February 9, 2021 by Grace Panter


Becky Brown and myself recently attended an interesting webinar by European Food Safety Authority/European Chemicals Agency (EFSA/ECHA) on how to consider the Xenopus eleuthereombryonic thyroid assay (XETA) in the assessment strategy of the endocrine disrupter (ED) guidance1. Although the XETA is included in the EFSA/ECHA ED guidance it had not been fully validated at the time this guidance came into force and was only adopted as an Organisation for Economic Cooperation and Development (OECD) test guideline in June 20192.

The XETA uses transgenic Xenopus eleutheroembyros, carrying a thyroid hormone promoter (THbZIP) cloned upstream of green fluorescent protein (GFP). The assay can detect thyroid agonists and antagonists through the modulation of GFP expression. The XETA sits in the OECD conceptual framework at Level 3, the same as the amphibian metamorphosis assay (AMA) which was adopted a decade earlier (OECD 2313), providing information on thyroid activity. The XETA, compared to the AMA, is shorter in duration and uses animals at a non-protected life stage, thereby reducing animal numbers, resources and time, but is not able to detect modulation of thyroid hormone transport or clearance. There are advantages and disadvantages in performing each assay, so which assay should you undertake? When would the XETA be considered sufficient to investigate thyroid activity?

In the webinar, two case studies were given as examples to demonstrate when the XETA, rather than an AMA, may be considered sufficient to investigate thyroid activity. It was acknowledged that there is high conservation of the thyroid receptor across the vertebrates and the decision on which assay would, in part, depend on the findings from a complete mammalian data set. However, if the XETA was positive the next step would be a conceptual framework Level 4 study incorporating population level endpoints (i.e. the larval amphibian growth and development assay; LAGDA; OECD 2414). Following the presentation there was a Q&A session with several points and questions raised by the audience seeking more clarification on the use of the XETA in the ED assessment and interpretation of the findings. These points will be addressed by EFSA/ECHA, who following the webinar will include the learning from the discussions to update an annex will be added to the ED guidance on how to use the XETA. No time frame was provided on when this annex would be available.

If you would like to find out more about how wca can help study monitor fish and amphibian studies for your ED assessments, or in the compiling of your ED assessments (ecotoxicology and toxicology) please do not hesitate to contact myself or Becky Brown.


1ECHA/EFSA. 2018. Guidance for the identification of endocrine disruptors in the context of Regulations (EU) No 528/2012 and (EC) No 1107/2009

2OECD (Organisation for Economic Cooperation and Development). 2019. Xenopus Eleutheroembryonic Thyroid Assay (XETA). In: OECD series on testing and assessment. Test guideline number 248. OECD Publishing, Paris. https://doi.org/10.1787/a13f80ee-en

3OECD (Organisation for Economic Cooperation and Development). 2009. The Amphibian Metamorphosis Assay. In: OECD series on testing and assessment. Test guideline number 231. OECD Publishing, Paris. https://doi.org/10.1787/9789264076242-en

4OECD (Organisation for Economic Cooperation and Development). 2015. The Larval Amphibian Growth and Development Assay. In: OECD series on testing and assessment. Test guideline number 241. OECD Publishing, Paris. https://doi.org/10.1787/9789264242340-en

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