This AOP links inhibition of thyroid peroxidase to reproductive toxicity in fish. This AOP describes one adverse outcome that may result from the inhibition of thyroid peroxidase (TPO). Thyroid peroxidase (TPO) is an enzyme for thyroid hormone (TH) synthesis. Chemical inhibition of TPO, the molecular-initiating event (MIE), results in decreased thyroid hormone (TH) synthesis. Reduction of TH induces the decline of E2 and VTG, which leads to decreased cumulative fecundity and spawning. Cumulative fertility is major endpoint for evaluation of reproductive toxicity caused by endocrine disruption. It is used as an endpoint for endocrine disruptor screening in OECD 229. Therefore, this AOP would be useful as a means to identify chemicals with known potential to adversely affect fish populations.