Iodine

Chemical Properties#

Halogen (Group 17); exists as iodide (I⁻) in biological systems. Absorbed in the stomach and upper small intestine with >90% efficiency. Concentrated in the thyroid via the sodium-iodide symporter (NIS); also concentrated in breast tissue, salivary glands, and gastric mucosa.

Normal serum iodine: 40-80 μg/L; urinary iodine concentration (UIC) is the primary population biomarker.

Dietary#

Seafood: Highest natural source (cod: ~99 μg per 3 oz; seaweed: 16-2,984 μg per sheet depending on type). Dairy: Iodophor sanitizers in dairy processing contribute significantly (1 cup milk: ~56 μg). Iodized salt: 45 μg per 1/4 teaspoon (primary public health intervention against deficiency).

Eggs: ~24 μg per egg. Bread: Iodate-conditioned dough contains variable iodine.

Environmental#

Iodine content in food is highly geography-dependent—soils near coastlines have more iodine than inland/mountainous regions. Goiter belts (Alps, Himalayas, Great Lakes region historically) reflect soil iodine depletion.

Selenium-Iodine Axis#

The most important metal interaction for iodine. Three selenium-dependent enzyme families are required for iodine utilization.^{[1]McGregor Brock 2015 — The Role of Selenium in Thyroid Autoimmunity: A ReviewMcGregor Brock · 2015Open reference 1 ↓}

Deiodinases (DIO1, DIO2, DIO3)—convert T4 to T3 (the active hormone). Without selenium, the thyroid produces T4 it cannot activate. Glutathione peroxidases (GPx)—neutralize the H₂O₂ generated during thyroid hormone synthesis.

Without selenium, iodine-driven H₂O₂ damages thyroid tissue. Thioredoxin reductases—maintain intracellular redox balance in thyrocytes.

Combined selenium-iodine deficiency produces more severe thyroid dysfunction than either alone. Selenium supplementation reduces anti-TPO antibodies in autoimmune thyroiditis.^{[1]McGregor Brock 2015 — The Role of Selenium in Thyroid Autoimmunity: A ReviewMcGregor Brock · 2015Open reference 1 ↓}

Iron-Iodine Interaction#

Thyroid peroxidase (TPO) is an iron-dependent heme enzyme—it catalyzes the iodination of thyroglobulin. Iron deficiency impairs TPO activity, reducing iodine utilization even when iodine status is adequate. Iron-deficient populations respond poorly to iodine supplementation.^{[2]Kravchenko 2023 — Thyroid hormones and minerals in immunocorrection of disorders in autoimmune thyroid diseasesKravchenko V, Zakharchenko T · 2023Open reference 2 ↓}

Heavy Metal Interference#

Cadmium and Mercury compete with iodide for uptake via NIS and interfere with thyroid hormone synthesis. Lead disrupts HPT (hypothalamic-pituitary-thyroid) axis signaling.

These interferences mean that populations with high heavy metal exposure may develop functional iodine insufficiency despite adequate dietary intake—the metals block iodine's biological utilization.

Iodine and the Gut Microbiome#

An emerging area connecting iodine to the gut-thyroid axis.^{[3]Effects of iodine intake on gut microbiota and gut metabolites in Hashimoto thyroiditis-diseased humans and miceGong B, Meng F, Wang X et al. · 2024Open reference 3 ↓}

Excess iodine alters gut microbial composition—high iodine intake shifts microbiota toward increased Enterobacteriaceae and decreased Lactobacillus and Bifidobacterium. This dysbiotic pattern mirrors the Hashimoto's disease microbiome signature. The gut microbiome modulates immune tolerance—Dysbiosis may trigger the autoimmune response against TPO that defines Hashimoto's Thyroiditis.

Thyroid hormones (T3, T4) regulate gut motility; hypothyroidism slows transit time, promoting bacterial overgrowth and further dysbiosis.

Dietary Considerations#

Iodine content in food intersects with other metal exposures in complex ways.^{[4]Role of Food and Nutrition in Pathogenesis and Prevention of Hashimoto's ThyroiditisPuszkarz I, Guty E, Stefaniak I et al. · 2018Open reference 4 ↓}

Seaweed delivers high iodine but may also contain Arsenic (especially hijiki seaweed). Dairy provides iodine with relatively low heavy metal contamination. Seafood provides iodine + selenium (protective combination) but also Mercury (depending on species).

Cruciferous vegetables contain goitrogens (thiocyanates) that competitively inhibit iodide uptake—relevant for populations eating both high-cruciferous and low-iodine diets.