Feedback loop reasoning is an essential part of systems thinking, which includes the analysis and description of system behaviour and regulative measures. In feedback loops, every change can simultaneously represent a cause and an effect. Research on reasoning in feedback loops is limited to investigating students' existing mental models. This thinking-aloud study extends insight to the field by providing causal loop diagrams to examine reasoning related to feedback loops. High school students (N = 30, 14-16 years) analysed system behaviours and regulative measures based on the regulation of blood glucose. We developed a category system examining students' prior knowledge and different reasoning patterns (general feedback loop reasoning, reversing cause-effect relationships, and explanations of equilibrium). Our results indicate that students consider direct, or direct and incomplete indirect cause-effect relationships. However, their response patterns vary depending on different task types (system behaviour, regulative measures, and complex system behaviour). Regarding equilibrium, most students explained it through opposing external factors, while only a few considered feedback loop mechanisms that maintain equilibrium. Therefore, we would recommend that teachers distinguish between the hidden dynamics of regulative processes and the visible maintenance of stable conditions by using appropriate external representations to explain phenomena such as equilibrium.