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This is a legacy representation of this AOP. Please see the current version here:

AOP Title

Type II iodothyronine deiodinase (DIO2) inhibition leading to altered amphibian metamorphosis
Short name: DIO2 inhib alters metamorphosis


Sally A. Mayasich, National Health and Environmental Effects Research Laboratory, US EPA, Duluth, MN, USA <>

Jonathan T. Haselman, National Health and Environmental Effects Research Laboratory, US EPA, Duluth, MN, USA <>

Sigmund J. Degitz, National Health and Environmental Effects Research Laboratory, US EPA, Duluth, MN, USA <>

Michael W. Hornung, National Health and Environmental Effects Research Laboratory, US EPA, Duluth, MN, USA <>


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OECD Project 1.29: A catalog of putative AOPs that will enhance the utility of US EPA Toxcast high throughput screening data for hazard identification

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This putative AOP describes the potential for an adverse outcome resulting from the inhibition of Type 2 iodothyronine deiodinase (DIO2) during amphibian metamorphosis. Initial development of this AOP is based largely on literature in which amphibian deiodinases are genetically disrupted or blocked by the deiodinase inhibitor iopanoic acid, and prediction from tissue expression patterns. Thyroid hormones (THs) are essential for normal sequential development of amphibian tissues and organs, and activities of the three deiodinases found in amphibians, as in mammals, function in a highly regulated balance. Chemical inhibition of DIO2, the molecular-initiating event (MIE), results in decreased transformation of thyroxine (T4) to the active form, 3,5,3’-triiodothyronine (T3) in peripheral tissues. Chemicals that interfere with the DIO2 catalyzing reaction of T4 to T3 have the potential to cause insufficiency of the active form that may result in altered metamorphosis. Adverse consequences of T3 insufficiency may vary based on timing of exposure and produce different effects at different developmental stages. For example, T3 insufficiency due to DIO2 inhibition in the African clawed frog, Xenopus laevis, within several days post-fertilization (pre-metamorphosis) could affect brain development, and could alter T4/T3 feedback. It has been found that DIO2 does not regulate T3 levels in serum. However, D2 inhibition in peripheral tissues through the larval phase and post-metamorphic climax may cause alterations in limb development, intestinal remodeling, gill resorption and/or tail resorption.

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Summary of the AOP

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Molecular Initiating Event

Molecular Initiating Event Support for Essentiality
Deiodinase 2, Inhibition Moderate

Key Events

Event Support for Essentiality
Triiodothyronine (T3) in tissues, Decreased Strong

Adverse Outcome

Adverse Outcome
Amphibian metamorphosis, Altered

Relationships Among Key Events and the Adverse Outcome

Event Description Triggers Weight of Evidence Quantitative Understanding
Deiodinase 2, Inhibition Directly Leads to Triiodothyronine (T3) in tissues, Decreased Moderate Weak
Triiodothyronine (T3) in tissues, Decreased Directly Leads to Amphibian metamorphosis, Altered Strong Moderate

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Life Stage Applicability

Life Stage Evidence Links
development Strong

Taxonomic Applicability

Name Scientific Name Evidence Links
African clawed frog Xenopus laevis Moderate NCBI

Sex Applicability

Sex Evidence Links
Unspecific Strong

Graphical Representation

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Overall Assessment of the AOP

Domain of Applicability

Life Stage Applicability, Taxonomic Applicability, Sex Applicability
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Essentiality of the Key Events

Molecular Initiating Event Summary, Key Event Summary
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Weight of Evidence Summary

Summary Table
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Quantitative Considerations

Summary Table
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Considerations for Potential Applications of the AOP (optional)


Becker, K.B., Stephens, K.C., Davey, J.C., Schneider, M.J., Galton, V.A. (1997). “The Type 2 and Type 3 iodothyronine deiodinases play important roles in coordinating development in Rana catesbeiana tadpoles.” Endocrinology 138(7): 2989-2997.

Cai, L. Q., Brown, D.D. (2004). "Expression of type II iodothyronine deiodinase marks the time that a tissue responds to thyroid hormone-induced metamorphosis in Xenopus laevis." Developmental Biology 266(1): 87-95.

Galton, V.A., Schneider, M.J., Clark, A.S., St. Germain, D.L. (2009). “Life without thyroxine to 3,5,3’-triiodothyronine conversion: studies in mice devoid of the 5’-deiodinases.” Endocrinology 150(6): 2957–2963.

Huang, H., Cai, L., Remo, B. F., Brown, D. D.. (2001). "Timing of metamorphosis and the onset of the negative feedback loop between the thyroid gland and the pituitary is controlled by type II iodothyronine deiodinase in Xenopus laevis." Proc Natl Acad Sci U S A 98(13): 7348-7353.

Morvan-Dubois, G., Demeneix, B.A., Sachs, L.M. (2008). “Xenopus laevis as a model for studying thyroid hormone signaling: From development to metamorphosis.” Mol Cell Endocrinol. 293: 71-79.

Morvan-Dubois, G., Sebillot, A., Kuiper, G.G.J.M., Verhoelst, C.H.J., Darras, V.M., Visser, T.J., Demeneix, B.A. (2006). “Deiodinase activity is present in Xenopus laevis during early embryogenesis.” Endocrinolgy 147(10): 4941-4949.

Schneider MJ, Fiering SN, Pallud SE, Parlow AF, St Germain DL, GaltonVA (2001) Targeted disruption of the type 2 selenodeiodinase gene (DIO2) results in a phenotype of pituitary resistance to T4. Mol Endocrinol 15:2137–2148.