Autoimmune Thyroid Diseases and Physical Activity and Sports—More Unknowns than Facts

<b>Background:</b> Chronic autoimmune thyroiditis, also known as Hashimoto’s thyroiditis (HT) or chronic lymphocytic thyroiditis whose pathophysiology includes both cellular (T-cell mediated) and humoral (B-cell mediated) immune responses, leads to the destruction of thyroid follicular cells and progressive fibrosis of the thyroid gland. While hypothyroidism is a common autoimmune disease, athletes may experience unique challenges related to its diagnosis and management within the context of training programme, competition and anti-doping regulations. In turn, it is known that moderate physical exercise can have a positive effect on the immune system, while excessive exercise can cause unfavourable changes in this system. Therefore, we aimed (1) to identify the interplay between physical activity and autoimmune thyroid disease, (2) to quantify changes in thyroid function associated with physical activity, and (3) to explain the underlying mechanisms of autoimmune thyroiditis in athletes. <b>Methods:</b> The medical database PubMed/MEDLINE was searched in the time period 2004–2025, where 12 publications met the inclusion criteria and were ultimately included for further evaluation according to the RAMESES (Realist and Meta-narrative Evidence Syntheses: Evolving Standards). <b>Results:</b> The reviewed studies have clearly indicated that physical exercise has a beneficial effect on thyroid function, and two studies reported that non-excessive physical exercise leads to a decrease in TPO-Ab concentrations. <b>Conclusions:</b> The beneficial effect of physical exercise on thyroid function and immune response underlines the need for further well-designed studies to formulate specific guidelines for patients with HT, as well as for athletes with autoimmune thyroid disease. Similarly, there is a need to evaluate the prevalence of thyroid hormone use among amateur and professional athletes in order to establish prevention strategies.