Iodine and Thyroid Dysfunction in Ageing: Nutritional, Pharmacologic, and Microbial Modifiers in Older Adults

Background: Ageing profoundly alters endocrine regulation and nutrient metabolism, predisposing older adults to thyroid dysfunction. Iodine, an essential micronutrient, lies at the center of this vulnerability due to its narrow physiological range and multiple interactions with nutrition, medications, renal function, and, presumably, gut microbiota. Objective: This narrative review integrates evidence on how ageing modifies iodine–thyroid homeostasis, emphasizing the roles of dietary intake, pharmacologic exposures, microbiota composition, and age-related metabolic alterations that influence iodine handling and thyroid hormone economy. Main Findings: Physiological ageing reduces renal iodine clearance, thyroidal reserve, and peripheral hormone conversion, while chronic inflammation and multimorbidity increase susceptibility to both iodine deficiency and excess. Polypharmacy, including amiodarone, lithium, and proton pump inhibitors, further destabilizes thyroid function. Age-related dysbiosis may impair micronutrient absorption and immune tolerance, linking gut ecology to thyroid autoimmunity. The gut microbiota may influence thyroid function through immune and metabolic pathways, although current evidence in older adults remains limited. Together, these factors shift the balance between iodine intake and utilization, heightening the risk of subclinical or overt hypothyroidism in older adults. Conclusions: Overall, variations in iodine intake emerge as one of the main determinants of thyroid dysfunction in ageing with nutritional, pharmacologic, and other modifiers primarily influencing iodine-related thyroid vulnerability. The adoption of age-adjusted thyroid reference ranges and preventive monitoring can reduce overtreatment and improve metabolic resilience in later life.