To the extent possible under law, AOP-Wiki has waived all copyright and related or neighboring rights to KER:2378

Relationship: 2378


The title of the KER should clearly define the two KEs being considered and the sequential relationship between them (i.e., which is upstream and which is downstream). Consequently all KER titles take the form “upstream KE leads to downstream KE”.  More help

Decreased, Triiodothyronine (T3) in serum leads to Altered, Photoreceptor patterning

Upstream event
Upstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help
Downstream event
Downstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes. Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

This table is automatically generated upon addition of a KER to an AOP. All of the AOPs that are linked to this KER will automatically be listed in this subsection. Clicking on the name of the AOP in the table will bring you to the individual page for that AOP. More help
AOP Name Adjacency Weight of Evidence Quantitative Understanding Point of Contact Author Status OECD Status
Thyroperoxidase inhibition leading to altered visual function via altered photoreceptor patterning adjacent Cataia Ives (send email) Under development: Not open for comment. Do not cite

Taxonomic Applicability

Select one or more structured terms that help to define the biological applicability domain of the KER. In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. Authors can indicate the relevant taxa for this KER in this subsection. The process is similar to what is described for KEs (see pages 30-31 and 37-38 of User Handbook) More help
Term Scientific Term Evidence Link
zebrafish Danio rerio NCBI

Sex Applicability

Authors can indicate the relevant sex for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of the User Handbook). More help

Life Stage Applicability

Authors can indicate the relevant life stage for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of User Handbook). More help

Key Event Relationship Description

Provide a brief, descriptive summation of the KER. While the title itself is fairly descriptive, this section can provide details that aren’t inherent in the description of the KEs themselves (see page 39 of the User Handbook). This description section can be viewed as providing the increased specificity in the nature of upstream perturbation (KEupstream) that leads to a particular downstream perturbation (KEdownstream), while allowing the KE descriptions to remain generalised so they can be linked to different AOPs. The description is also intended to provide a concise overview for readers who may want a brief summation, without needing to read through the detailed support for the relationship (covered below). Careful attention should be taken to avoid reference to other KEs that are not part of this KER, other KERs or other AOPs. This will ensure that the KER is modular and can be used by other AOPs. More help

Thyroid hormone signaling coordinates cell fate of photoreceptors in the visual system, especially during development and growth. Although different taxonomic groups differ in their photoreceptor subtypes, in general across species, thyroid hormone action promotes a shift of spectral sensitivity of opsins toward longer wavelengths. Decreased serum levels of triiodothyronine (T3), the more biologically active thyroid hormone, can alter photoreceptor patterning.


Evidence Supporting this KER

Assembly and description of the scientific evidence supporting KERs in an AOP is an important step in the AOP development process that sets the stage for overall assessment of the AOP (see pages 49-56 of the User Handbook). To do this, biological plausibility, empirical support, and the current quantitative understanding of the KER are evaluated with regard to the predictive relationships/associations between defined pairs of KEs as a basis for considering WoE (page 55 of User Handbook). In addition, uncertainties and inconsistencies are considered. More help
Biological Plausibility
Define, in free text, the biological rationale for a connection between KEupstream and KEdownstream. What are the structural or functional relationships between the KEs? For example, there is a functional relationship between an enzyme’s activity and the product of a reaction it catalyses. Supporting references should be included. However, it is recognised that there may be cases where the biological relationship between two KEs is very well established, to the extent that it is widely accepted and consistently supported by so much literature that it is unnecessary and impractical to cite the relevant primary literature. Citation of review articles or other secondary sources, like text books, may be reasonable in such cases. The primary intent is to provide scientifically credible support for the structural and/or functional relationship between the pair of KEs if one is known. The description of biological plausibility can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured (see page 40 of the User Handbook for further information).   More help

It is widely accepted that thyroid hormones play a role in the development of the visual system, and specifically in the development of the normal photoreceptor pattern in the retina. It follows that decreased availability of T3 in serum disrupts the normal photoreceptor pattern. 


Uncertainties and Inconsistencies
In addition to outlining the evidence supporting a particular linkage, it is also important to identify inconsistencies or uncertainties in the relationship. Additionally, while there are expected patterns of concordance that support a causal linkage between the KEs in the pair, it is also helpful to identify experimental details that may explain apparent deviations from the expected patterns of concordance. Identification of uncertainties and inconsistencies contribute to evaluation of the overall WoE supporting the AOPs that contain a given KER and to the identification of research gaps that warrant investigation (seep pages 41-42 of the User Handbook).Given that AOPs are intended to support regulatory applications, AOP developers should focus on those inconsistencies or gaps that would have a direct bearing or impact on the confidence in the KER and its use as a basis for inference or extrapolation in a regulatory setting. Uncertainties that may be of academic interest but would have little impact on regulatory application don’t need to be described. In general, this section details evidence that may raise questions regarding the overall validity and predictive utility of the KER (including consideration of both biological plausibility and empirical support). It also contributes along with several other elements to the overall evaluation of the WoE for the KER (see Section 4 of the User Handbook).  More help
  • Mackin et al. (2019): All 4 cone opsins are regulated by T4. However, in athyroid juvenile zebrafish, sws1 and sws2 levels were not different compared to controls, findings which are not consistent with endogenous functions for TH signaling in regulation of these genes in juvenile zebrafish.

  • Some studies show that TH can still alter opsin expression in later life stages after retinal development, while other studies report that opsin expression remains unaltered but the wavelength where maximal absorbance occurs increases.

Response-response Relationship
This subsection should be used to define sources of data that define the response-response relationships between the KEs. In particular, information regarding the general form of the relationship (e.g., linear, exponential, sigmoidal, threshold, etc.) should be captured if possible. If there are specific mathematical functions or computational models relevant to the KER in question that have been defined, those should also be cited and/or described where possible, along with information concerning the approximate range of certainty with which the state of the KEdownstream can be predicted based on the measured state of the KEupstream (i.e., can it be predicted within a factor of two, or within three orders of magnitude?). For example, a regression equation may reasonably describe the response-response relationship between the two KERs, but that relationship may have only been validated/tested in a single species under steady state exposure conditions. Those types of details would be useful to capture.  More help
This sub-section should be used to provide information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). This can be useful information both in terms of modelling the KER, as well as for analyzing the critical or dominant paths through an AOP network (e.g., identification of an AO that could kill an organism in a matter of hours will generally be of higher priority than other potential AOs that take weeks or months to develop). Identification of time-scale can also aid the assessment of temporal concordance. For example, for a KER that operates on a time-scale of days, measurement of both KEs after just hours of exposure in a short-term experiment could lead to incorrect conclusions regarding dose-response or temporal concordance if the time-scale of the upstream to downstream transition was not considered. More help
Known modulating factors
This sub-section presents information regarding modulating factors/variables known to alter the shape of the response-response function that describes the quantitative relationship between the two KEs (for example, an iodine deficient diet causes a significant increase in the slope of the relationship; a particular genotype doubles the sensitivity of KEdownstream to changes in KEupstream). Information on these known modulating factors should be listed in this subsection, along with relevant information regarding the manner in which the modulating factor can be expected to alter the relationship (if known). Note, this section should focus on those modulating factors for which solid evidence supported by relevant data and literature is available. It should NOT list all possible/plausible modulating factors. In this regard, it is useful to bear in mind that many risk assessments conducted through conventional apical guideline testing-based approaches generally consider few if any modulating factors. More help
Known Feedforward/Feedback loops influencing this KER
This subsection should define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits? In some cases where feedback processes are measurable and causally linked to the outcome, they should be represented as KEs. However, in most cases these features are expected to predominantly influence the shape of the response-response, time-course, behaviours between selected KEs. For example, if a feedback loop acts as compensatory mechanism that aims to restore homeostasis following initial perturbation of a KE, the feedback loop will directly shape the response-response relationship between the KERs. Given interest in formally identifying these positive or negative feedback, it is recommended that a graphical annotation (page 44) indicating a positive or negative feedback loop is involved in a particular upstream to downstream KE transition (KER) be added to the graphical representation, and that details be provided in this subsection of the KER description (see pages 44-45 of the User Handbook).  More help

Domain of Applicability

As for the KEs, there is also a free-text section of the KER description that the developer can use to explain his/her rationale for the structured terms selected with regard to taxonomic, life stage, or sex applicability, or provide a more generalizable or nuanced description of the applicability domain than may be feasible using standardized terms. More help


  • Taxonomic applicability

  • The function of thyroid hormones in regulating eye development including photoreceptor patterning is highly conserved across vertebrates (Viets et al., 2016)

  • Species that undergo noticeable metamorphosis seem to have more plasticity in opsin expression both at the embryonic stage and when the retina is fully differentiated (Suliman and Flamarique, 2014).

  • Life-stage applicability

  • Mackin et al. (2019): Lws and Rh2 differential Expression Remains Plastic to the Effects of TH Signaling through Juvenile Growth.

  • Mackin et al. (2019): components of the zebrafish rh2 opsin gene array can also be regulated by exogenous T3 in larval zebrafish.

  • Sex applicability

  • Zebrafish are undifferentiated gonochorists since both sexes initially develop an immature ovary (Maack and Segner, 2003). Immature ovary development progresses until approximately the onset of the third week. Later, in female fish immature ovaries continue to develop further, while male fish undergo transformation of ovaries into testes. Final transformation into testes varies among male individuals, however finishes usually around 6 weeks post fertilization. Effects on photoreceptor patterning resulting from altered T3 levels during early development are therefore expected to be independent of sex.

  • Glaschke et al. (2011) showed that TH also controls adult cone opsin expression in mice and rats.

  • Mader and Cameron (2006): Premetamorphic winter flounder express only RH2 opsin. During metamorphosis they develop a new repertoire of opsins (RH1, SWS2, RH2, and LWS). the phenotypic organization of the premetamorphic retina, which is produced during low TH conditions, is consistent with the premetamorphic-like retina produced by the growing postmetamorphic retina during induced hypothyroidic conditions. Additionally, a similar effect of TH upon photoreceptor production was observed for regenerating postmetamorphic retina. This suggests that regeneration of the adult vertebrate retina involves a recapitulation of the mechanisms that drive and direct cytogenesis during normal development and growth

  • While in early life stages during retinal development, TH alters opsin expression and photoreceptor fate, during later stages TH treatment does not always result in altered opsin expression:

    • Allison et al. (2004) showed that thyroid hormone treatment increases the wavelength of maximum absorbance of photoreceptors in adult zebrafish, and this could not be explained by changes in opsin expression.

    • Suliman and Novales Flamarique (2014): Opsin expression did not change in young juveniles of zebrafish or killifish treated with TH.


List of the literature that was cited for this KER description using the appropriate format. Ideally, the list of references should conform, to the extent possible, with the OECD Style Guide (OECD, 2015). More help

Allison, W.T., Dann, S.G., Veldhoen, K.M., Hawryshyn, C.W., 2006. Degeneration and regeneration of ultraviolet cone photoreceptors during development in rainbow trout. Journal of Comparative Neurology 499, 702-715.

Allison, W.T., Haimberger, T.J., Hawryshyn, C.W., Temple, S.E., 2004. Visual pigment composition in zebrafish: Evidence for a rhodopsin-porphyropsin interchange system. Visual Neuroscience 21, 945-952.

Cheng, C.L., Flamarique, I.N., Harosi, F.I., Rickers-Haunerland, J., Haunerland, N.H., 2006. Photoreceptor layer of salmonid fishes: Transformation and loss of single cones in juvenile fish. Journal of Comparative Neurology 495, 213-235.

Ding XQ, Ma H. (2016) Thyroid Hormone Signaling and Cone Photoreceptor Viability. In: Bowes Rickman C., LaVail M., Anderson R., Grimm C., Hollyfield J., Ash J. (eds) Retinal Degenerative Diseases. Advances in Experimental Medicine and Biology, vol 854. Springer, Cham.

DuVal, M.G., Allison, W.T., 2018. Photoreceptor Progenitors Depend Upon Coordination of gdf6a, thr beta, and tbx2b to Generate Precise Populations of Cone Photoreceptor Subtypes. Investigative Ophthalmology & Visual Science 59, 6089-6101.

Gamborino, M. J., Sevilla-Romero, E., Muñoz, A., Hernández-Yago, J., Renau-Piqueras, J., & Pinazo-Durán, M. D. (2001). Role of thyroid hormone in craniofacial and eye development using a rat model. Ophthalmic Research, 33(5), 283–291.

Gan, K.J., Flamarique, I.N., 2010. Thyroid Hormone Accelerates Opsin Expression During Early Photoreceptor Differentiation and Induces Opsin Switching in Differentiated TR alpha-Expressing Cones of the Salmonid Retina. Developmental Dynamics 239, 2700-2713.

Glaschke, A., Glosmann, M., Peichl, L., 2010. Developmental Changes of Cone Opsin Expression but Not Retinal Morphology in the Hypothyroid Pax8 Knockout Mouse. Investigative Ophthalmology & Visual Science 51, 1719-1727.

Glaschke, A., Weiland, J., Del Turco, D., Steiner, M., Peichl, L., Glosmann, M., 2011. Thyroid Hormone Controls Cone Opsin Expression in the Retina of Adult Rodents. Journal of Neuroscience 31, 4844-4851.

Houbrechts, A.M., Vergauwen, L., Bagci, E., Van Houcke, J., Heijlen, M., Kulemeka, B., Hyde, D.R., Knapen, D., Darras, V.M., 2016. Deiodinase knockdown affects zebrafish eye development at the level of gene expression, morphology and function. Molecular and Cellular Endocrinology 424, 81-93.

Maack, G., Segner, H., 2003. Morphological development of the gonads in zebrafish. Journal of Fish Biology 62, 895-906.

Mackin, R.D., Frey, R.A., Gutierrez, C., Farre, A.A., Kawamura, S., Mitchell, D.M., Stenkamp, D.L., 2019. Endocrine regulation of multichromatic color vision. Proceedings of the National Academy of Sciences of the United States of America 116, 16882-16891.

Mader, M., Cameron, D., 2006. Effects of induced systemic hypothyroidism upon the retina: Regulation of thyroid hormone receptor alpha and photoreceptor production. Molecular Vision 12, 915-930.

Ng, L., Hurley, L.B., Dierks, B., Srinivas, M., Salto, C., Vennstrom, B., Reh, T.A., Forrest, D., 2001. A thyroid hormone receptor that is required for the development of green cone photoreceptors. Nature Genetics 27, 94-98.

Ng, L., Lu, A., Swaroop, A., Sharlin, D.S., Swaroop, A., Forrest, D., 2011. Two transcription factors can direct three photoreceptor outcomes from rod precursor cells in mouse retinal development. J Neurosci 31, 11118-11125.

Roberts, M.R., Srinivas, M., Forrest, D., Morreale de Escobar, G., Reh, T.A., 2006. Making the gradient: thyroid hormone regulates cone opsin expression in the developing mouse retina. Proc Natl Acad Sci U S A 103, 6218-6223.

Suliman, T., Flamarique, I.N., 2014. Visual Pigments and Opsin Expression in the Juveniles of Three Species of Fish (Rainbow Trout, Zebrafish, and Killifish) Following Prolonged Exposure to Thyroid Hormone or Retinoic Acid. Journal of Comparative Neurology 522, 98-117.

Suzuki, S.C., Bleckert, A., Williams, P.R., Takechi, M., Kawamura, S., Wong, R.O.L., 2013. Cone photoreceptor types in zebrafish are generated by symmetric terminal divisions of dedicated precursors. Proceedings of the National Academy of Sciences of the United States of America 110, 15109-15114.

Vancamp, P., Houbrechts, A.M., Darras, V.M., 2019. Insights from zebrafish deficiency models to understand the impact of local thyroid hormone regulator action on early development. General and Comparative Endocrinology 279, 45-52.

Viets, K., Eldred, K.C., Johnston, R.J., 2016. Mechanisms of Photoreceptor Patterning in Vertebrates and Invertebrates. Trends in Genetics 32, 638-659.