The AOP describes the effects of inhibition of deiodinase 1 (DIO1) on anterior swim bladder inflation leading to reduced young of year survival and population trajectory decline. The inhibition of DIO1 is the molecular-initiating event (MIE), which results in decreased circulating concentrations of triiodothyronine (T3) in serum. Disruption of the thyroid hormone (TH) system is increasingly being recognized as an important MoA that can lead to adverse outcomes, especially during embryonic development. In fish, many different adverse effects during early development resulting from disruption of the TH endocrine system have been reported (e.g., effects on body and eye size, head-to-trunk angle, heartbeat, otolith formation, pigmentation index, swim bladder inflation, hatching time, somite formation, escape response and photoreceptor development). As in amphibians, the transition in fish between the different developmental phases, including maturation and inflation of the swim bladder, have been shown to be mediated by THs. Fish larvae exposed to PTU, a known DIO1 inhibitor, has been known to result in impairment of swim bladder inflation. Chemicals interfering with the conversion of T4 to T3 have the potential to inhibit anterior chamber inflation which may result in reduced auditory capacity and reduced swimming capacity of the fish, a relevant adverse outcome that can affect feeding behaviour and predator avoidance, resulting in lower survival probability and ultimately population trajectory decline (Czesny et al., 2005; Woolley and Qin, 2010).