According to a narrative review published in the journal Nutrients by researchers at the University of Guelph and ImmunoCeutica Inc., iodine intake follows a narrow U-shaped relationship that dictates how the micronutrient influences immune defenses across humans and domesticated mammals. While adequate intake supports leukocyte metabolism and thyroid-driven immunity, both chronic deficiency and sustained excess disrupt immune homeostasis, according to the cross-species analysis.
Mechanisms of Iodine and Leukocyte Function
Active iodide transport occurs far beyond the thyroid gland in tissues expressing transporters like the sodium iodide symporter (NIS) or pendrin, according to the review. These sites include salivary glands, gastric mucosa, and immune structures such as the bone marrow, thymus, and spleen, while iodine also accumulates in circulating leukocytes.
Under sufficient iodine intake, thyroid hormone production supports balanced natural killer cell and T and B cell responses. Furthermore, povidone-iodine use promotes effective myeloperoxidase-mediated microbial killing via controlled reactive iodine and oxygen species, while dietary iodide enhances nuclear factor erythroid 2-related factor 2-dependent antioxidant capacity in the blood, mammary gland, and milk, according to the study authors.
Pro Tip: Standard population dietary guidelines from groups like the World Health Organization and Health Canada rely primarily on thyroid-centric parameters to prevent goiter, often leaving immune-specific tissue thresholds undefined, according to the Guelph and ImmunoCeutica researchers.
Immune Disruption From Deficiency and Excess
Iodine imbalance skews T helper populations and activates nuclear factor kappa-light-chain-enhancer of activated B cells, predisposing the body to autoimmune thyroiditis. Under iodine deficiency, reduced thyroid hormone signaling impairs neutrophil and macrophage chemotaxis and reduces T cell proliferation markers, such as CD25 and CD69, according to experimental evidence cited in the review.
Reduced iodide availability also limits myeloperoxidase-driven production of reactive species, weakening pathogen defense. Conversely, chronic iodine excess promotes excessive intrathyroidal iodination that generates reactive oxygen species, causing cell injury and the release of alarmins that trigger local inflammation.
Did You Know? In healthy individuals, acute high-dose iodide exposure triggers the self-limiting Wolff-Chaikoff effect, transiently blocking thyroid hormone synthesis for 24 to 50 hours before NIS downregulation restores homeostasis, according to the review.
Autoimmune Risks and Epigenetic Patterns
Susceptible individuals with structurally abnormal thyroids may develop the Jod-Basedow phenomenon, where affected tissue fails to suppress iodide transport and produces sustained thyrotoxicosis, though the review notes this evidence stems mainly from case reports and small series. In genetically susceptible hosts carrying HLA-DR alleles, heavily iodinated thyroglobulin and thyroid peroxidase peptides are presented more efficiently to autoreactive CD4+ T cells.
Variants affecting CTLA4 and PTPN22 further reduce control over autoreactive responses, while epigenetic analyses associate iodine exposure with altered methylation of chemokine signaling genes, including CCL5, CXCL8, and CXCR5, according to cross-sectional studies evaluated in the paper.
Comparative Impacts in Domesticated Mammals
Comparative veterinary data analyzed in the review identified distinct species-specific outcomes. In dairy cattle, iodine supplementation improved udder health, reduced milk somatic cell counts, and enhanced Fc-γ receptor-mediated phagocytosis, according to cited research.
However, in pregnant ewes, iodine intake several-fold above requirements was accompanied by lower lamb plasma IgG after colostrum consumption, suggesting impaired passive transfer. In companion animals, iodine excess produced goiter and potential primary hypothyroidism in canine puppies, while long-term excess has been identified as a potential contributor to nodular thyroid hyperplasia and hyperthyroidism in aging cats.
Frequently Asked Questions
What is the relationship between iodine and the immune system?
According to the Nutrients review, iodine follows a U-shaped relationship where adequate intake supports leukocyte metabolism and antioxidant capacity, while deficiency or excess disrupts immune homeostasis and increases inflammation.
How does iodine affect pathogen defense?
Sufficient dietary iodide and povidone-iodine use support myeloperoxidase-mediated microbial killing via controlled reactive iodine and oxygen species, whereas deficiency limits these pathogen-clearing mechanisms, according to the study.
What happens during iodine deficiency?
Deficiency reduces thyroid hormone signaling, which impairs neutrophil and macrophage chemotaxis, lowers T cell proliferation markers, and compromises B cell-mediated adaptive immunity, according to experimental and clinical evidence cited in the paper.
Can too much iodine cause thyroiditis?
Yes. Sustained dietary excess promotes oxidative stress, altered cytokine profiles, and autoimmune thyroiditis in genetically susceptible individuals carrying specific risk alleles like HLA-DR, CTLA4, and PTPN22, according to the authors.
Explore More: To stay updated on nutritional immunology research, subscribe to our newsletter or explore our archives for more breakdowns of peer-reviewed studies.