Kelp and Iodine: Benefits, Forms, Dosing, and Side Effects

Iodine is a trace element that is an essential component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) [1][2]. These hormones regulate many critical biochemical reactions, including protein synthesis and enzymatic activity, and are the principal determinants of metabolic rate throughout the body. Thyroid hormones are also required for proper skeletal and central nervous system development in fetuses and infants [1].

Thyroid function is primarily regulated by thyroid-stimulating hormone (TSH), secreted by the pituitary gland. In the absence of sufficient iodine, TSH levels remain elevated, leading to goiter — an enlargement of the thyroid gland. Beyond thyroid function, iodine appears to play a role in immune response and may have beneficial effects on fibrocystic breast disease [2].

Kelp refers to several genera and species of large brown algae (order Laminariales) that grow in cold, nutrient-rich ocean waters [5][6]. It is best known as one of the richest natural sources of iodine — accumulating iodine from seawater more efficiently than any other living organism [5][7]. Kelp and other edible seaweeds such as kombu, nori, and wakame are staples of traditional East Asian cuisine. Iodine deficiency remains the most common cause of preventable intellectual disability worldwide [1][10], and while the general US population has adequate intake, subgroups remain at risk — particularly pregnant women, vegans, and those who do not use iodized salt.

Table of Contents

• Overview
• Forms and Bioavailability
• Evidence for Benefits
• Recommended Dosing
• Safety and Side Effects
• Drug Interactions
• Dietary Sources
• References

Overview

The iodine-replete healthy adult has approximately 15–20 mg of iodine in the body, 70%–80% of which is contained in the thyroid gland [1][3]. Iodine in food and supplements is present in several chemical forms — sodium and potassium salts, inorganic iodine (I2), iodate, and iodide (the reduced form). Iodide is quickly and almost completely absorbed in the stomach and duodenum [2][4]. Iodate is reduced in the gastrointestinal tract and absorbed as iodide. When iodide enters the circulation, the thyroid gland concentrates it for hormone synthesis, and most of the remaining amount is excreted in the urine [2].

Common kelp species found in supplements include Laminaria hyperborea, Laminaria digitata, Laminaria setchelli, Macrocystis integrifolia, and Macrocystis pyrifera [5]. Another brown alga found in supplements is Ascophyllum nodosum (commonly called rockweed, sometimes referred to as "Norwegian kelp"), which grows on the northwest coast of Europe and the northeast coast of the United States [5]. Laminaria digitata accumulates the highest amounts of iodine, though the same species also has the highest levels of arsenic among seaweed species [5][8].

On a dry weight basis, kelp contains approximately 40–60% carbohydrates (largely soluble fibers such as alginates and laminarin), 5–12% protein, and 1–3% lipids including small amounts of omega-3 fatty acids like EPA [6]. Iodine content is highly variable: 500–2,500 mcg per 100 g dry weight on average, with some species reaching 8,000 mcg per 100 g [6][9].

Median urinary iodine concentrations of 100–199 mcg/L in children and non-pregnant adults, 150–249 mcg/L in pregnant women, and >100 mcg/L in lactating women indicate adequate iodine status [1][12]. Values below 100 mcg/L in non-pregnant individuals indicate insufficient intake, though severe deficiency is not classified until levels fall below 20 mcg/L.

Forms and Bioavailability

Potassium Iodide

Potassium iodide (KI) is the most common form of iodine in dietary supplements. It is well absorbed — a small study found that people absorb potassium iodide almost completely (96.4%) [13]. This is the form most commonly included in multivitamin/multimineral supplements, typically at a dose of 150 mcg iodine [14]. Potassium iodide is also the form recommended by the American Thyroid Association for pregnant and lactating women and is the form approved by the FDA as a thyroid-blocking agent in radiation emergencies [15][16].

Sodium Iodide

Similar to potassium iodide in terms of bioavailability. Used less frequently in dietary supplements than potassium iodide. Both forms provide iodide ions that are rapidly absorbed in the gastrointestinal tract [2].

Kelp-Derived Iodine

Kelp supplements are an alternative iodine source, but iodine from kelp is significantly less bioavailable than from potassium iodide. In a study of healthy young British women (ages 22–34) with low dietary iodine intakes, researchers compared iodine absorption from a 500 mg capsule of Ascophyllum nodosum seaweed (providing 356 mcg of iodine) to an equivalent amount from potassium iodide [17]. Only 33% of iodine from the seaweed was bioavailable compared to 59% from potassium iodide — meaning one absorbs roughly half as much iodine from kelp as from KI supplements [17].

Kelp powders (dried kelp) are about 0.1–0.5% iodine by weight [5]. To obtain 150 mcg of iodine, one needs approximately 30–150 mg of kelp powder, or less if using a kelp extract. However, iodine content in kelp supplements can be highly variable and may not match label claims, making consistent dosing more challenging than with standardized potassium iodide [9][18].

Molecular Iodine (I2)

Molecular (elemental) iodine has been studied in the context of fibrocystic breast disease at doses of 3,000–6,000 mcg/day [19]. This form is not commonly found in standard supplements but has shown efficacy in reducing breast pain and nodularity in clinical trials.

Iodized Salt

In the United States, iodized table salt contains approximately 45 mcg iodine per gram of salt (roughly 78 mcg per 1/4 teaspoon) [2][20]. However, most salt intake in the US comes from processed foods, and food manufacturers almost always use non-iodized salt [2]. Specialty salts — sea salt, kosher salt, Himalayan salt, and fleur de sel — are typically not iodized and provide virtually no iodine [2].

Iodate vs. Iodide in Foods

Iodized salt in the US uses potassium iodide or cuprous iodide [21]. The WHO recommends potassium iodate for salt iodization in tropical climates due to its greater stability [12]. This distinction matters because iodide (I-) can react with chloramine in chlorinated tap water during cooking to form iodinated trihalomethanes (associated with cancer), whereas iodate (IO3-) does not produce these byproducts as readily [22][23].

Comparison Table

Iodine Source	Bioavailability	Iodine Content	Key Advantages	Key Limitations
Potassium Iodide (KI)	~96%	Standardized	Highest absorption, consistent dosing, FDA-approved	Synthetic; no additional sea nutrients
Kelp powder	~33%	Variable (0.1–0.5% by weight)	Natural source; contains other minerals and fiber	Lower bioavailability, variable iodine, arsenic risk
Kelp extract	~33%	More concentrated than powder	Higher iodine per gram of product	Same bioavailability limitations as kelp powder
Iodized salt	High	~45 mcg/g salt	Widely available, inexpensive, population-level impact	Discretionary use; processed foods use non-iodized salt
Molecular iodine (I2)	Moderate	Defined	Studied for fibrocystic breast disease	Not widely available; high-dose applications only

Evidence for Benefits

Preventing and Treating Iodine Deficiency

Iodine supplementation effectively prevents and treats iodine deficiency, which causes hypothyroidism, goiter, impaired mental function, and — during pregnancy — severe developmental consequences including cretinism [1][2][10].

Case report of kelp treating deficiency: A nine-year-old boy with a restrictive diet (including no dairy) was diagnosed with hypothyroidism caused by iodine deficiency. His mother supplemented him with a kelp supplement providing 400 mcg of iodine per capsule. After three weeks, his thyroid hormone levels returned to normal and remained normal after 7 months of continued supplementation [24].

Seaweed for mild deficiency: In a study of healthy young British women with low iodine intakes, daily supplementation for two weeks with one 500 mg capsule of Ascophyllum nodosum (providing 356 mcg of iodine) significantly increased average urinary iodine excretion from 78 mcg/L (mildly insufficient) to 145 mcg/L (adequate) [17]. This confirms that kelp can effectively raise iodine status even with its lower bioavailability.

Impact of deficiency on thyroid: If iodine intake falls below approximately 10–20 mcg/day, hypothyroidism occurs, frequently accompanied by goiter [1][2]. Under normal conditions, TSH secretion increases when iodine intake drops below about 100 mcg/day, stimulating thyroidal iodine uptake from the blood [4]. However, very low iodine intakes can reduce thyroid hormone production even in the presence of elevated TSH.

Fetal and Infant Development

Iodine sufficiency during pregnancy is critically important for proper fetal development. During early pregnancy, when the fetal thyroid gland is still developing, the fetus depends entirely on maternal T4 — and therefore on maternal iodine intake [25]. Production of T4 increases by approximately 50% during pregnancy, requiring a concomitant increase in maternal iodine intake [25][26].

Meta-analysis of IQ effects: A meta-analysis of 6,180 mother-child pairs from three birth cohorts in the Netherlands, Spain, and the United Kingdom found that verbal IQ assessed in children at 1.5–8 years of age was lower if their mothers had lower iodine status in their first trimester of pregnancy [27].

Severe deficiency consequences: In pregnant women, severe iodine deficiency can cause major neurodevelopmental deficits and growth retardation in the fetus, as well as miscarriage and stillbirth [4]. Chronic severe deficiency in utero causes cretinism — characterized by intellectual disability, deaf mutism, motor spasticity, stunted growth, delayed sexual maturation, and other neurological abnormalities [4].

ADHD risk: Mild to moderate maternal iodine deficiency has been associated with an increased risk of attention deficit hyperactivity disorder (ADHD) in offspring [28].

Supplementation trial results: However, two randomized clinical trials providing iodine from early pregnancy to delivery (150 or 200 mcg/day as potassium iodide) found no statistically significant effect on child cognitive, language, or motor scores assessed at ages 1.5–6 years [29][30][31]. This suggests that supplementation may be most beneficial in populations with more substantial deficiency rather than mild insufficiency.

Breastfeeding: Breast milk contains iodine, though concentrations vary based on maternal iodine levels. A study of 57 healthy lactating women from the Boston area found a median breast milk iodine content of 155 mcg/L, and calculated that 47% of the women may have been providing their infants insufficient iodine through breast milk alone [32].

Professional recommendations: The American Thyroid Association recommends that women who are planning a pregnancy, currently pregnant, or lactating supplement with 150 mcg/day iodine as potassium iodide [15]. The American Academy of Pediatrics similarly recommends at least 150 mcg iodine supplementation plus use of iodized salt for pregnant and lactating women [33]. Of note, pregnant and nursing women should use potassium iodide supplements rather than kelp supplements due to the risk of arsenic contamination in kelp [5].

Cognitive Function in Children

Chronic moderate to severe iodine deficiency in children reduces IQ by approximately 12–13.5 points [26]. A 2004 Cochrane Review concluded that iodine supplementation in children living in areas of deficiency positively affects both physical and mental development and decreases mortality [34].

Randomized trial in mildly deficient children: In a 2009 RCT, 184 children aged 10–13 years in New Zealand with a median urinary iodine concentration of 63 mcg/L received either iodine supplements (150 mcg/day) or placebo for 28 weeks [35]. Iodine supplementation improved urinary iodine status (median 145 mcg/L post-treatment) and significantly improved measures of perceptual reasoning and overall cognitive score compared to placebo.

Fibrocystic Breast Disease

Fibrocystic breast disease is a benign condition characterized by lumpy, painful breasts and palpable fibrosis, commonly affecting women of reproductive age [19]. Breast tissue has a high concentration of iodine, especially during pregnancy and lactation [36].

Double-blind RCT (n=56): Women with fibrocystic breast disease received daily iodine supplements (70–90 mcg I2/kg body weight) or placebo for 6 months. At completion, 65% of women receiving iodine reported decreased pain compared to 33% in the placebo group [19].

Dose-response RCT (n=111): Women aged 18–50 with fibrosis and a history of breast pain were randomized to 0, 1,500, 3,000, or 6,000 mcg of molecular iodine (I2) per day for 5 months [37]. Women receiving 3,000 or 6,000 mcg had a significant decrease in breast pain, tenderness, and nodularity compared to placebo or 1,500 mcg. The study also showed a dose-dependent reduction in self-assessed pain. None of the doses was associated with major adverse events or changes in thyroid function.

Important caveat: The doses used in these studies (1,500–6,000 mcg/day) are several times higher than the UL of 1,100 mcg for adults. Such doses should only be used under the guidance of a physician [2][37].

Thyroid Cancer Prevention (Radiation Emergencies)

Nuclear accidents can release radioactive iodine (I-131) into the environment, increasing the risk of thyroid cancer in exposed individuals, especially children [16][38]. The FDA has approved potassium iodide as a thyroid-blocking agent: pharmacological doses (16–130 mg depending on age) saturate the thyroid and prevent radioactive iodine uptake [16][38].

Chernobyl evidence: Potassium iodide was widely distributed in Poland following the 1986 Chernobyl accident, and childhood thyroid cancer rates did not substantially increase in subsequent years [39]. In contrast, in Belarus and Ukraine — where iodide prophylaxis was not used and many children were mildly iodine-deficient — thyroid cancer incidence in children and adolescents sharply increased [16].

Energy and Weight Loss

Kelp supplements are sometimes promoted for energy and weight loss, partly because iodine is needed for healthy thyroid function. Hypothyroidism can cause fatigue and weight gain [5]. However, there is no evidence that kelp supplementation increases energy or promotes weight loss in people who do not have underactive thyroids [5].

Alginate and appetite: Alginate, a gel-like substance derived from kelp cell walls, has shown some promise in reducing appetite and lowering postprandial blood sugar [40][41]. However, alginate is a specially prepared gel-forming extract — it is not known whether consuming dried kelp in regular supplements has the same effect [5].

Bread enrichment study: A small study in healthy young British men found that bread enriched with a branded Ascophyllum nodosum ingredient reduced energy intake at a subsequent meal (4 hours later) by 16.4% compared to unenriched bread. However, there were no significant reductions in hunger, fullness, or blood sugar levels [42].

Body fat RCT: A randomized trial found that daily ingestion of iodine-reduced kelp powder (providing approximately 150 mcg iodine) safely reduced body fat percentage by 1–2% over 8 weeks in overweight males, potentially via fiber-induced satiety and metabolic effects, without altering thyroid function [6][43]. These results are preliminary and require replication.

Interference Compounds and Iodine Absorption

Chemicals structurally similar to iodine — bromide, chloride, and fluoride — may compete with iodine for absorption based on animal studies [44]. However, there are no reported cases of iodine deficiency caused by these compounds in humans [5]. Perchlorate can interfere with thyroidal iodine uptake, but the FDA concluded that most Americans get enough dietary iodine and do not need supplemental iodine to counteract perchlorate effects [5].

Goitrogens — substances in foods that interfere with thyroidal iodine uptake — are found in soy, cassava, and cruciferous vegetables (cabbage, broccoli, cauliflower) [2]. Iron and vitamin A deficiencies may also be goitrogenic [45]. These are primarily a concern for people living in iodine-deficient regions [2].

Other Kelp Bioactive Compounds

Kelp contains several bioactive compounds beyond iodine with potential health implications, though clinical evidence in humans remains limited. Fucoidans are sulfated polysaccharides with documented anticoagulant and antiplatelet properties [46][47]. Fucoxanthin is a carotenoid with antioxidant and anti-inflammatory properties in preclinical studies [6]. Alginates and laminarin are soluble dietary fibers (comprising 40–60% of kelp dry weight) that may support glycemic control and satiety [6][40].

Recommended Dosing

Recommended Dietary Allowances (RDAs) for Iodine

From the Institute of Medicine / NIH Office of Dietary Supplements [2]:

Age Group	Male (mcg/day)	Female (mcg/day)	Pregnancy	Lactation
Birth to 6 months	110*	110*	—	—
7–12 months	130*	130*	—	—
1–3 years	90	90	—	—
4–8 years	90	90	—	—
9–13 years	120	120	—	—
14–18 years	150	150	220	290
19+ years	150	150	220	290

*Adequate Intake (AI). The WHO, UNICEF, and ICCIDD recommend a slightly higher intake of 250 mcg/day during pregnancy and lactation [12][48].

Tolerable Upper Intake Levels (ULs) for Iodine

From the Institute of Medicine [2]:

Age Group	UL (mcg/day)
1–3 years	200
4–8 years	300
9–13 years	600
14–18 years	900
19+ years (including pregnancy and lactation)	1,100

Infants: No UL established; formula and food should be the only iodine sources.

American Thyroid Association Recommendation

The ATA recommends that no individual consume an iodine or kelp daily supplement containing more than 500 mcg of iodine unless advised by a healthcare professional [49]. This is more conservative than the IOM UL of 1,100 mcg and reflects the risk of iodine-induced thyroid dysfunction.

FDA Limit for Kelp Supplements

The FDA limits iodine from kelp supplements to a daily serving of no more than 225 mcg for adults and children over age four [5].

Practical Dosing Guidance

General adult supplementation: 150 mcg/day from potassium iodide (standard multivitamin dose) is adequate for most adults. If using kelp, choose a supplement that provides a documented iodine content. Kelp supplements with 225 mcg listed iodine are not excessive for adults needing 150 mcg daily, given kelp's lower bioavailability (~50% compared to KI) [5][17].

Pregnancy and lactation: 220–290 mcg/day total from diet and supplements. The ATA recommends 150 mcg/day supplementation as potassium iodide for pregnant and lactating women [15]. Use KI rather than kelp during pregnancy due to arsenic contamination concerns [5].

People on low-salt diets: Those who avoid iodized salt (including users of Himalayan salt, sea salt, or kosher salt) may be at particular risk for inadequate intake and should ensure iodine from other dietary sources or supplements [2][5].

Vegans and plant-based dieters: Seafood, eggs, milk, and milk products are among the best dietary iodine sources. People who consume none or minimal amounts of these foods may not obtain sufficient iodine and may benefit from supplementation [50].

Dr Brad Stanfield's MicroVitamin includes 112.5 mcg of iodine as potassium iodide — a well-absorbed form that provides 75% of the adult RDA, designed to complement dietary iodine intake without exceeding safe levels.

How to Assess Iodine Status

Urinary iodine is the standard measure. Median urinary iodine concentrations above 100 mcg/L in non-pregnant adults indicate adequacy [1][12]. However, spot urine samples are suitable only for population-level assessment — multiple 24-hour collections or repeat spot samples are needed for reliable individual assessment [51][52].

Safety and Side Effects

Iodine-Induced Thyroid Dysfunction

The most significant risk of iodine supplementation — whether from kelp or potassium iodide — is disruption of thyroid function. Both hypo- and hyperthyroidism can result from excessive iodine intake [2][53].

Hypothyroidism from excess iodine (Wolff-Chaikoff effect): A 54-year-old woman with hypothyroidism who was not taking levothyroxine consumed excessive iodine (2,730 mcg/day) from supplements including kelp for 11 days. Her TSH increased from 8.4 to 21.3 mU/mL. She was hospitalized with altered mental state, combativeness, screaming spells, slow heart rate, and low blood pressure. Symptoms improved after IV levothyroxine [54].

Hyperthyroidism from kelp (iodine-induced thyrotoxicosis): A 58-year-old woman with 2 years of "substantial" kelp supplement use developed tremors, palpitations, and was diagnosed with elevated thyroid hormone levels and low TSH. She also developed osteoporosis (T-score of -3.0), attributed to excess iodine leading to elevated thyroid hormones that accelerate bone loss [55].

Congenital hypothyroidism: Cases of congenital hypothyroidism have been reported in infants born to mothers taking herbal supplements high in iodine-containing kelp [56].

Thyroid inflammation and cancer: High iodine intakes can cause thyroid gland inflammation (thyroiditis) and studies have shown associations with thyroid papillary cancer [2][53]. Observational data link high seaweed consumption to elevated thyroid cancer risk in postmenopausal women, though causality is not established [6].

Population susceptibility: Responses to excess iodine vary. People with autoimmune thyroid disease (Hashimoto's thyroiditis, Graves' disease) and those with pre-existing iodine deficiency may experience adverse effects at iodine intakes considered safe for the general population [2][4].

Arsenic Contamination in Kelp

Kelp can accumulate arsenic from seawater, with Laminaria digitata showing the highest levels among seaweed species tested [5][8]. This is a particular concern for pregnant and nursing women (arsenic crosses the placenta), long-term kelp supplement users, and people consuming whole kelp products rather than standardized extracts. For this reason, if iodine supplementation is needed during pregnancy, potassium iodide is preferred over kelp [5].

Skin Reactions

In people sensitive to iodine, excessive intake may cause skin reactions (iododermas) including itching, rash, hives, blisters, purpuric hemorrhagic lesions, and acne outbreaks [57].

Case report (acne): A 24-year-old woman whose moderately severe acne had nearly cleared with treatment experienced a sudden, severe breakout after consuming 10–12 kelp tablets daily containing 15 mg of iodine each; the outbreak resolved 2 weeks after stopping [58]. Another report described a 28-year-old man whose treatment-resistant acne resolved 6 weeks after eliminating iodized salt, then returned within 3 weeks of reintroduction [59]. However, research suggests that in most people there is no association between iodine consumption and acne [60][61].

Acute Iodine Poisoning

Very large doses of iodine (several grams per day) can cause burning of the mouth, throat, and stomach; fever; abdominal pain; nausea; vomiting; diarrhea; weak pulse; and coma [2][5]. Acute poisoning is rare and typically requires doses far exceeding any supplement amount.

Autoimmune Reaction from Kelp

An autoimmune reaction resulting in abnormal bleeding, bruising, and low blood platelet count was reported in a woman taking three 550 mg kelp tablets daily for six weeks; symptoms resolved after medical treatment and stopping supplementation [62].

Disinfection Byproduct Risk from Cooking with Kelp

A laboratory study found that heating seaweed salt in chlorinated tap water for 1 hour increased concentrations of iodinated trihalomethanes (associated with cancer) to near or above EPA limits [22]. This occurs because iodide (I-) in seaweed reacts with residual chloramine in tap water. Heating iodized salt (which contains iodate) did not produce this effect [22][23]. This is relevant for people who use kelp-based salt substitutes in cooking with tap water.

Safety in Pregnancy

The recommended approach is 150 mcg/day supplemental iodine as potassium iodide (not kelp) [15]. A cross-sectional study of pregnant women in Spain found a significantly increased risk of elevated TSH in women taking iodine supplements at doses of 200 mcg/day or more compared to those taking less than 100 mcg/day [63]. This suggests that even moderately high supplemental iodine doses during pregnancy can induce thyroid dysfunction in some women.

Drug Interactions

Antithyroid Medications

The iodine in kelp and iodine supplements can interact with antithyroid medications used to treat hyperthyroidism (e.g., methimazole/Tapazole). Taking high doses of iodine with antithyroid medications can have an additive effect and could cause hypothyroidism [5][53][64].

Blood Thinners / Anticoagulants

Kelp contains fucoidans — sulfated polysaccharides with documented antiplatelet and anticoagulant effects [46][47]. People taking blood-thinning medications should use kelp with caution:

Medication	Concern
Aspirin	Fucoidan-mediated additive antiplatelet effect
Clopidogrel (Plavix)	Additive antiplatelet effect
Heparin	Additive anticoagulant effect
Ticlopidine (Ticlid)	Additive antiplatelet effect
Warfarin (Coumadin)	Additive anticoagulant effect

This interaction is specific to kelp supplements (which contain fucoidans), not to potassium iodide supplements [46][47].

ACE Inhibitors

Taking potassium iodide with ACE inhibitors (benazepril/Lotensin, lisinopril/Prinivil/Zestril, fosinopril/Monopril) can increase the risk of hyperkalemia (elevated blood potassium) [64].

Potassium-Sparing Diuretics

Taking potassium iodide with potassium-sparing diuretics (spironolactone/Aldactone, amiloride/Midamor) can increase the risk of hyperkalemia [64].

Levothyroxine and Other Thyroid Medications

Iodine supplements can interfere with thyroid medication dosing. People with hypothyroidism on levothyroxine should consult their physician before starting iodine or kelp supplements [5][53].

Lithium

Lithium and iodine both affect thyroid function. Concurrent use may increase the risk of hypothyroidism [53].

Shellfish Allergy Consideration

As kelp is harvested from the sea and may have contaminants from other marine organisms, it may be prudent to avoid kelp supplements if allergic to shellfish [5].

Dietary Sources

Seaweed (kelp, nori, kombu, wakame) is one of the best food sources of iodine, though the iodine content is highly variable — commercially available seaweeds range from 16 mcg/g to 2,984 mcg/g [9][18]. Other good sources include fish, seafood, dairy products, eggs, and bread made with iodate dough conditioners.

Iodine Content of Selected Foods

Food	Serving	Iodine (mcg)	% DV (150 mcg)
Bread, white, made with iodate dough conditioner	2 slices	296	197%
Bread, whole-wheat, made with iodate dough conditioner	2 slices	273	182%
Cod, baked	3 oz	146	97%
Seaweed, nori, dried	2 tbsp, flaked (5 g)	116	77%
Oysters, cooked	3 oz	93	62%
Yogurt, Greek, plain, nonfat	3/4 cup	87	58%
Milk, nonfat	1 cup	84	56%
Iodized table salt	1/4 teaspoon	78	52%
Fish sticks, cooked	3 oz	57	38%
Egg, hard boiled	1 large	31	21%
Pasta, enriched (boiled in iodized salt water)	1 cup	30	20%
Ice cream, chocolate	2/3 cup	28	19%
Cheese, cheddar	1 oz	14	9%
Shrimp, cooked	3 oz	13	9%
Tuna, canned in water	3 oz	7	5%
Sea salt, non-iodized	1/4 teaspoon	0	0%

Source: USDA, FDA, and ODS-NIH Database for the Iodine Content of Common Foods, Release 4.0 (2024) [20].

Key Notes on Dietary Iodine

• Dairy products contain iodine, but amounts vary widely (38–160 mcg per cup of nonfat milk) depending on whether cows received iodine feed supplements and whether iodophor sanitizing agents were used [2][20].
• Bread can be a significant source if manufacturers use iodate dough conditioners. Approximately 20% of bread products in the US list iodate conditioners; those made without these conditioners contain very little iodine [65].
• Plant-based milk substitutes (soy, almond) contain relatively small amounts of iodine [2].
• Fruits and vegetables are generally poor iodine sources, and their content varies based on soil iodine levels, fertilizer use, and irrigation practices [2].
• Seaweed iodine content varies enormously by species: Laminaria digitata has among the highest concentrations, while nori (Porphyra) tends to have lower, more moderate levels [9][18].

Kelp Nutritional Profile (per 100 g dry weight)

Nutrient	Amount
Iodine	500–2,500 mcg (up to 8,000 mcg in some species)
Potassium	2,000–6,000 mg
Magnesium	500–1,200 mg
Calcium	200–800 mg
Iron	10–50 mg
Vitamin B12	Trace to 1–2 mcg
Dietary fiber	30–50 g

Values vary significantly by species, harvest location, and processing. Kelp also provides vitamins A, C, E, and B-group vitamins in lower concentrations than terrestrial vegetables [6].

Groups at Risk of Inadequate Iodine

• People who do not use iodized salt: About 88% of households worldwide use iodized salt, but iodine insufficiency persists in Southeast Asia, sub-Saharan Africa, and Eastern Europe [66][67].
• Pregnant women: Many pregnant women in the US consume insufficient iodine. NHANES data from 2007–2014 showed a median urinary iodine concentration of 144 mcg/L in pregnant women — below the 150 mcg/L threshold for adequacy [11][68]. Pregnant women who consumed no dairy products had median urinary iodine of only 100 mcg/L [69].
• Vegans and people avoiding dairy, seafood, and eggs: These are among the richest dietary iodine sources [50].
• People in iodine-deficient soil regions: Mountainous areas (Himalayas, Alps, Andes) and flood-prone river valleys have iodine-deficient soils that produce low-iodine crops [4].
• People with marginal iodine status who consume goitrogens: Soy, cassava, and cruciferous vegetables contain goitrogens that can exacerbate deficiency [2][45].

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