Iodine
MineralPreclinicalAlso known as: I, I-, Iodide, Potassium iodide, KI, Sodium iodide, NaI, Molecular iodine, I2, Lugol's solution, Lugol's iodine, Iodoral, SSKI, Nascent iodine, Iosol, Kelp iodine, Bladderwrack iodine
Iodine is a halogen trace mineral and an obligate substrate for thyroid hormone synthesis — the single biochemical fact that dominates all clinical thinking about iodine. Without adequate iodine the thyroid gland cannot synthesize thyroxine (T4) or triiodothyronine (T3), and the downstream consequences of thyroid hormone deficiency range from fatigue and weight gain in mild adult deficiency to irreversible neurodevelopmental impairment (endemic cretinism) in offspring of severely deficient mothers.
Overview
At A Glance
Iodine's fundamental mechanism is its incorporation into thyroid hormone synthesis, a tightly regulated process confined almost entirely to the thyroid follicular cell. Dietary iodine is absorbed as iodide (I⁻) in the small intestine, enters systemic circulation, and is concentra…
Mechanism of Action
Iodine's fundamental mechanism is its incorporation into thyroid hormone synthesis, a tightly regulated process confined almost entirely to the thyroid follicular cell. Dietary iodine is absorbed as iodide (I⁻) in the small intestine, enters systemic circulation, and is concentrated in the thyroid gland via the sodium-iodide symporter (NIS, SLC5A5) on the basolateral membrane of the thyrocyte. NIS uses the sodium gradient to pump iodide into the thyrocyte against a 20–40-fold concentration gradient, and this concentrating activity is what allows scintigraphic thyroid imaging with radiotracers and the therapeutic uptake of I-131 — the thyroid literally has the most active iodine uptake of any tissue in the body. Once inside the thyrocyte, iodide is transported to the apical membrane via pendrin (SLC26A4), oxidized by thyroid peroxidase (TPO) in the presence of hydrogen peroxide generated by DUOX2, and incorporated (organified) onto tyrosine residues of thyroglobulin — a massive 660 kDa protein scaffold stored in the colloid of the thyroid follicle. Iodination produces monoiodotyrosine (MIT) and diiodotyrosine (DIT), which then undergo TPO-catalyzed coupling: DIT + DIT → T4 (thyroxine, four iodine atoms), DIT + MIT → T3 (triiodothyronine, three iodine atoms). Thyroglobulin with its iodinated hormone residues is then reabsorbed into the thyrocyte by endocytosis, proteolyzed in lysosomes, and T4 and T3 are released into circulation. Circulating T4 is the major thyroid product (90%+ of output by mass); the biologically active hormone is T3, which is generated primarily from T4 in peripheral tissues by selenium-dependent deiodinases (D1, D2, D3). This is where the Selenium dependency becomes critical — without adequate selenium, T4 activation to T3 is impaired even when iodine intake and thyroid gland function are preserved. The thyroid gland itself has the highest selenium concentration per gram of any tissue precisely because of this deiodinase activity, along with glutathione peroxidase's role in protecting the gland from the oxidative stress of hormone synthesis. Iodine is also regulated at the uptake level by the Wolff-Chaikoff effect: when intrathyroidal iodide concentrations exceed a threshold (roughly corresponding to acute doses above 1–2 mg iodine), thyroid hormone synthesis is transiently suppressed to protect against hormone overproduction. Most people escape the Wolff-Chaikoff block within 24–48 hours via NIS downregulation, but patients with underlying thyroid autoimmunity (Hashimoto) may fail to escape and develop persistent iodine-induced hypothyroidism. Conversely, in iodine-deficient populations or patients with autonomous thyroid nodules, a sudden iodine load can trigger Jod-Basedow hyperthyroidism when previously-suppressed autonomous nodules encounter abundant substrate. Iodine also has NIS-mediated uptake in lactating breast tissue (essential for fetal/neonatal thyroid development via breast milk), salivary glands, gastric mucosa, lacrimal glands, thymus, and ovary — this broader distribution underlies the "extrathyroidal iodine" discussions in the high-dose iodine literature, though strong clinical outcome data for high-dose iodine in breast health remains limited. Beyond thyroid hormone synthesis, iodine has direct antimicrobial activity (the basis of povidone-iodine antiseptics) and has been shown in vitro to modulate estrogen receptor activity and induce apoptosis in some cell lines, but these effects are pharmacologic rather than nutritional and do not translate into established oral supplementation indications. The bottom line mechanistically is that iodine is the irreplaceable substrate for thyroid hormone and its biology is dominated by thyroid physiology; dosing above what the thyroid can use or handle produces thyroid dysfunction in both directions, which is why the therapeutic window is narrow compared to most trace minerals.
Overview
Iodine is a halogen trace mineral and an obligate substrate for thyroid hormone synthesis — the single biochemical fact that dominates all clinical thinking about iodine. Without adequate iodine the thyroid gland cannot synthesize thyroxine (T4) or triiodothyronine (T3), and the downstream consequences of thyroid hormone deficiency range from fatigue and weight gain in mild adult deficiency to irreversible neurodevelopmental impairment (endemic cretinism) in offspring of severely deficient mothers. Iodine deficiency is the leading global cause of preventable intellectual disability, which the WHO has pursued aggressively through universal salt iodization programs since the 1990s, driving dramatic reductions in goiter prevalence and cretinism in previously affected regions. The adult RDA is 150 mcg/day, pregnancy 220 mcg/day, lactation 290 mcg/day, and the tolerable upper limit for adults is 1,100 mcg/day — a relatively narrow therapeutic window compared to most minerals, reflecting the fact that both deficiency and excess can destabilize thyroid function. The iodine story is geographically and historically uneven: populations living far from coastlines and consuming iodine-poor soils (the Alpine countries, the Great Lakes region of the United States, central Africa, parts of Southeast Asia, and the Himalayan plateau) developed endemic goiter and cretinism over generations before iodized salt became routine. The United States began salt iodization in 1924 in response to the "goiter belt" around the Great Lakes, and iodine deficiency became rare in subsequent decades — but recent NHANES data show that median urinary iodine concentrations have been trending downward since the 1970s, mild-to-moderate iodine insufficiency has re-emerged in some US subpopulations especially pregnant women, and the iodine content of commercial dairy (historically a major American iodine source via iodophor sanitizers and iodine-supplemented feed) has become more variable. At the same time a vocal subculture promotes high-dose iodine supplementation — Lugol's solution, Iodoral, "SSKI" (saturated solution of potassium iodide), and nascent iodine products delivering 12.5–50 mg per dose, which is 100–400× the RDA — citing Abraham's "iodine project" claims of breast and thyroid benefits. This high-dose iodine philosophy is not supported by mainstream endocrinology: the tolerable upper limit of 1,100 mcg/day exists because doses above that have caused iodine-induced hyperthyroidism in susceptible individuals (Jod-Basedow phenomenon) and paradoxical iodine-induced hypothyroidism (Wolff-Chaikoff effect escape failure) in others, and chronic high-dose iodine has precipitated or worsened autoimmune thyroid disease in people with underlying Hashimoto susceptibility. Iodine is also the substrate for radioiodine therapy (I-131) used to treat hyperthyroidism and thyroid cancer, and stable iodine (potassium iodide, KI) is the FDA-approved prophylaxis against radioactive iodine uptake in nuclear emergencies. Dietary sources concentrate in seafood (seaweed especially kelp and kombu, which can deliver 1–2 mg per gram dry weight, followed by cod, tuna, shrimp), iodized salt (the dominant population-level source), dairy from iodine-supplemented herds, and eggs. See also Selenium for the obligate thyroid cofactor partnership (selenium-dependent deiodinases activate T4 to T3), Vitamin D for the broader thyroid-autoimmunity discussion, Zinc for the pituitary-thyroid axis trace mineral contributions, and Vitamin B12 for pernicious anemia's association with Hashimoto disease in polyglandular autoimmunity. This overview is educational only and is not medical advice — iodine sits in a narrower therapeutic window than most nutrients, and patients with thyroid disease should not self-dose iodine without endocrinology guidance.
Chemical Information
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Contraindications
Iodine supplementation above the RDA is contraindicated or requires special caution in several clinical contexts. Active Graves hyperthyroidism is a relative contraindication for supplemental iodine (SSKI used preoperatively is specialist-directed, short-term, and aimed at thyroid gland reduction before surgery — not general supplementation). Autonomous thyroid nodules or multinodular goiter in older adults predispose to Jod-Basedow iodine-induced hyperthyroidism; iodine supplementation in these patients should be limited to RDA range and avoid Lugol's/Iodoral/SSKI forms without endocrinology evaluation. Known Hashimoto thyroiditis is a relative contraindication for high-dose iodine: multiple case series document worsening autoimmunity, rising TPO antibodies, and progression to overt hypothyroidism in Hashimoto patients exposed to mg-range iodine, and mainstream endocrinology recommends against supra-RDA iodine in these patients. Dermatitis herpetiformis is a specific iodine contraindication — iodine exposure can trigger or exacerbate the blistering rash. Acneiform "iododerma" can occur with chronic high-dose iodine and is a reason to stop supplementation. Pregnancy: RDA-range iodine (150–220 mcg/day) is strongly recommended given the fetal neurodevelopmental stakes, but high-dose iodine in pregnancy (above 1,100 mcg/day routinely, or certainly above 12.5 mg/day as in Lugol's) can cause fetal goiter and transient neonatal hypothyroidism and is contraindicated outside specific specialist-directed use. Breastfeeding: moderate iodine supplementation (290 mcg/day lactation RDA) is recommended; high-dose supplementation transfers to breast milk and can cause neonatal hypothyroidism. Neonates and premature infants are exquisitely iodine-sensitive and should not be exposed to povidone-iodine antiseptics on skin or mucous membranes except for specific clinical indications — topical povidone-iodine has caused iodine-induced hypothyroidism in NICU populations. Pediatric iodine supplementation should be RDA-range and pediatrician-directed. Thyroid cancer patients scheduled for I-131 ablation or scintigraphy must follow a low-iodine diet for 2–4 weeks prior and avoid all iodine-containing supplements. Patients on amiodarone (37% iodine by weight) should not take supplemental iodine; amiodarone alone already delivers pharmacologic iodine loads and thyroid dysfunction is managed through amiodarone management and endocrinology. Lithium-treated patients (both lithium and iodine suppress thyroid function) should avoid high-dose iodine and have thyroid function monitored. Shellfish allergy is not an iodine contraindication (the allergen is tropomyosin, not iodine), but IgE-mediated reactions to iodinated contrast media, povidone-iodine antiseptics (often the PVP carrier), and some topical products are real and patients with documented reactions should avoid those specific products. Drug interactions with thionamide antithyroid drugs (methimazole, PTU), radioiodine, amiodarone, and lithium are the clinically important ones. For most healthy euthyroid adults without the above risk factors, RDA-range iodine is safe and beneficial, with the cautious framework that supplementation above 1,100 mcg/day should have a specific clinical indication and endocrinology oversight. This is general educational content, not medical advice, and iodine dosing decisions in patients with thyroid disease or family history should involve an endocrinologist.
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This interaction data is compiled from published research and community reports. It may not be exhaustive. Always consult a healthcare professional before combining compounds.
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Related Compounds
View AllBoron
MineralPreclinicalBoron is an ultra-trace element whose nutritional status in humans sits in a distinctive regulatory gray zone: the Institute of Medicine (US) has not established a recommended dietary allowance (RDA) or estimated average requirement (EAR) for boron because the evidence for essentiality in humans does not meet the strict criteria applied to calcium, iron, or zinc, yet the IOM, the European Food Safety Authority (EFSA), and the World Health Organization (WHO) all set tolerable upper intake levels (ULs) — implicitly acknowledging that boron has biological activity and dose-response safety concerns.
Calcium
MineralPreclinicalCalcium is the most abundant mineral in the human body — roughly 1,000 to 1,500 grams in a 70 kg adult, with 99% sequestered in the skeleton and teeth as crystalline hydroxyapatite [Ca10(PO4)6(OH)2], and the remaining 1% distributed across extracellular fluid, intracellular cytoplasm, mitochondria, and the endoplasmic/sarcoplasmic reticulum.
Chromium
MineralPreclinicalChromium is a transition metal that occupies one of the more peculiar positions in human nutrition: long marketed as essential for carbohydrate metabolism and insulin sensitization, the evidence for chromium essentiality has progressively softened over the past two decades, and both the European Food Safety Authority (EFSA 2014) and multiple independent reviews have concluded that chromium III is not definitively essential for humans.
Copper
MineralPreclinicalCopper is an essential trace mineral that most adults get in adequate amounts from a varied omnivorous diet — but that routinely drops into functional insufficiency when people take long-term high-dose zinc supplements, consume highly processed diets, undergo bariatric surgery, or use copper-chelating therapies.
Iron
MineralPreclinicalIron is a trace mineral with a biochemistry dominated by a single chemical property — the reversible one-electron redox between Fe²⁺ (ferrous) and Fe³⁺ (ferric) — that makes it indispensable for oxygen transport, electron transfer, and hundreds of enzymatic reactions, and simultaneously dangerous when unchaperoned in cells.
Manganese
MineralPreclinicalManganese is an essential trace mineral and redox-active transition metal occupying a peculiar place in human nutrition: absolutely required at milligram doses for mitochondrial antioxidant defense, gluconeogenesis, urea cycle function, and connective tissue synthesis — yet potently neurotoxic at the hundredfold-higher doses encountered occupationally (welders, miners, battery workers) and in patients on long-term parenteral nutrition with inadequately controlled trace mineral content.
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This information is for educational and research purposes only. Not intended as medical advice. Consult a healthcare professional before use.
Frequently Asked Questions
How much iodine do I need per day?
The adult RDA is 150 mcg/day; pregnancy 220 mcg/day; lactation 290 mcg/day; the tolerable upper limit for adults is 1,100 mcg/day. If you use iodized salt regularly and eat seafood or dairy, you likely meet the 150 mcg RDA from food. For pregnancy and lactation, most prenatals should contain 150 mcg iodine as potassium iodide — but many commonly-prescribed prenatals still do not, so check the label. The American Thyroid Association 2017 position statement recommends 150 mcg iodine in prenatal vitamins (PMID 28056690). Exceeding 1,100 mcg/day without specific indication is not appropriate for most people.
Should I take Lugol's solution or Iodoral?
For most healthy adults, no. Lugol's (typically 6.25 mg total iodine per drop) and Iodoral (12.5–50 mg per tablet) deliver 40–400× the RDA in a single dose. This is not supplementation — it's pharmacologic dosing. The high-dose iodine subculture promotes these products for fibrocystic breast disease, breast cancer prevention, fibromyalgia, chronic fatigue, and general 'iodine sufficiency,' but the rigorous clinical trial evidence does not support efficacy for these indications and multiple case series document iodine-induced hyperthyroidism and iodine-induced hypothyroidism at these doses, especially in patients with underlying Hashimoto or autonomous nodules (PMID 24601693, 22095470). If you are considering Lugol's or Iodoral, do so only under endocrinology supervision with baseline and follow-up thyroid function and antibody testing.
Is iodized salt enough for me?
For most people eating a normal diet with regular iodized salt use, yes. Iodized salt in the US typically contains 45–76 mcg iodine per gram, and the average American salt intake from home cooking plus processed foods provides adequate iodine. However, commercial food processors often use non-iodized salt, sea salt is not naturally iodized (despite folk belief), and people on low-sodium diets or cooking predominantly from fresh ingredients with non-iodized salt can develop marginal deficiency. Pregnant women have increased requirements (220 mcg/day) that may not be met by iodized salt alone, and most prenatal vitamins should contain 150 mcg iodine to close the gap. NHANES data show median US urinary iodine concentrations have trended downward since the 1970s and a substantial fraction of pregnant women have UIC below the recommended threshold (PMID 23386645).
Do I need selenium with iodine?
Yes, this is one of the most important stacking pairs in thyroid biology. Selenium-dependent deiodinases (D1, D2, D3) convert thyroxine (T4) to active triiodothyronine (T3) in peripheral tissues, and selenium-dependent glutathione peroxidase protects the thyroid gland from the oxidative stress of hormone synthesis. Iodine repletion without adequate selenium produces impaired peripheral T3 generation, and selenium in iodine-deficient regions has been associated with worsening hypothyroidism. The practical approach: pair selenium 100–200 mcg/day (selenomethionine preferred) with iodine at RDA range. In pregnancy both matter for fetal neurodevelopment (PMID 31218075). See the Selenium entry for the full trial and mechanism discussion.
Can iodine cure Hashimoto thyroiditis?
No — and iodine supplementation above the RDA can worsen Hashimoto. Hashimoto is an autoimmune condition driven by antibodies against thyroid peroxidase and thyroglobulin, and iodine substrate availability is not the core problem. Epidemiologic and clinical data show that iodine intake has a U-shaped relationship with Hashimoto incidence — both deficiency and excess increase risk (PMID 31218075). Case series document worsening autoimmunity and progression to overt hypothyroidism in Hashimoto patients exposed to Lugol's or Iodoral doses (PMID 22095470). If you have Hashimoto, keep iodine at RDA range through iodized salt and a standard prenatal/multivitamin, prioritize selenium 100–200 mcg/day and vitamin D sufficiency, and work with endocrinology rather than self-dosing high-dose iodine.
What does iodine do for the thyroid?
Iodine is the obligate substrate for thyroid hormone synthesis — the thyroid gland cannot make T4 or T3 without iodine. The process: dietary iodide is concentrated in the thyroid via the sodium-iodide symporter (NIS), oxidized by thyroid peroxidase (TPO), and incorporated onto tyrosine residues of thyroglobulin, then coupled to form T4 and T3. T4 is the major output and is activated to T3 in peripheral tissues by selenium-dependent deiodinases. Without adequate iodine, hormone output falls, TSH rises, the gland enlarges (goiter) in an attempt to capture more iodine, and in severe deficiency especially during pregnancy, fetal neurodevelopment is compromised, producing cretinism at the extreme or more subtle IQ reductions at the milder end (PMID 17920027, 23706508).
Can iodine help with fibrocystic breast disease?
Modest signal but not established as standard therapy. Ghent 1993 showed molecular iodine (I2, as opposed to iodide) reduced fibrocystic breast pain in a small RCT, and there is cell-line data suggesting iodine modulates estrogen signaling in breast tissue (PMID 8221402). However, larger trials have been limited, molecular iodine is not the standard form in most supplements (which use iodide), and mainstream breast specialists do not routinely recommend iodine for fibrocystic disease. If symptoms are significant, discuss with your breast health provider; caffeine reduction, vitamin E trials, and standard analgesics have comparable evidence bases and fewer thyroid concerns than mg-range iodine dosing.
Should I stock potassium iodide for a nuclear emergency?
Reasonable if you live near a nuclear power plant or in an area at elevated risk of radiologic/nuclear incidents. The FDA-approved adult dose is 130 mg potassium iodide (~100 mg elemental iodine) once at the time of exposure, repeated daily if exposure continues; pediatric doses are 65 mg (ages 3–18) and lower for younger children per FDA/CDC tables. KI works by saturating the thyroid NIS transporter so that circulating radioactive iodine (I-131) cannot be taken up by the thyroid, preventing radiation-induced thyroid cancer — dramatically demonstrated in the Chernobyl thyroid cancer cohort where Polish children who received KI had much lower rates of thyroid cancer than unprotected Ukrainian/Belorussian children (PMID 8995025). Stock the tablets in original sealed foil packaging for 5+ year shelf life; use only when instructed by public health authorities during an actual release.
What foods are highest in iodine?
Seaweed is the densest dietary source by a wide margin: kelp and kombu can deliver 1–2 mg (1,000–2,000 mcg) iodine per gram dry weight, which is above the tolerable upper limit in a single serving — this is the reason for caution about seaweed in pregnancy. Nori (sushi seaweed) is more moderate at ~200 mcg per gram. Other strong sources: cod (~100 mcg per 3 oz), shrimp (~35 mcg per 3 oz), canned tuna (~17 mcg per 3 oz), dairy (~70 mcg per cup milk), eggs (~25 mcg per egg), and iodized salt (~60–100 mcg per gram). Non-dairy vegans who don't use iodized salt or regular seaweed can be at risk of iodine inadequacy and should use iodized salt in cooking or a 150 mcg iodine supplement. Most omnivores using iodized salt and eating any seafood or dairy meet the RDA from food alone.
Can I take iodine if I'm on levothyroxine?
Generally yes at dietary and RDA-range intake, but avoid high-dose iodine supplements without endocrinology guidance. Patients on levothyroxine can continue normal iodized salt use, seafood consumption, and standard multivitamins containing 150 mcg iodine. What to avoid: Lugol's, Iodoral, SSKI, and large seaweed doses can destabilize thyroid hormone requirements and worsen underlying Hashimoto, which is the most common reason people are on levothyroxine in the first place. If you're scheduled for radioiodine imaging or therapy, you'll need to follow a low-iodine diet for 2–4 weeks before the procedure. Coordinate any planned iodine supplementation above the standard multivitamin dose with your prescribing clinician.
Research Tools
Related Compounds
View AllBoron
MineralPreclinicalBoron is an ultra-trace element whose nutritional status in humans sits in a distinctive regulatory gray zone: the Institute of Medicine (US) has not established a recommended dietary allowance (RDA) or estimated average requirement (EAR) for boron because the evidence for essentiality in humans does not meet the strict criteria applied to calcium, iron, or zinc, yet the IOM, the European Food Safety Authority (EFSA), and the World Health Organization (WHO) all set tolerable upper intake levels (ULs) — implicitly acknowledging that boron has biological activity and dose-response safety concerns.
Calcium
MineralPreclinicalCalcium is the most abundant mineral in the human body — roughly 1,000 to 1,500 grams in a 70 kg adult, with 99% sequestered in the skeleton and teeth as crystalline hydroxyapatite [Ca10(PO4)6(OH)2], and the remaining 1% distributed across extracellular fluid, intracellular cytoplasm, mitochondria, and the endoplasmic/sarcoplasmic reticulum.
Chromium
MineralPreclinicalChromium is a transition metal that occupies one of the more peculiar positions in human nutrition: long marketed as essential for carbohydrate metabolism and insulin sensitization, the evidence for chromium essentiality has progressively softened over the past two decades, and both the European Food Safety Authority (EFSA 2014) and multiple independent reviews have concluded that chromium III is not definitively essential for humans.
Copper
MineralPreclinicalCopper is an essential trace mineral that most adults get in adequate amounts from a varied omnivorous diet — but that routinely drops into functional insufficiency when people take long-term high-dose zinc supplements, consume highly processed diets, undergo bariatric surgery, or use copper-chelating therapies.
Iron
MineralPreclinicalIron is a trace mineral with a biochemistry dominated by a single chemical property — the reversible one-electron redox between Fe²⁺ (ferrous) and Fe³⁺ (ferric) — that makes it indispensable for oxygen transport, electron transfer, and hundreds of enzymatic reactions, and simultaneously dangerous when unchaperoned in cells.
Manganese
MineralPreclinicalManganese is an essential trace mineral and redox-active transition metal occupying a peculiar place in human nutrition: absolutely required at milligram doses for mitochondrial antioxidant defense, gluconeogenesis, urea cycle function, and connective tissue synthesis — yet potently neurotoxic at the hundredfold-higher doses encountered occupationally (welders, miners, battery workers) and in patients on long-term parenteral nutrition with inadequately controlled trace mineral content.
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