From Diet to Drugs: Integrative Perspectives on Iodine, Thyroid Dysfunction, and Microbiota in the Elderly

Background/Objectives: Iodine intake demonstrates a U-shaped relationship with thyroid dysfunction, with both deficiency and excess contributing to adverse outcomes. Older adults are especially susceptible due to age-related alterations in thyroid physiology, reduced functional reserve, impaired adaptation to iodine excess, comorbidities, and polypharmacy. This review aims to synthesize evidence on how ageing influences iodine–thyroid interactions and to identify factors that complicate clinical management in older populations. Methods: A narrative review of the literature was conducted, focusing on studies addressing iodine metabolism, thyroid function, and the modifying roles of gut microbiota, nutrient cofactors, pharmacological exposures, renal function, and metabolic ageing in older adults. Results: Ageing affects iodine handling and thyroid function through multiple mechanisms. Dysbiosis may contribute to thyroid autoimmunity and hormone metabolism via immune modulation, micronutrient utilization, and enterohepatic recycling. Declining renal clearance prolongs iodide retention, while the frequent use of amiodarone, iodinated contrast, lithium, and medications interfering with levothyroxine absorption increases iatrogenic risk. Concurrent metabolic changes—such as adiposity, insulin resistance, and chronic low-grade inflammation—further impair iodine utilization and thyroid hormone action. Conclusions: Recognition of the complex interactions between ageing, iodine metabolism, and thyroid function is essential for accurate diagnosis and individualized management in older adults. Strategies should incorporate age-adjusted reference ranges, systematic medication review, micronutrient optimization, and iodine prophylaxis policies compatible with salt-reduction initiatives. Emerging microbiome-targeted interventions warrant further investigation as potential modulators of iodine–thyroid dynamics in ageing populations.