiouracil, PTU) that induces thyroid-mediated developmental neurotoxicity. We show that PFHxS dramatically reduces maternal serum thyroxine (T4), nearly equivalently to PTU (-55 and -51%, respectively). However, only PTU increases thyroid stimulating hormone. The lactational transfer of PFHxS is significant and reduces pup serum T4 across the postnatal period. Surprisingly, brain THs are only minimally decreased by PFHxS, whereas PTU drastically diminishes them. Evaluation of brain TH action by phenotyping, RNA-Sequencing, and quantification of radial glia cell morphology supports that PTU interrupts TH signaling while PFHxS has limited to no effect. These data show that PFHxS induces abnormal serum TH profiles; however, there were no indications of hypothyroidism in the postnatal brain. We suggest the stark differences between the neurodevelopmental effects of PFHxS and a typical antithyroid agent may be due to its interaction with TH distributing proteins like transthyretin.

Tetrabromobisphenol A exerts thyroid disrupting effects but has little overt impact on postnatal brain development and neurobehaviors in mice

Tetrabromobisphenol A (TBBPA) is a widely used brominated flame retardant. There is evidence showing that TBBPA can exert thyroid disrupting effects in mammals, but different results were also reported, along with inconsistent reports regarding its neurotoxicity. Here, we investigated thyroid disrupting effects and neurotoxicity of TBBPA (5, 50, 500 µg/(kg·day)) to male mice following maternal and direct exposure through drinking water, with the anti-thyroid drug propylthiouracil (PTU) as the positive control. On postnatal day (PND) 15, we expectedly observed severe thyroid compensatory hyperplasia and cerebellar developmental retardation in PTU-treated pups. The highest dose of TBBPA also caused thyroid histological alteration but had no effects on cerebellar development in terms of Purkinje cell morphology and the thickness of the internal granular layer and the molecular layer of the cerebellum. During puberty and adulthood, the thyroid morphological alterations became more pronounced in the TBBPA-treated animals, accompanied by decreased serum thyroid hormone levels. Furthermore, the 50 and 500 µg/(kg·day) TBBPA groups showed a significant decrease in the serum level of serotonin, a neurotransmitter associated with anxiety behaviors. Correspondingly, the highest dose group displayed anxiety-like behaviors in the elevated plus-maze test on PND 35, but this neurobehavioral alteration disappeared on PND 56. Moreover, no changes in neurobehavioral parameters tested were found in TBBPA-treated animals at puberty and adulthood. Altogether, all observations show that TBBPA can exert thyroid disrupting effects but has little overt impact on brain development and neurobehaviors in mice, suggesting that thyroid disruption does not necessarily cause overtly adverse neurodevelopmental outcomes.

Prenatal test cohort of a modified rat comparative thyroid assay adding brain thyroid hormone measurements and histology but lowering group size appears able to detect disruption by sodium phenobarbital

The Comparative Thyroid Assay (CTA, USEPA) is a screening test for thyroid hormone (TH) disruption in peripheral blood of dams and offspring. Recently, we began investigating feasible improvements to the CTA by adding examination of offspring brain TH concentrations and brain histopathology. In addition, we hypothesize that the number of animals required could be reduced by 50 % while still maintaining sensitivity to characterize treatment related changes in THs. Previously, we showed that the prenatal test cohort of the modified CTA could detect 1000 ppm sodium phenobarbital (NaPB)-induced suppression of brain T3 (by 9 %) and T4 (by 33 %) with no significant changes in serum T3 and T4 (less than 8 %). In the current study we expanded the dose response in a prenatal test cohort. Pregnant SD rats (N = 10/group) were exposed to 0, 1000 or 1500 ppm NaPB in the diet from gestational days (GD) 6 to GD20. Serum THs concentrations in GD20 dams together with serum/brain THs concentrations and brain histopathology in the GD20 fetuses were examined. NaPB dose-dependently suppressed serum T3 (up to -26 %) and T4 (up to -44 %) in dams, with suppression of T3 in serum (up to -26 %) and brain (up to -18 %) and T4 in serum (up to -26 %) and brain (up to -29 %) of fetuses but without clear dose dependency. There were no remarkable findings that deviated significantly from controls in GD20 fetal brain by qualitative histopathology. Overall, the present study suggests that the prenatal test cohort of this modified CTA is able to detect the expected fetal TH disruptions by prenatal exposure to NaPB, while also reducing the number of animals used by 50 %, consistent with the results of our previous study. These findings add to the suggestion that lowering group sizes and adding endpoints may be a useful alternative to the original CTA design.

Perinatal maternal exposure to high-dose sodium phenobarbital in the modified Comparative Thyroid Assay: no significant reduction in thyroid hormones in pups despite notable effects in dams

We propose a modified Comparative Thyroid Assay (CTA, USEPA) utilizing a smaller number of Sprague-Dawley rats (N=10/group) that assesses brain thyroid hormone (TH) concentrations and periventricular heterotopia while maintaining assay sensitivity. Our recent findings demonstrated that a prenatal test cohort of the modified CTA detected a dose-dependent decrease in maternal serum T3 (up to -26%) and T4 (up to -44%) with sodium phenobarbital (NaPB) exposure at 1000 ppm and 1500 ppm, equivalent to intakes of 60 and 84 mg/kg/day, respectively. On gestation day (GD) 20, fetuses exhibited reduced serum (-26%) and brain (-29%) TH concentrations, although these reductions were not dose dependent. The present study expanded the treatment in a postnatal test cohort, with maternal exposure to NaPB (81-93 mg/kg/day) from GD6 to lactation day (LD) 21. We assessed serum and brain TH concentrations, and periventricular heterotopia in pups on postnatal days (PND) 4, 21, and 28. While LD21 dams showed significant reductions in serum T3 (up to -34%) and T4 (up to -54%), the pups did not exhibit significant TH suppression or periventricular heterotopia at any test point. Instead, a compensatory increase in T4 was observed in serum and brain of PND21 pups. The present study confirmed that perinatal maternal exposure to high doses of NaPB leads to a moderate decrease in maternal TH concentrations; however, the exposure of maternal rats to a similar dose of NaPB did not significantly reduce serum or brain TH concentrations in their postnatal offspring.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - Part IV: the ECETOC and CLE Proposal for a Thyroid Function-Related Neurodevelopmental Toxicity Testing and Assessment Scheme (Thyroid-NDT-TAS)

Following the European Commission Endocrine Disruptor Criteria, substances shall be considered as having endocrine disrupting properties if they (a) elicit adverse effects, (b) have endocrine activity, and (c) the two are linked by an endocrine mode-of-action (MoA) unless the MoA is not relevant for humans. A comprehensive, structured approach to assess whether substances meet the Endocrine Disruptor Criteria for the thyroid modality (EDC-T) is currently unavailable. Here, the European Centre for Ecotoxicology and Toxicology of Chemicals Thyroxine Task Force and CropLife Europe propose a Thyroid Function-Related Neurodevelopmental Toxicity Testing and Assessment Scheme (Thyroid-NDT-TAS). In Tier 0, before entering the Thyroid-NDT-TAS, all available in vivo, in vitro and in silico data are submitted to weight-of-evidence (WoE) evaluations to determine whether the substance of interest poses a concern for thyroid disruption. If so, Tier 1 of the Thyroid-NDT-TAS includes an initial MoA and human relevance assessment (structured by the key events of possibly relevant adverse outcome pathways) and the generation of supportive in vitro/in silico data, if relevant. Only if Tier 1 is inconclusive, Tier 2 involves higher-tier testing to generate further thyroid- and/or neurodevelopment-related data. Tier 3 includes the final MoA and human relevance assessment and an overarching WoE evaluation to draw a conclusion on whether, or not, the substance meets the EDC-T. The Thyroid-NDT-TAS is based on the state-of-the-science, and it has been developed to minimise animal testing. To make human safety assessments more accurate, it is recommended to apply the Thyroid-NDT-TAS during future regulatory assessments.

Thyroid hormone action controls multiple components of cell junctions at the ventricular zone in the newborn rat brain

Thyroid hormone (TH) action controls brain development in a spatiotemporal manner. Previously, we demonstrated that perinatal hypothyroidism led to formation of a periventricular heterotopia in developing rats. This heterotopia occurs in the posterior telencephalon, and its formation was preceded by loss of radial glia cell polarity. As radial glia mediate cell migration and originate in a progenitor cell niche called the ventricular zone (VZ), we hypothesized that TH action may control cell signaling in this region. Here we addressed this hypothesis by employing laser capture microdissection and RNA-Seq to evaluate the VZ during a known period of TH sensitivity. Pregnant rats were exposed to a low dose of propylthiouracil (PTU, 0.0003%) through the drinking water during pregnancy and lactation. Dam and pup THs were quantified postnatally and RNA-Seq of the VZ performed in neonates. The PTU exposure resulted in a modest increase in maternal thyroid stimulating hormone and reduced thyroxine (T4). Exposed neonates exhibited hypothyroidism and T4 and triiodothyronine (T3) were also reduced in the telencephalon. RNA-Seq identified 358 differentially expressed genes in microdissected VZ cells of hypothyroid neonates as compared to controls (q-values ≤0.05). Pathway analyses showed processes like maintenance of the extracellular matrix and cytoskeleton, cell adhesion, and cell migration were significantly affected by hypothyroidism. Immunofluorescence also demonstrated that collagen IV, F-actin, radial glia, and adhesion proteins were reduced in the VZ. Immunohistochemistry of integrin αvβ3 and isoforms of both thyroid receptors (TRα/TRβ) showed highly overlapping expression patterns, including enrichment in the VZ. Taken together, our results show that TH action targets multiple components of cell junctions in the VZ, and this may be mediated by both genomic and nongenomic mechanisms. Surprisingly, this work also suggests that the blood-brain and blood-cerebrospinal fluid barriers may also be affected in hypothyroid newborns.

Lissencephaly with Congenital Hypothyroidism: A Case Report

Lissencephaly is a malformation of cortical development associated with deficient neuronal migration and abnormal formation of cerebral convolutions or gyri. The lissencephaly spectrum consists of agyria, pachygyria, and subcortical band heterotopia. At least 19 genes have been identified in the causation of lissencephaly and related syndrome. Lissencephaly is associated with many other congenital disorders but the association of lissencephaly with congenital hypothyroidism is rarely reported. We report a case of a 10-year-old girl having lissencephaly with congenital hypothyroidism. Early diagnosis of lissencephaly and genetic counselling can be made in suspected cases and further possible interventions can be taken. Also, regular follow-up, monitoring, and better conservative management lead to a better prognosis.

Keywords

congenital abnormalities; hypothyroidism; lissencephaly; neuronal migration disorders.

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.

Gestational Exposure to Perchlorate in the Rat: Thyroid Hormones in Fetal Thyroid Gland, Serum, and Brain

Iodine is essential for the production of thyroid hormones. Perchlorate is an environmental contaminant that interferes with iodine uptake into the thyroid gland to reduce thyroid hormone synthesis. As thyroid hormones are critical for brain development, exposure to perchlorate during pregnancy is of concern for the developing fetal brain. In this study, we (1) define profiles of thyroid hormone in the maternal and fetal compartments of pregnant rats in response to inhibition of the sodium-iodide symporter (NIS) by perchlorate and (2) expand inquiry previously limited to serum to include fetal thyroid gland and brain. Perchlorate was added to the drinking water (0, 1, 30, 300, and 1000 ppm) of pregnant rat dams from gestational days (GD) 6-20. On GD20, blood, thyroid gland, and brain were collected from the fetus and dam for thyroid hormone and molecular analyses. Thyroid gland and serum thyroid hormones were dose-dependently reduced, with steeper declines evident in the fetus than in the dam. The thyroid gland revealed perturbations of thyroid hormone-action with greater sensitivity in the fetus than the dam. Thyroid hormones and thyroid hormone-responsive gene expression were reduced in the fetal cortex portending effects on brain development. These findings are the first quantitative assessments of perchlorate-induced deficits in the fetal thyroid gland and fetal brain. We provide a conceptual framework to develop a quantitative NIS adverse outcome pathway for serum thyroid hormone deficits and the potential to impact the fetal brain. Such a framework may also serve to facilitate the translation of in vitro bioactivity to the downstream in vivo consequences of NIS inhibition in the developing fetus.

Adverse effects of bisphenol B exposure on the thyroid and nervous system in early life stages of zebrafish

Bisphenol B (BPB), a widely used alternative of bisphenol A (BPA), has been detected in various environmental media and foodstuffs. However, the knowledge of the health risks about BPB is still limited. In this study, the effects of BPB on thyroid hormone homeostasis and neuronal development were evaluated by exposure of embryos 2 h post-fertilization (hpf) to BPB (0, 1, 10, 100 and 1000 μg/L) until 144 hpf. The results showed that 100 and 1000 μg/L BPB exposed larvae exhibited abnormal morphologies in phenotype and brain histological patterns. Significant decline of thyroid hormone thyroxine (T4) content and elevation of 3,5,3'-triiodothyronine (T3) content, along with the up-regulated expression of tg, trhr1, dio1, dio2, thrα, thrβ genes and down-regulated expression of tsh, ttr and trh genes in BPB exposed zebrafish larvae were observed. Moreover, locomotor activity of larvae was decreased, and the transcription of genes (e.g., elavl3, gap43, zn5, α-tubulin, syn2a and mbp) related to neuronal development were inhibited after exposure to BPB. The mechanism of neurotoxicity and thyroid disruption in zebrafish larvae induced by BPB were discussed.

Using High-Throughput Screening to Evaluate Perturbations Potentially Linked to Neurobehavioral Outcomes: A Case Study Using Publicly Available Tools on FDA Batch-Certified Synthetic Food Dyes

There is growing evidence from human and animal studies indicating an association between exposure to synthetic food dyes and adverse neurobehavioral outcomes in children. However, data gaps persist for potential mechanisms by which the synthetic food dyes could elicit neurobehavioral impacts. We developed an approach to evaluate seven US FDA-batch-certified food dyes using publicly available high-throughput screening (HTS) data from the US EPA's Toxicity Forecaster to assess potential underlying molecular mechanisms that may be linked to neurological pathway perturbations. The dyes were screened through 270 assays identified based on whether they had a neurological-related gene target and/or were mapped to neurodevelopmental processes or neurobehavioral outcomes, and were conducted in brain tissue, targeted specific hormone receptors, or targeted oxidative stress and inflammation. Some results provided support for neurological impacts found in human and animal studies, while other results showed a lack of correlation with in vivo findings. The azo dyes had a range of activity in assays mapped to G-protein-coupled receptors and were active in assays targeting dopaminergic, serotonergic, and opioid receptors. Assays mapped to nuclear receptors (androgen, estrogen, and thyroid hormone) also exhibited activity with the food dyes. Other molecular targets included the aryl hydrocarbon receptor, acetylcholinesterase, and monoamine oxidase. The Toxicological Prioritization Index tool was used to visualize the results of the Novascreen assays. Our results highlight certain limitations of HTS assays but provide insight into potential underlying mechanisms of neurobehavioral effects observed in in vivo animal toxicology studies and human clinical studies.

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

Perinatal exposure to endocrine disrupting chemicals and neurodevelopment: How articles of daily use influence the development of our children

Substances that interfere with the body's hormonal balance or their function are called endocrine disrupting chemicals (EDCs). Many EDCs are ubiquitous in the environment and are an unavoidable aspect of daily life, including during early embryogenesis. Developmental exposure to these chemicals is of critical relevance, as EDCs can permanently alter developmental programs, including those that pattern and wire the brain. Of emerging interest is how these chemicals may also affect the immune response, given the cross-talk between the endocrine and immune systems. As brain development is strongly dependent on hormones including thyroid, androgens, and estrogens, and can also be affected by immunomodulation, this complicated interplay may have long-lasting neurodevelopmental consequences. This review focuses on data available from human cohorts, in vivo models, and in vitro assays regarding the impact of EDCs after a gestational and/or lactational exposure, and how they may impact the immune system and/or neurodevelopment.

Thyroid Disruptors: Extrathyroidal Sites of Chemical Action and Neurodevelopmental Outcome-An Examination Using Triclosan and Perfluorohexane Sulfonate

Many xenobiotics are identified as potential thyroid disruptors due to their action to reduce circulating levels of thyroid hormone, most notably thyroxine (T4). Developmental neurotoxicity is a primary concern for thyroid disrupting chemicals yet correlating the impact of chemically induced changes in serum T4 to perturbed brain development remains elusive. A number of thyroid-specific neurodevelopmental assays have been proposed, based largely on the model thyroid hormone synthesis inhibitor propylthiouracil (PTU). This study examined whether thyroid disrupting chemicals acting distinct from synthesis inhibition would result in the same alterations in brain as expected with PTU. The perfluoroalkyl substance perfluorohexane sulfonate (50 mg/kg/day) and the antimicrobial Triclosan (300 mg/kg/day) were administered to pregnant rats from gestational day 6 to postnatal day (PN) 21, and a number of PTU-defined assays for neurotoxicity evaluated. Both chemicals reduced serum T4 but did not increase thyroid stimulating hormone. Both chemicals increased expression of hepatic metabolism genes, while thyroid hormone-responsive genes in the liver, thyroid gland, and brain were largely unchanged. Brain tissue T4 was reduced in newborns, but despite persistent T4 reductions in serum, had recovered in the PN6 pup brain. Neither treatment resulted in a low dose PTU-like phenotype in either brain morphology or neurobehavior, raising questions for the interpretation of serum biomarkers in regulatory toxicology. They further suggest that reliance on serum hormones as prescriptive of specific neurodevelopmental outcomes may be too simplistic and to understand thyroid-mediated neurotoxicity we must expand our thinking beyond that which follows thyroid hormone synthesis inhibition.

Maternal Perfluoroalkyl Substances, Thyroid Hormones, and DIO Genes: A Spanish Cross-sectional Study

Results of studies on perfluoroalkyl substances (PFASs) and thyroid hormones (THs) are heterogeneous, and the mechanisms underlying the action of PFASs to target THs have not been fully characterized. We examined the relation between first-trimester maternal PFAS and TH levels and the role played by polymorphisms in the iodothyronine deiodinase 1 (DIO1) and 2 (DIO2) genes in this association. Our sample comprised 919 pregnant Spanish women (recruitment = 2003-2008) with measurements of perfluorohexanesulfonic acid (PFHxS), perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), perfluorononanoic acid (PFNA), thyroid-stimulating hormone (TSH), total triiodothyronine (TT3), and free thyroxine (FT4), and we genotyped for single-nucleotide polymorphisms in the DIO1 (rs2235544) and DIO2 (rs12885300) genes. We performed multivariate regression analyses between PFASs and THs and included the interaction term PFAS-genotypes in the models. PFHxS was associated with an increase in TSH (% change in outcome [95% CI] per 2-fold PFAS increase = 6.09 [-0.71, 13.4]), and PFOA and PFNA were associated with a decrease in TT3 (-7.17 [-13.5, -0.39] and -6.28 [-12.3, 0.12], respectively). We found stronger associations between PFOA, PFNA, and TT3 for DIO1-CC and DIO2-CT genotypes, although interaction p-values were not significant. In conclusion, this study found evidence of an inverse association between PFOA and TT3 levels. No clear effect modification by DIO enzyme genes was observed.

Regulation of Thyroid-disrupting Chemicals to Protect the Developing Brain

Synthetic chemicals with endocrine disrupting properties are pervasive in the environment and are present in the bodies of humans and wildlife. As thyroid hormones (THs) control normal brain development, and maternal hypothyroxinemia is associated with neurological impairments in children, chemicals that interfere with TH signaling are of considerable concern for children's health. However, identifying thyroid-disrupting chemicals (TDCs) in vivo is largely based on measuring serum tetraiodothyronine in rats, which may be inadequate to assess TDCs with disparate mechanisms of action and insufficient to evaluate the potential neurotoxicity of TDCs. In this review 2 neurodevelopmental processes that are dependent on TH action are highlighted, neuronal migration and maturation of gamma amino butyric acid-ergic interneurons. We discuss how interruption of these processes by TDCs may contribute to abnormal brain circuitry following developmental TH insufficiency. Finally, we identify issues in evaluating the developmental neurotoxicity of TDCs and the strengths and limitations of current approaches designed to regulate them. It is clear that an enhanced understanding of how THs affect brain development will lead to refined toxicity testing, reducing uncertainty and improving our ability to protect children's health.