Objective <p>Thyroid function tests are frequently overused. This systematic review aims to summarise the effectiveness of behaviour change interventions to reduce low-value thyroid testing and to identify theoretical foundations and contextual factors associated with their success.</p> Design <p>We conducted a comprehensive search of Medline, Embase, Scopus, and the Cochrane Library for randomised and non-randomised controlled trials as well as before-and-after studies. We followed PRISMA guidelines, critically appraised study quality, and applied the GRADE approach to assess certainty of evidence. We categorised interventions as soft (education, reminders, feedback, guidelines) or structural (change in funding, clinical decision support systems).</p> Results <p>We included 47 studies (54 interventions) including five randomised trials. Structural interventions, particularly clinical decision support systems, were the most common (<i>n</i> = 28). Most interventions reported a reduction in low-value thyroid testing (<i>n</i> = 52), with 40 of them having effects ≥ 20%. However, the certainty of evidence was very low to moderate. Among 49 interventions assessing volume reduction (test rates, expenditure), only two reported increased test rates. All 24 studies that measured improvement of care (appropriateness, shift in ordering pattern, coefficient of variation among physicians) indicated positive developments. Only four interventions referenced theoretical foundations or contextual factors.</p> Conclusions <p>Structural interventions, especially clinical decision support systems, were most effective in reducing thyroid testing. While most interventions showed positive effects, the certainty of evidence remains limited, highlighting the need for more high-quality studies to support robust clinical practice changes. Our results may inform targeted interventions to reduce low-value thyroid testing at national, regional, and local levels.</p>