Iodine in Health and Disease: A Comprehensive Review

Iodine is an essential micronutrient required for the synthesis of thyroid hormones and the maintenance of metabolic, neurodevelopmental and immune function. As iodine cannot be synthesized endogenously, adequate intake depends on dietary sources and environmental availability. Despite decades of progress in improving iodine supply, both iodine deficiency and excess remain significant global public health challenges. This review summarizes iodine physiology, covering both its role in thyroid hormone synthesis and emerging evidence for extrathyroidal immunomodulatory and antioxidant actions. It summarizes major dietary sources, global intake patterns and current approaches to iodine status assessment, including urinary biomarkers, salivary iodide measurement and dietary screening tools. The clinical consequences of iodine imbalance are examined, ranging from goiter, hypothyroidism and impaired neurocognitive development associated with deficiency, to iodine-induced thyroid dysfunction, autoimmunity and adverse systemic effects linked to excess intake. Special attention is given to vulnerable populations, particularly pregnant women and infants. This review further evaluates public health strategies, including salt iodization and targeted supplementation, while addressing the emerging challenge posed by salt-reduction initiatives. Achieving optimal iodine intake remains essential for thyroid health and population well-being, underscoring the need for coordinated monitoring and policy adaptation.