Thyroid hormones (THs) are involved in the regulation of many important physiological and developmental processes, including vertebrate eye development. Thyroid hormone system–disrupting chemicals (THSDCs) may have severe consequences, because proper functioning of the visual system is a key factor for survival in wildlife. However, the sequence of events leading from TH system disruption (THSD) to altered eye development in fish has not yet been fully described. The development of this adverse outcome pathway (AOP) was based on an intensive literature review of studies that focused on THSD and impacts on eye development, mainly in fish. In total, approximately 120 studies (up to the end of 2021) were used in the development of this AOP linking inhibition of the key enzyme for TH synthesis, thyroperoxidase (TPO), to effects on retinal layer structure and visual function in fish (AOP‐Wiki, AOP 363). In a weight‐of‐evidence evaluation, the confidence levels were overall moderate, with ample studies showing the link between reduced TH levels and altered retinal layer structure. However, some uncertainties about the underlying mechanism(s) remain. Although the current weight‐of‐evidence evaluation is based on fish, the AOP is plausibly applicable to other vertebrate classes. Through the re‐use of several building blocks, this AOP is connected to the AOPs leading from TPO and deiodinase inhibition to impaired swim bladder inflation in fish (AOPs 155–159), together forming an AOP network describing THSD in fish. This AOP network addresses the lack of thyroid‐related endpoints in existing fish test guidelines for the evaluation of THSDCs. Environ Toxicol Chem 2022;41:2632–2648. © 2022 SETAC

Thyroid hormones (THs) regulate a multitude of developmental and metabolic processes, which are responsible for vertebrate development, growth, and maintenance of homeostasis. THs also play a key role in neurogenesis of vertebrates and thus affect eye development, which is vital for foraging efficiency and for effective escape from predation. Currently, there are no validated test guidelines for the assessment of TH system-disrupting chemicals (THSDCs) in fish. Consequently, the present study was designed to demonstrate the suitability of novel thyroid-related endpoints in early life-stages of fish. Embryos of a transgenic zebrafish (Danio rerio) line expressing the reporter gene tg:mCherry in their thyrocytes were used to investigate the effects of the environmental THSDCs triclosan (TCS, antibacterial agent) and benzophenone-2 (BP-2, UV filter) on thyroid follicle and eye development. Both BP-2 and TCS caused thyroid follicle hyperplasia in transgenic zebrafish, thus confirming their role as THSDCs. The effect intensity on follicle size and fluorescence was comparable with a 1.7-fold increase for BP-2 and 1.6-fold for TCS. Alterations of the cellular structures of the retina indicate an impact of both substances on eye development, with a stronger impact of TCS. With respect to guideline development, results provide further evidence for the suitability of morphological changes in thyroid follicles and the eyes as novel endpoints for the sensitive assessment of THSD-related effects in fish.

Early vertebrate development is partially regulated by thyroid hormones (THs). Environmental pollutants that interact with the TH system (TH system–disrupting chemicals [THSDCs]) can have massively disrupting effects on this essential phase. Eye development of fish is directly regulated by THs and can, therefore, be used as a thyroid‐related endpoint in endocrine disruptor testing. To evaluate the effects of THSDC‐induced eye malformations during early development, zebrafish (Danio rerio) embryos were exposed for 5 days postfertilization (dpf) to either propylthiouracil, a TH synthesis inhibitor, or tetrabromobisphenol A, which interacts with TH receptors. Subsequently, one half of the embryos were exposed further to the THSDCs until 8 dpf, while the other half of the embryos were raised in clean water for 3 days to check for reversibility of effects. Continued THSDC exposure altered eye size and pigmentation and induced changes in the cellular structure of the retina. This correlated with morphological alterations of thyroid follicles as revealed by use of a transgenic zebrafish line. Interestingly, effects were partly reversible after a recovery period as short as 3 days. Results are consistent with changes in TH levels measured in different tissues of the embryos, for example, in the eyes. The results show that eye development in zebrafish embryos is very sensitive to THSDC treatment but able to recover quickly from early exposure by effective repair mechanisms. Environ Toxicol Chem 2023;42:1276–1292. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.