Iodine and Its Impact on the Immune System of Humans and Domesticated Mammals: A Narrative Review.

Iodine is an essential micronutrient required for coordinating thyroid hormone synthesis, growth, neurodevelopment, metabolism, and immune function across humans and domesticated mammals. Molecular iodine can act as an antioxidant, anti-inflammatory, and anti-proliferative agent in several tissues, but these mechanistic effects require confirmation in well-designed human studies. Recommended intakes fall within a narrow optimal range, reflecting a U-shaped response curve in which adequate intake supports systemic homeostasis, while both deficient and excessive intakes can impact immune function. Chronic deficiency impairs neurodevelopment and reproductive performance and weakens the host microbial defences, while sustained excess promotes oxidative stress, alters cytokine profiles, and in genetically susceptible individuals, increases the risk of subclinical or overt thyroid dysfunction and autoimmune thyroid disease. This review summarizes iodine nutrikinetics, dose-dependent outcomes and susceptibility in humans and domesticated mammalian species, emphasizing the importance of maintaining appropriate iodine status to support immunocompetence while minimizing adverse effects.