Aop: 8


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

Upregulation of Thyroid Hormone Catabolism via Activation of Hepatic Nuclear Receptors, and 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
Nuclear receptor induced TH Catabolism and Developmental Hearing Loss

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

Katie Paul Friedman, National Center for Computational Toxicology, US EPA, RTP, NC USA <>

Mary E. Gilbert, National Health and Environmental Effects Research Laboratory, US EPA, RTP, NC USA <>

Kevin M. Crofton, National Center for Computational Toxicology, US EPA, RTP, NC USA <>

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
Katie Paul Friedman   (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
  • Katie Paul Friedman
  • Kevin Crofton
  • Mary Gilbert


Provides users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. OECD Status - Tracks the level of review/endorsement the AOP has been subjected to. OECD Project Number - Project number is designated and updated by the OECD. SAAOP Status - Status managed and updated by SAAOP curators. More help
Author status OECD status OECD project SAAOP status
Open for adoption Under Development 1.9 Included in OECD Work Plan
This AOP was last modified on March 10, 2022 07:41

Revision dates for related pages

Page Revision Date/Time
Activation, Pregnane-X receptor, NR1l2 September 16, 2017 10:14
Loss, Cochlear function September 16, 2017 10:14
Thyroxine (T4) in neuronal tissue, Decreased April 04, 2019 09:13
Thyroxine (T4) in serum, Decreased October 10, 2022 08:52
Induction, Upregulation of glucuronyltransferase activity September 16, 2017 10:14
Hippocampal anatomy, Altered May 20, 2022 05:45
Hippocampal Physiology, Altered August 11, 2018 09:41
Hippocampal gene expression, Altered August 11, 2018 09:26
Increased, Clearance of thyroxine from serum January 26, 2021 10:41
Activation, Pregnane-X receptor, NR1l2 leads to Induction, Upregulation of glucuronyltransferase activity September 14, 2021 12:29
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 anatomy, Altered August 11, 2018 19:05
Hippocampal anatomy, Altered leads to Hippocampal Physiology, Altered August 11, 2018 19:21
Hippocampal Physiology, Altered leads to Loss, Cochlear function September 14, 2021 12:35


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

Data from rodent studies demonstrate that thyroid hormone disruption during cochlear development culminates in ototoxicity. Developmental exposure of rats to polychlorinated biphenyls (PCBs) results in a low-frequency hearing loss in adult offspring (Goldey et al., 1995a; Herr et al., 1996; 2001; Crofton and Rice, 1999; Laskey et al., 2002). A body of work now supports the hypothesis that this ototoxicity results from PCB-induced hypothyroxinemia during a critical period of auditory development. Evidence for this hypothesis includes: a correlation between the severity of functional auditory impairment and the degree of thyroid hormone depletion (Goldey et al., 1995a; 1995b; Goldey and Crofton, 1998; Crofton, 2004), a cross-fostering study demonstrating that the critical exposure period is postnatal (Crofton et al., 2000a), and amelioration of the hearing loss following postnatal thyroxine replacement (Goldey and Crofton, 1998). Below an adverse outcome pathway is described for chemicals that activate xenobiotic nuclear receptors, including AhR, CAR, and PXR, leading to thyroid hormone disruption during cochlear development and resulting in permanent auditory loss.

This AOP is a revision and update of the original started on the Chemical Mode of Action wiki sponsored by WHO/IPCS. This MOA was described and published by Crofton and Zoeller 2005).

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 is an update of the WHO/IPCS MOA developed in 2005 by Crofton and Zoeller (Crit Rev Toxicol 2005).

Crofton KM, Zoeller RT. Mode of action: neurotoxicity induced by thyroid hormone disruption during development--hearing loss resulting from exposure to PHAHs. Crit Rev Toxicol. 2005 Oct-Nov;35(8-9):757-69. PMID: 6417043


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

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 239 Activation, Pregnane-X receptor, NR1l2 Activation, Pregnane-X receptor, NR1l2
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 757 Hippocampal anatomy, Altered Hippocampal anatomy, Altered
KE 758 Hippocampal Physiology, Altered Hippocampal Physiology, Altered
AO 319 Loss, Cochlear function Loss, Cochlear function

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
Fetal to Parturition Moderate
Nursing Child 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 NCBI

Sex Applicability

The sex for which the AOP is known to be applicable. More help

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

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

Evidence Assessment

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

Summary Table Provide an overall summary of the weight of evidence based on the evaluations of the individual linkages from the Key Event Relationship pages.

Concordance of dose-response relationships

Multiple studies provide limited (2-3 doses) dose-response data for many of the key events. These studies demonstrate similar magnitudes of effect on circulating hormones for doses of PCBs that are within an order of magnitude (3-25 mg/kg/day for Aroclor 1254) (e.g.,Morse et al., 1996; Goldey et al., 1998). Very limited data are available correlating any of the key events. One exception is the relationship between circulating serum T4 concentrations during development and the magnitude of hearing loss (Crofton, 2004). There is a very good correlation between total serum T4 concentrations on postnatal day (PND) 14 and hearing loss assessed in adult offspring of PCB exposed dams (Figure 2). All of these events occur within a 2-3 fold dose range.

Temporal concordance among the key events and the adverse outcome Strength, consistency, and specificity of association of adverse effect and initiating event Biological plausibility, coherence, and consistency of the experimental evidence Alternative mechanism(s) or MIE(s) that logically present themselves and the extent to which they may detract from the AOP Uncertainties, inconsistencies, and data gaps

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

Quantitative Understanding

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

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


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