The ocean is the planet’s main reservoir of iodine, and the foods most reliably rich in it reflect that origin: seaweed, fish, shellfish, and the dairy products and eggs of animals fed iodine-supplemented diets. For most people in industrialized countries, though, the single largest contributor is iodized salt, a public health measure introduced roughly a century ago. The picture gets more interesting once you look at how dramatically iodine content varies within each category and how easily certain dietary patterns can leave you short.
Seaweed Stands in a Class of Its Own
No food comes close to seaweed for raw iodine concentration, but the range across species is staggering. Kombu (a type of kelp) averages around 2,500 milligrams per kilogram of dry weight, while nori, the thin sheets wrapped around sushi rolls, averages closer to 37 mg/kg, roughly seventy times less.1Journal of Food and Drug Analysis. Analysis of iodine content in seaweed by GC-ECD and estimation of iodine intake – Section: 3.2. Iodine contents in seaweed products Wakame falls somewhere in between. Even within a single species, iodine content swings wildly depending on growing conditions, harvest timing, and processing. One analysis of twelve seaweed species found concentrations ranging from 16 micrograms per gram in nori to over 8,000 µg/g in processed kelp granules sold as a salt substitute.2PubMed. Variability of iodine content in common commercially available edible seaweeds Young, freshly harvested kelp blades contained far more iodine than sun-bleached older ones from the same location.
Brown seaweeds like kelp and Saccharina tend to have the highest concentrations, but red seaweeds can also be meaningful sources. Palmaria, a red species, has been measured at up to 293 µg/g dry weight, putting it in a range that could meaningfully contribute to daily intake.3Food Chemistry. Iodine content in bulk biomass of wild-harvested and cultivated edible seaweeds: Inherent variations determine species-specific daily allowable consumption The practical takeaway is that “eat seaweed for iodine” is incomplete advice without specifying which seaweed and how much. A small serving of kombu in a soup broth can deliver dozens of times the recommended daily intake, while the same weight of nori barely makes a dent.
Dairy, Eggs, and the Feed Connection
In countries like the United States, the United Kingdom, and much of Europe, dairy is one of the top dietary iodine sources for the general population, not because milk naturally contains a lot of iodine, but because of how cows are fed and managed. Milk iodine concentrations in industrialized countries range from about 33 to 534 micrograms per liter, shaped by factors including the iodine content of cattle feed, the season, milk yield, and whether iodine-containing disinfectants are used during milking.4PubMed. Iodine in dairy milk: Sources, concentrations and importance to human health Winter milk tends to have more iodine than summer milk in many regions, partly because cattle eat more supplemented feed during winter and graze less on pasture.
Eggs follow a similar pattern. The iodine in an egg depends heavily on the hen’s diet. Studies feeding laying hens increasing levels of iodine have shown that egg iodine content rises in a dose-dependent way, and the form of iodine in the feed matters too. Potassium iodide in hen diets leads to higher iodine deposition in eggs compared to calcium iodate.5PubMed. The effects of iodine level and source on iodine carry-over in eggs and body tissues of laying hens Iodine-enriched yeast fed to hens can boost egg iodine content by roughly 80 to 90 percent compared to standard feed.6Poultry Science. Effect of iodine-enriched yeast supplementation of diet on performance of laying hens, egg traits, and egg iodine content This means the eggs at your grocery store could vary meaningfully in iodine depending on how the birds were raised, and there is rarely any labeling to help you tell.
Iodized Salt and the Public Health Story
The single most impactful source of iodine worldwide is not a food at all but a fortification policy. Universal salt iodization programs, now adopted in some form by most countries, have driven dramatic reductions in iodine deficiency disorders. Globally, the prevalence of clinical iodine deficiency disorders dropped from about 13 percent to roughly 3 percent, preventing an estimated 720 million cases. The resulting improvements in cognitive development and future earnings represent an estimated economic benefit of nearly $33 billion.7PubMed Central. Estimating the Health and Economic Benefits of Universal Salt Iodization Programs to Correct Iodine Deficiency Disorders
That said, the iodine in your salt is not indestructible. How you cook with it matters. One study found that iodine losses from iodized salt during cooking ranged from about 7 percent in shallow frying (where the salt is exposed to heat for only a minute or so) to over 50 percent in pressure cooking, where exposure times are much longer. Boiling caused losses around 40 percent, while roasting, deep frying, and microwaving fell somewhere in the middle.8PubMed Central. Effect of different cooking methods on iodine losses So the iodine content listed on the salt label is a ceiling, not a guarantee of what ends up on your plate.
For processed foods made with iodized salt, the picture is somewhat better. In meat and fish products prepared with potassium iodate-fortified salt, iodine content remained stable through fermenting, freezing, smoking, and storage. The one exception was ham after heating, where iodine was lost along with the cooking brine.9PubMed. Influence of processing and storage on the iodine content of meat and fish products using iodized salt Baking also preserved iodine well in vegetables, while boiling with iodized salt did not effectively transfer iodine into the food being cooked.10PubMed. Stability of iodine during cooking: investigation on biofortified and not fortified vegetables
Drinking Water as a Wildcard
Most people do not think of tap water as an iodine source, but depending on where you live, it can be significant. Iodine concentrations in groundwater and surface water vary enormously with local geology. In some regions of Brazil, the iodine in drinking water alone can cover more than 70 percent of daily requirements for all age groups.11PubMed Central. Iodine Concentration in Brazilian Drinking Water and Its Possible Contribution to Iodine Intake for Different Physiological Groups A study modeling Danish drinking water found that the contribution to daily intake ranged from essentially zero in some areas to over 100 percent of the WHO recommendation for adults in others.12PubMed. Assessment of spatial variation in drinking water iodine and its implications for dietary intake: a new conceptual model for Denmark This geographic variability is one reason why populations inland or at high altitude have historically been more prone to goiter and other deficiency disorders: the farther you get from the coast and its iodine-rich soils, the less iodine tends to show up in local food and water.
The Vegan and Plant-Based Gap
If you remove dairy, eggs, and fish from your diet, your remaining natural food sources of iodine become thin. Fruits, grains, and vegetables contain relatively little iodine, and the amount they do contain depends on how much iodine was in the soil where they were grown. A narrative review of the evidence concluded that people following a strict plant-based diet cannot reach recommended iodine intake from food alone and are reliant on supplements.13PubMed Central. Iodine and plant-based diets: a narrative review and calculation of iodine content
This is not a theoretical concern. A cross-sectional study of children in the Czech Republic found that roughly 42 percent of vegan children and 35 percent of vegetarian children had urinary iodine concentrations below 100 µg/L (the threshold for deficiency), compared to about 20 percent of omnivore children.14European Journal of Clinical Nutrition. Prevalence of iodine deficiency among vegan compared to vegetarian and omnivore children in the Czech Republic: cross-sectional study Severe deficiency was found exclusively among vegan and vegetarian children in that sample.
One would think plant-based milk alternatives could fill the gap, but the reality is disappointing. A UK market survey found that only 28 percent of plant-based milk alternatives were fortified with iodine, even though 88 percent were fortified with calcium. Swapping three daily portions of dairy with unfortified alternatives would slash the iodine those portions provide by about 98 percent.15PubMed Central. Iodine fortification of plant-based dairy and fish alternatives: the effect of substitution on iodine intake based on a market survey in the UK Measured iodine concentrations in plant-based milks were about ten times lower than in dairy milk.16PubMed. Iodine in plant-based dairy products is not sufficient in the UK: A market survey If you are switching to plant-based products, checking labels specifically for iodine fortification is worth the extra few seconds.
How Well Your Body Absorbs Iodine from Different Sources
Knowing the iodine content of a food only tells you part of the story. Bioavailability, how much your body actually takes up, varies with the source. The good news is that iodine from cow’s milk appears to be absorbed about as efficiently as iodine from a simple supplement. A randomized crossover study in healthy adults found no significant difference in urinary iodine excretion between iodine naturally present in milk, iodine externally added to milk, or an aqueous iodine supplement.17The American Journal of Clinical Nutrition. Iodine bioavailability from cow milk: a randomized, crossover balance study in healthy iodine-replete adults
Seaweed-based iodine is a bit different. A randomized trial comparing a sushi and wakame meal to a potassium iodide supplement found that iodine bioavailability from the seaweed meal was about 75 percent over 24 hours, compared to 97 percent from the supplement.18PubMed Central. Bioavailability of iodine from a meal consisting of sushi and a wakame seaweed salad-A randomized crossover trial That gap is small enough that seaweed is still considered a useful iodine source, but it means you cannot assume that all the iodine measured in a seaweed product translates directly into absorbed iodine.
Foods That Can Interfere with Iodine Uptake
Certain vegetables, particularly those in the Brassica family (broccoli, cabbage, kale, Brussels sprouts, cauliflower), contain compounds called glucosinolates that can interfere with iodine metabolism. These compounds and their breakdown products can inhibit the mechanism thyroid cells use to absorb iodine, as well as the enzyme involved in making thyroid hormones.19PubMed Central. Do Brassica Vegetables Affect Thyroid Function?—A Comprehensive Systematic Review For people with adequate iodine intake, eating normal amounts of these vegetables is not a problem. The concern is more relevant for people who are already borderline deficient and eating large quantities of raw cruciferous vegetables regularly. Cooking reduces the goitrogenic activity of these foods considerably.
The Risk of Getting Too Much
Iodine deficiency gets most of the attention, but too much iodine carries its own risks, and the most common culprit is kelp-based supplements and diet products. Both deficiency and excess can lead to thyroid dysfunction, which can show up as either an underactive or overactive thyroid.20PubMed Central. Commercially available kelp and seaweed products – valuable iodine source or risk of excess intake? Case reports have documented thyrotoxicosis (an overactive thyroid) in people consuming kelp-containing diet products, even in individuals with no prior thyroid disease.21PubMed Central. Thyroid dysfunction following a kelp-containing marketed diet Complementary supplements containing kelp have triggered the same problem.22PubMed Central. Transient Hyperthyroidism following the ingestion of complementary medications containing kelp seaweed A case-report
The issue is compounded by the inherent variability in kelp products. Because iodine content differs so much between batches, a seaweed supplement labeled at one dose might deliver several times that amount in practice. Long-term excess intake has been linked to hypothyroidism, goiter, and autoimmune thyroid disease in larger studies.23PubMed Central. Commercially available kelp and seaweed products – valuable iodine source or risk of excess intake? If you are taking a kelp supplement, doing so under medical supervision with periodic thyroid checks is the cautious approach.
Why Iodine Matters So Much During Pregnancy
Iodine’s main role in the body is as a building block for thyroid hormones, which regulate metabolism, growth, and development.24PubMed Central. Iodine: Its Role in Thyroid Hormone Biosynthesis and Beyond That role becomes especially critical during pregnancy and early childhood. When a pregnant woman does not get enough iodine, both she and the fetus cannot produce adequate thyroid hormones, and because those hormones are essential for fetal brain development, the consequences can be severe and irreversible. Iodine deficiency disorders remain the single greatest cause of preventable brain damage in fetuses and infants worldwide.25PubMed Central. Iodine consumption and cognitive performance: Confirmation of adequate consumption
In children, even moderate deficiency can lower IQ and impair psychomotor development.26PubMed Central. Health consequences of iodine deficiency Animal studies have demonstrated that iodine deficiency reduces brain weight, slows brain cell maturation, and disrupts the development of the cerebral cortex and cerebellum, with effects detectable well before birth.27The Journal of Nutrition. Iodine and Neuropsychological Development Despite global progress through salt iodization, an estimated 4.8 million newborns are still affected by iodine deficiency disorders each year.28PubMed Central. Estimating the Health and Economic Benefits of Universal Salt Iodization Programs to Correct Iodine Deficiency Disorders
How Iodine Status Is Actually Measured
If you are wondering whether you get enough iodine, there is no convenient home test. The standard method used in public health surveys is urinary iodine concentration, because most ingested iodine ends up being excreted through urine within 24 hours. A 24-hour urine collection is considered the gold standard for assessing iodine status, and recent work suggests it may be useful as an individual marker as well, not just a population average.29PubMed Central. Reflection of Dietary Iodine in the 24 h Urinary Iodine Concentration, Serum Iodine and Thyroglobulin as Biomarkers of Iodine Status: A Pilot Study That said, reliable individual-level biomarkers remain a gap in the field. Spot urine samples, the kind collected in a doctor’s office, bounce around too much from day to day to tell you your personal status with confidence.30The American Journal of Clinical Nutrition. Urinary iodine, thyroid function, and thyroglobulin as biomarkers of iodine status The practical implication is that instead of testing, most people are better served by ensuring their diet includes reliable iodine sources consistently.
Biofortification of Crops
Beyond salt iodization, researchers have been exploring ways to boost iodine levels in staple crops themselves. The idea is to add iodine to soil or irrigation water so that plants take it up and deliver it to consumers who might not use enough iodized salt. This agronomic biofortification approach works well in some plant species, effectively raising their iodine content, though certain important staple crops may require genetic engineering to achieve meaningful iodine accumulation.31PubMed. Iodine biofortification of crops: agronomic biofortification, metabolic engineering and iodine bioavailability The strategy is still mostly experimental, but it represents a potential complement to salt programs, particularly in regions where iodized salt distribution is inconsistent or where dietary salt reduction campaigns are simultaneously underway.
The broader pattern across all of these sources is that iodine availability is ultimately tied to geography and food-system choices. The ocean is where iodine is abundant, and every step away from it, whether through inland geography, plant-based eating, or processing, tends to reduce what ends up on your plate. Understanding which sources actually deliver iodine, and in what amounts, puts you in a much better position to ensure you are getting enough without accidentally getting too much.