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AOP: 458


A descriptive phrase which references both the Molecular Initiating Event and Adverse Outcome.It should take the form “MIE leading to AO”. For example, “Aromatase inhibition leading to reproductive dysfunction” where Aromatase inhibition is the MIE and reproductive dysfunction the AO. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE.  More help

AhR activation in the liver leading to Subsequent Adverse Neurodevelopmental Outcomes in Mammals

Short name
A name that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
AhR activation in the liver leading to Adverse Neurodevelopmental Outcomes in Mammals
The current version of the Developer's Handbook will be automatically populated into the Handbook Version field when a new AOP page is created.Authors have the option to switch to a newer (but not older) Handbook version any time thereafter. More help
Handbook Version v2.0

Graphical Representation

A graphical representation of the AOP.This graphic should list all KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs. More help
Click to download graphical representation template Explore AOP in a Third Party Tool


The names and affiliations of the individual(s)/organisation(s) that created/developed the AOP. More help

Prakash Patel PhD; Mathematical Modeller, Cyprotex Discovery Ltd., An Evotec Company, Alderley Park, Cheshire, SK10 4TG, UK

Simon Thomas PhD; Head of Modelling and Simulation, Cyprotex Discovery Ltd., An Evotec Company, Alderley Park, Cheshire, SK10 4TG, UK

Point of Contact

The user responsible for managing the AOP entry in the AOP-KB and controlling write access to the page by defining the contributors as described in the next section.   More help
Cataia Ives   (email point of contact)


Users with write access to the AOP page.  Entries in this field are controlled by the Point of Contact. More help
  • Prakash Patel
  • Simon Thomas
  • Cataia Ives


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OECD Information Table

Provides users with information concerning how actively the AOP page is being developed and whether it is part of the OECD Workplan and has been reviewed and/or endorsed. OECD Project: Assigned upon acceptance onto OECD workplan. This project ID is managed and updated (if needed) by the OECD. OECD Status: For AOPs included on the OECD workplan, ‘OECD status’ tracks the level of review/endorsement of the AOP . This designation is managed and updated by the OECD. Journal-format Article: The OECD is developing co-operation with Scientific Journals for the review and publication of AOPs, via the signature of a Memorandum of Understanding. When the scientific review of an AOP is conducted by these Journals, the journal review panel will review the content of the Wiki. In addition, the Journal may ask the AOP authors to develop a separate manuscript (i.e. Journal Format Article) using a format determined by the Journal for Journal publication. In that case, the journal review panel will be required to review both the Wiki content and the Journal Format Article. The Journal will publish the AOP reviewed through the Journal Format Article. OECD iLibrary published version: OECD iLibrary is the online library of the OECD. The version of the AOP that is published there has been endorsed by the OECD. The purpose of publication on iLibrary is to provide a stable version over time, i.e. the version which has been reviewed and revised based on the outcome of the review. AOPs are viewed as living documents and may continue to evolve on the AOP-Wiki after their OECD endorsement and publication.   More help
OECD Project # OECD Status Reviewer's Reports Journal-format Article OECD iLibrary Published Version
This AOP was last modified on May 26, 2024 20:39

Revision dates for related pages

Page Revision Date/Time
Activation, AhR February 28, 2024 05:12
Induction, Upregulation of glucuronyltransferase activity September 16, 2017 10:14
Increased, Clearance of thyroxine from serum January 26, 2021 10:41
Thyroxine (T4) in serum, Decreased October 10, 2022 08:52
Thyroxine (T4) in neuronal tissue, Decreased April 04, 2019 09:13
Hippocampal gene expression, Altered August 11, 2018 09:26
Hippocampal Physiology, Altered August 11, 2018 09:41
Cognitive Function, Decreased August 09, 2018 11:55
Activation, AhR leads to Induction, Upregulation of glucuronyltransferase activity April 12, 2023 11:25
T4 in serum, Decreased leads to Cognitive Function, Decreased August 11, 2018 19:44
Induction, Upregulation of glucuronyltransferase activity leads to Increased, Clearance of thyroxine from serum September 10, 2021 08:32
Increased, Clearance of thyroxine from serum leads to T4 in serum, Decreased January 26, 2021 10:42
T4 in serum, Decreased leads to T4 in neuronal tissue, Decreased April 04, 2019 10:50
T4 in neuronal tissue, Decreased leads to Hippocampal gene expression, Altered August 11, 2018 19:18
Hippocampal gene expression, Altered leads to Hippocampal Physiology, Altered April 12, 2023 11:28
Hippocampal Physiology, Altered leads to Cognitive Function, Decreased August 11, 2018 19:24
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) February 09, 2017 14:32
Polychlorinated biphenyl November 29, 2016 18:42
PCB 126 April 20, 2023 20:20


A concise and informative summation of the AOP under development that can stand-alone from the AOP page. The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance. More help

Polychlorinated biphenyls (PCBs) and dioxins are environmental contaminatants whose prenatal exposure in humans and exposure in breast milk is correlated to a specific set of mental impairments (Boucher et al. 2009, Boersma and Lanting 2000, Koopman-Esseboom et al. 1996). What the main mechanism(s) of causing these delays for the various PCBs and dioxins is unclear, but studies in rodents regarding 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) and "dioxin-like" PCBs like PCB-126 that it could be caused by lowering maternal and subsequently foetal thyroxine (T4) levels in the brain.

This AOP summarises the findings that chemicals that are AhR receptor agonists can reduce T4 levels in the plasma in rodents and possibly humans and lead to brain changes in neonates, via activation of liver enzymes that increase the metabolism of T4. 

This AOP was created in order to collate and summarise information around the reduction of T4 levels in the plasma by endocrine disruptors, distinct from the reduction of binding to plasma proteins or interfering with thyroid hormone secretion. A short summary:

  1. Chemical activates AhR
  2. UGT activity in the liver increases
  3. More T4 is metabolised and reduces plasma T4 levels
  4. Adverse outcomes (such as cognitive impairment)

The problem is that AhR agonists TCDD and dioxin-like PCBs (PCB-126, PCB-169) have multiple effects on several organs as AhR receptor is present in these organs whose influence cannot be ruled out (complementary to AOP 459: AhR activation in the thyroid leading to Subsequent Adverse Neurodevelopmental Outcomes in Mammals) but its effect on the liver has been relatively well-characterised.

Also, one needs to understand that all AhR agonists do not produce toxic effects - some are found in vegetables, fruits and teas and are important for normal physiology. AhR -/- mice develop immune and liver defects (Quintana 2012). For the case of 2-(1'H-indole-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE) which is a potent agonist of AhR in vitro and in vivo but does not cause toxic effects (Henry et al. 2006)

AOP Development Strategy


Used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development.The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. More help

This AOP was developed as part of the ScreenED project, which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 825745


Provides a description of the approaches to the identification, screening and quality assessment of the data relevant to identification of the key events and key event relationships included in the AOP or AOP network.This information is important as a basis to support the objective/envisaged application of the AOP by the regulatory community and to facilitate the reuse of its components.  Suggested content includes a rationale for and description of the scope and focus of the data search and identification strategy/ies including the nature of preliminary scoping and/or expert input, the overall literature screening strategy and more focused literature surveys to identify additional information (including e.g., key search terms, databases and time period searched, any tools used). More help

This AOP summarises the findings of Kohn et al. 1996 and other papers that corroborate its findings, whilst also summarising evidence of hippocampal and mental effects on rodents of TCDD and various PCBs, although their effects on the thyroid directly and other organs cannot be ruled out. Relevant publications were found by searching through PubMed for terms such as "AhR" "thyroxine" "TCDD" "T4".

Summary of the AOP

This section is for information that describes the overall AOP.The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help


Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a prototypical stressor and the biological system) of an AOP. More help
Key Events (KE)
A measurable event within a specific biological level of organisation. More help
Adverse Outcomes (AO)
An AO is a specialized KE that represents the end (an adverse outcome of regulatory significance) of an AOP. More help
Type Event ID Title Short name
MIE 18 Activation, AhR Activation, AhR
KE 295 Induction, Upregulation of glucuronyltransferase activity Induction, Upregulation of glucuronyltransferase activity
KE 961 Increased, Clearance of thyroxine from serum Increased, Clearance of thyroxine from serum
KE 281 Thyroxine (T4) in serum, Decreased T4 in serum, Decreased
KE 280 Thyroxine (T4) in neuronal tissue, Decreased T4 in neuronal tissue, Decreased
KE 756 Hippocampal gene expression, Altered Hippocampal gene expression, Altered
KE 758 Hippocampal Physiology, Altered Hippocampal Physiology, Altered
AO 402 Cognitive Function, Decreased Cognitive Function, Decreased

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarizes all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP.Each table entry acts as a link to the individual KER description page. More help

Network View

This network graphic is automatically generated based on the information provided in the MIE(s), KEs, AO(s), KERs and Weight of Evidence (WoE) summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help

Prototypical Stressors

A structured data field that can be used to identify one or more “prototypical” stressors that act through this AOP. Prototypical stressors are stressors for which responses at multiple key events have been well documented. More help

Life Stage Applicability

The life stage for which the AOP is known to be applicable. More help
Life stage Evidence
Embryo Moderate
Foetal Moderate
Birth to < 1 month Moderate

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected.In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available. More help
Term Scientific Term Evidence Link
rat Rattus norvegicus High NCBI
mouse Mus musculus High NCBI
Monkey Monkey Low NCBI
human Homo sapiens Low NCBI

Sex Applicability

The sex for which the AOP is known to be applicable. More help
Sex Evidence
Mixed Moderate

Overall Assessment of the AOP

Addressess the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and Weight of Evidence (WoE) for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). More help

Domain of Applicability

Addressess the relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context. More help

Domain of Applicability

  • Chemicals: This AOP applies to a wide range of chemicals structures that activate AhR either in vivo or in vitro, but one needs to understand that not all AhR agonists do not produce toxic effects
  • Sex: This AOP applies to males and females. Disruption of thyroid hormone regulation during foetal and early postnatal development, but the subsequent adverse impacts on nervous system development may differ and be more severe in males than females (Seo et al. 1999, Rice 1999).
  • Life stages: The relevant life stages for this AOP are fetal and early postnatal ages during critical windows of nervous system development where thyroid hormones guide normal development of the brain. There are clear windows of developmental susceptibility and different brain regions show distinct ontogenetic profiles for TH requirements. Distinct phenotypes have been described in both humans and animal models for different periods of TH insufficiency. The influence of maternal thyroid status prior to onset of fetal thyroid function is an important consideration. This AOP does not apply to adult life states.
  • Taxonomic: Based on the majority of the available evidence the taxonomic applicability domains of this AOP is mammals. Most evidence for this AOP has been gathered primarily from laboratory rodents and humans.

Essentiality of the Key Events

The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently, evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence. The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs. More help

Key Events

Direct evidence

Indirect evidence

No or contradictory evidence

MIE 18

TCDD and various PCBs can be confirmed to be agonists of AhR from both the EROD (Petrulis et al. 1999) and CALUX assay (Murk et al. 1996) to estimate exposure and activation of the AhR-ARNT pathways.

KE 295

TCDD – upregulation of UGT1A6 in mouse pups heterozygous for AhR +/- gene but not for AhR -/- gene whose mothers were administered 10 ug/kg TCDD on gestation day 12.5 (Nishimura et al. 2005)

Induction of the UGT-1 gene in Holtzman rats (TCDD-sensitive strain) in pups whose mothers were given a single oral dose of 200 ng or 800 ng TCDD/kg on gestational day 15 (Nishimura et al. 2003)

TCDD - liver UGT mRNA increased in female Sprague-Dawley rats compared to controls after a TCDD dose of 3.5 and 100 ng/kg/day for 31 weeks (Sewall et al. 1995)

PCB-126 - increase in T4-glucuronide production in adult male SD rats compared to controls (Fisher et al. 2006)

KE 961

TCDD - rats cleared more T4 via the biliary route (Bastomsky 1977) and more T4-glucuronide excreted (Bastomsky 1977, Henry and Gasiewicz 1987)

KE 281

TCDD – reduction of serum T4 in pups heterozygous for AhR +/- gene but not for AhR -/- gene whose mothers were administered 10 ug/kg TCDD on gestation day 12.5 (Nishimura et al. 2005)

Reduction in serum T4 in Holtzman rats (TCDD-sensitive strain) in pups whose mothers were given a single oral dose of 200 ng or 800 ng TCDD/kg on gestational day 15 (Nishimura et al. 2003)

PCB-126:  Pregnant albino rats received PCB 126 (20 or 40μg/kgb.wt.) by oral gavage from gestation day (GD) 1 to 20.  Both administrations of PCB 126 elevated serum thyrotropin (TSH) concentration, and decreased free thyroxine (FT4) and free triiodothyronine (FT3) concentrations, resulting in a maternofetal hypothyroidism  (Ahmed et al. 2018).

Arochlor 1254 - SD rat dams were dosed daily with 0, 1, or 4 mg/kg A1254 from gestational day 6 (GD6) until they were sacrificed on GD16. A1254 significantly reduced circulating levels of triiodothyronine(T3) and thyroxine (T4) in pregnant rats (Gauger et al. 2004)

TCDD - Marked dose-dependent reduction in T4 recorded in rats (Kohn et al. 1996, National Toxicology Program 2006)

PCB-126 - reductions in serum T4 on acute (Fisher et al. 2006) and chronic administration of PCB-126 (National Toxicology Program 2006) 

KE 280

Several studies have demonstrated that fetal brain TH levels, previously decreased by maternal exposure to TH synthesis inhibitors or thyroidectomy, recovered following maternal supply of T4 (e.g., Calvo et al., 1990). However, there are no studies with direct infusion of T4 or T3 directly into brain.

The upregulation of deiodinase has been shown to compensate for some loss of neuronal T3 (Escobar-Morreale et al. 1995; Escobar-Morreale et al. 1997).

Indirect evidence shows that T4 replacement that brings circulating T4 concentration back to physiological levels normal, leads to recovery of brain TH and prevents downstream effects including alterations in cell morphology, differentiation and function.

PCB-118 acted via the TH pathway in the rat foetus whilst PCB-126 did not and acted mainly via reducing maternal thyroid hormone levels (Fritsche et al. 2005, Gauger et al. 2004)

KE 759

Hippocampal Gene Expression, Altered: It is well established specific genomic pathways underlie the progression of a number of neurodevelopmental processes in the hippocampus. There is some evidence from ex vivo studies that administration of growth factors will reverse the hippocampal dysplasia seen in Jacob/Nsfm knockout mice (Spilker et al., 2016). Less is known about the impact of hormone replacement on TH-responsive gene expression and the qualitative and quantitative relationships between altered TH-dependent gene expression in this brain region and altered hippocampal cytoarchitectural anatomy.

KE 758

Hippocampal anatomy, altered: It is well accepted that normal hippocampal anatomy is critical for hippocampal physiological function, and that alterations in anatomy lead to altered neuronal activity in the hippocampus (Lee et al., 2015; Grant et al., 1992; Spilker et al., 2016).

Hippocampal physiology, altered: It is a well-accepted assertion that hippocampal synaptic integrity and neuronal plasticity are essential for spatial information processing in animals and spatial and episodic memory in humans. However, other brain regions also can influence these complex behaviors. Limited data from studies in BDNF knockout animals demonstrate that deficits in hippocampal synaptic transmission and plasticity, and downstream behaviors can be rescued with recombinant BDNF (Aarse et al. 2016; Andero et al. 2014). 

The relative size of the IIP-MF in females and males. The IIP-MF was smaller in TCDD exposed female AhR+/- mice with respect to their genotype control group. (Powers et al. 2005)

Pregnant Sprague-Dawley rats were given a consecutive daily dose of TCDD (200 or 800 ng/day/kg) or an equivalent volume of vehicle by gavage on gestational days 8-14 as the prenatal TCDD exposure model. When the male pups grew to adults, morphology and number of neurons in the hippocampus CA1 region was not affected, although the activity of astrocytes in the same region was significantly reduced (Zhang et al. 2018)

AO 402

TCDD exposed female AhR+/- mice performed worse on the spatial water maze task than non exposed mice (Powers et al. 2005)

Pregnant Sprague-Dawley rats were given a consecutive daily dose of TCDD (200 or 800 ng/day/kg) or an equivalent volume of vehicle by gavage on gestational days 8-14. The results of the behavioural tests showed that gestational TCDD exposure induced premature motor activity and earlier eyes-opening, but lead to serious deficits of spatial memory and learning ability in adult male offspring. (Zhang et al 2018)

Monkeys exposed to TCDD perinatally exhibited retarded learning of shape reversals (Shantz and Bowman 1994)

Pregnant Long-Evans rats were dosed by gavage with TCDD or at a dose of 0, 200, or 800 ng/kg on gestational day 15, and the offspring was tested during adulthood. Paired-associate learning was found to be impaired in the 200 ng/kg TCDD group, but not in either group exposed to 800 ng/kg TCDD (Kakeyama et al. 2014)

Long-Evans dams dosed at 0, 0.25, or 1.0 ug/kg/day PCB-126 Monday to Friday beginning 5 weeks before and continuing through gestation and lactation. On the spatial delayed alternation task, there was no convincing evidence for impairment as a result of PCB exposure on adult male pups, as assessed by overall accuracy of performance and measures of perseverative and other types of inappropriate responding. (Rice 1999)

Time-mated Sprague–Dawley rats were gavaged with either TCDD (0.1 ug/kg/day) or corn oil vehicle on gestation days 10–16. On day 80, both male and female TCDD-exposed grown-up pups showed a deficit in learning on the visual partial discrimination-reversal learning task, but TCDD-exposed male rats displayed a pronounced decrease in errors relative to control males in the Morris Water Maze task. There was no difference in the performance of TCDD exposed rats in the radial arm maze task. (Seo et al. 1999)

Pregnant Sprague–Dawley rats (10 per dose) received either 0 or 0.1 ug/kg TCDD orally in corn oil from GD 10 to GD 16. One male and one female from each litter were tested beginning at 100 days of age. the results of the current study underscore the fact that (1) alterations in cognitive function observed following early TCDD exposure are very subtle and (2) under some conditions, learning is actually facilitated, rather than impaired, in TCDD-exposed animals. (Widholm et al. 2003)

Monkeys exposed to TCDD perinatally showed a slight facilitation of learning on both delayed spatial alternation and spatial reversal learning tasks (Seegal and Schantz 1994)

Evidence Assessment

Addressess the biological plausibility, empirical support, and quantitative understanding from each KER in an AOP. More help

Biological plausibility

Biological plausibility refers to the structural or functional relationship between the key events based on our fundamental understanding of "normal biology".

In general, the biological plausibility and coherence linking AhR activation by PCBs and dioxins to decreases in circulating concentrations of THs is great. The biological plausibility of decreases in circulating concentrations of THs, to adverse impacts in the developing hippocampus and subsequent cognitive behaviors is very solid (AOP 42). The problem is the interaction with PCBs and dioxins directly with the foetal brain can act like thyroid hormones due to its similar shape to these compounds, and may compensate for the reduction of THs there. Some of the adverse effects have more in common with congenital hyperthyroidism than hypothyroidism e.g. that gestational TCDD exposure induced premature motor activity in neonatal rats and earlier eyes-opening (Zhang et al. 2018).

Concordance of dose-response relationships:

There are several studies that include correlative evidence between exposure to AhR agonists TCDD (Kohn et al. 1996, NTP 2006) and PCB-126 (NTP, 2006) to downstream KEs up until the reduction of T4 in the plasma (KE 281). Referring to the AOPs 42 and 54, there is ample evidence the reduction of plasma T4 correlates to neurological deficits. What is unclear is the effect of dioxin or dioxin-like PCB effects directly on the brain itself which can activate the TH receptors mitigate the effects of low T4 (Gauger et al. 2004, Fritsche et al. 2005, Kitamura et al. 2005, Zhang et al. 2018), so the dose concordance is still unproven due to this.

Temporal concordance among the key events and adverse effect:

There are two aspects of the temporal concordance of the key events in a developmental AOP.  The first is the temporal concordance refers to the degree to which the data support the hypothesized sequence of the key events; i.e., the effect on KE1 is observed before the effect on KE2, which is observed before the effect on KE3, and so on. This translates to the temporal concordance of the AOP from AhR activation to decreased TH synthesis, reduced circulating TH concentrations, decreased nervous system TH, altered gene expression and anatomy in the hippocampus, and subsequent alterations in hippocampal physiology that result in decrements in cognition. The strength of the temporal concordance between these KEs varies from weak to strong. There is strong evidence for the early direct KEs from both empirical and modelling studies, and for many of the later KEs via the indirect KERs. The temporal concordance between AhR activation and TH synthesis is clearly evidenced by data from ex vivo and in vitro studies, as well as computational models (Kohn et al. 1996). Data supporting the temporal concordance for the later KEs, i.e., from serum TH to changes in hippocampal physiology are limited or lacking.

The second aspect of temporal concordance for developmental AOPs is evidenced by demonstrations for critical windows of development where key events are perturbed, for which the effects are permanent and found during early development and throughout adulthood. It is a well-recognized fact that there are critical developmental windows for disruption of serum THs that result in subsequent alterations in all downstream KEs including the AO cognitive function later in development and adulthood. 


Uncertainties, inconsistencies, and data gaps:

There are several areas of uncertainty and data gaps in the current AOP, especially regarding the later KERs and AOs. The main uncertainty is that AhR agonists do not produce toxic effects and can’t be measured without in vivo testing. Also, hippocampal changes and effect on mental functions in mammals are hard to measure.

Known Modulating Factors

Modulating factors (MFs) may alter the shape of the response-response function that describes the quantitative relationship between two KES, thus having an impact on the progression of the pathway or the severity of the AO.The evidence supporting the influence of various modulating factors is assembled within the individual KERs. More help
Modulating Factor (MF) Influence or Outcome KER(s) involved
TCDD May act as agonists to thyroid receptors in the brain to mitigate lower T4 levels (Seo et al. 1999, Widholm et al. 2003)

KER 761

PCB-118 May act as agonists to thyroid receptors in the brain to mitigate lower T4 levels (Gauger et al. 2007) KER 761

Quantitative Understanding

Optional field to provide quantitative weight of evidence descriptors.  More help

Assessment of quantitative understanding of the AOP:

Currently, there are quantitative models for the early KERs from AhR activation to serum hormone concentrations (Kohn et al. 1996), but none for later KERs. One could link the reduction of thyroid hormones to malformations in the brain, as in Hassan et al. (2017) quantitatively linked PTU-induced TH synthesis declines in the dam and the foetus to decrements in serum and brain TH concentrations to a structural malformation in the postnatal brain. At present, the overall quantitative understanding of the AOP is insufficient to directly link a measure of chemical potency as a TPO inhibitor to a quantitative prediction of effect on cognitive function (e.g., IQ in humans, learning deficits in rodents).

Considerations for Potential Applications of the AOP (optional)

Addressess potential applications of an AOP to support regulatory decision-making.This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. More help

To determine whether a particular compound can potentially cause this AO, one must find out first if the compound is a activator of UGT isoforms in the liver that can glucuronidate T4. It is not sufficient to see whether it has a positive outcome from the EROD or CALUX assays as they only measure CYP inhibition and AhR nuclear activation, respectively. 

This could possibly done in hepatocyte cell lines or primary hepatocytes, but it is uncertain what level of activation would cause T4 levels to go sufficiently down in the brain in vivo to counteract that some AhR agonists can also activate thyroid hormone receptors in the brain and can mitigate the AO. 


List of the literature that was cited for this AOP. More help

Aarse J, Herlitze S, Manahan-Vaughan D. (2016).The requirement of BDNF for hippocampal synaptic plasticity is experience-dependent. Hippocampus. 26:739-51

Ahmed RG, El-Gareib AW, Shaker HM. Gestational 3,3',4,4',5-pentachlorobiphenyl (PCB 126) exposure disrupts fetoplacental unit: Fetal thyroid-cytokines dysfunction. Life Sci. 2018;192:213-220

Andero R, Choi DC, Ressler KJ. (2014). BDNF-TrkB receptor regulation of distributed adult neural plasticity, memory formation, and psychiatric disorders. Prog Mol Biol Transl Sci.122:169-92

Bastomsky CH. Enhanced thyroxine metabolism and high uptake goiters in rats after a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Endocrinology. 1977;101(1):292-6

Boersma ER, Lanting CI. Environmental exposure to polychlorinated biphenyls (PCBs) and dioxins. Consequences for longterm neurological and cognitive development of the child lactation. Adv Exp Med Biol. 2000;478:271-87

Boucher O, Muckle G, Bastien CH. Prenatal exposure to polychlorinated biphenyls: a neuropsychologic analysis. Environ Health Perspect. 2009;117(1):7-16

Calvo R, Obregón MJ, Ruiz de Oña C, Escobar del Rey F, Morreale de Escobar G.  Congenital hypothyroidism, as studied in rats. Crucial role of maternal thyroxine but not of 3,5,3'-triiodothyronine in the protection of the fetal brain.  J Clin Invest. 1990;86(3):889-99

Escobar-Morreale HF, Obregón MJ, Escobar del Rey F, Morreale de Escobar G.Replacement therapy for hypothyroidism with thyroxine alone does not ensure euthyroidism in all tissues, as studied in thyroidectomized rats. J Clin Invest. 1995;96(6):2828-38.

Escobar-Morreale HF, Obregón MJ, Hernandez A, Escobar del Rey F, Morreale de Escobar G. (1997). Regulation of iodothyronine deiodinase activity as studied in thyroidectomized rats infused with thyroxine or triiodothyronine. Endocrinology. 138(6):2559-68.

Fisher JW, Campbell J, Muralidhara S, Bruckner JV, Ferguson D, Mumtaz M, Harmon B, Hedge JM, Crofton KM, Kim H, Almekinder TL. Effect of PCB 126 on hepatic metabolism of thyroxine and perturbations in the hypothalamic-pituitary-thyroid axis in the rat. Toxicol Sci. 2006; 90(1):87-95

Fritsche E, Cline JE, Nguyen NH, Scanlan TS, Abel J. Polychlorinated biphenyls disturb differentiation of normal human neural progenitor cells: clue for involvement of thyroid hormone receptors. Environ Health Perspect. 2005 Jul;113(7):871-6,

Gauger KJ, Kato Y, Haraguchi K, Lehmler HJ, Robertson LW, Bansal R, Zoeller RT. Polychlorinated biphenyls (PCBs) exert thyroid hormone-like effects in the fetal rat brain but do not bind to thyroid hormone receptors. Environ Health Perspect. 2004;112(5):516-23

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Hassan I, El-Masri H, Kosian PA, Ford J, Degitz SJ, Gilbert ME. Neurodevelopment and Thyroid Hormone Synthesis Inhibition in the Rat: Quantitative Understanding Within the Adverse Outcome Pathway Framework. Toxicol Sci. 2017;160(1):57-73

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