The Broad Effect of Iodine in Graves' Hyperthyroidism and Its Relationship with the Gut Microbiota

Thyroid disorders are among the most common endocrine disorders worldwide and are classified as noncommunicable diseases. These disorders are associated with significant morbidity, impaired quality of life, and considerable socioeconomic burden. Like other noncommunicable diseases, thyroid disorders arise from complex interactions between genetic susceptibility and environmental factors, including diet and lifestyle. Despite growing interest in lifestyle-based approaches to noncommunicable disease prevention and management, thyroid disorders have received comparatively limited attention in this context. Graves' disease, the most common cause of hyperthyroidism, is a relevant condition for exploring dietary interventions. Current treatment strategies-anti-thyroid drugs, radioactive iodine and thyroidectomy-have remained largely unchanged for decades. Long-term remission following drug therapy is achieved in no more than approximately 50% of patients, while all treatment modalities carry potential adverse effects. These limitations underscore the need for alternative or adjunctive therapeutic strategies. Iodine intake plays a central role in thyroid hormone synthesis. Indeed, observational studies have shown inverse associations between iodine intake and remission rates, as well as achievement of euthyroidism, medication requirements and thyroid autoantibody titers. These findings suggest that dietary iodine restriction may enhance treatment efficacy and reduce medication-related risks. Beyond its direct effects on thyroid hormone synthesis, iodine may influence Graves' disease through indirect mechanisms involving the lipid profile and the gut-thyroid axis. Autoimmune thyroid diseases are associated with a dyslipidemic profile and with gut microbiota dysbiosis; the latter characterized by increased potentially pathogenic bacteria and reduced beneficial bacteria such as Lactobacillus and Bifidobacterium.