SACN + Iodine Global Network + Bath et al. Lancet 2013

Vegan vs Vegetarian Iodine: Why Plant-Based Diets Run Low

Iodine is the nutrient where the UK and US guidance most diverges, because the iodine vehicles in the two food systems differ. The British food chain delivers iodine primarily through dairy; the American food chain delivers it through mandatorily iodised salt. Cutting dairy hits the British case much harder. This page works through the published intake data and the safe dosing for vegans and vegetarians.

The short answer. Aim for 140 to 150 mcg of iodine per day from food or supplement, 200 mcg in pregnancy, 290 mcg in lactation. The cleanest vegan approaches are a 150 mcg potassium iodide tablet daily, or a half-sheet of nori a few times a week (about 50 to 100 mcg per sheet), or 1.5 g iodised salt (where available) per day. Avoid kelp supplements and kombu-heavy diets; they deliver erratic and often excessive iodine that can trigger hyperthyroidism.

Why iodine is in the UK food chain at all

The UK does not mandatorily iodise table salt, unlike most other European countries and the US. The country is classed as mildly iodine deficient by the Iodine Global Network, based on national surveys of urinary iodine concentration. The main pathway by which iodine enters the British diet is through cow milk: dairy farms feed cattle iodine-supplemented mineral mix and use iodine-based teat dips and udder washes for mastitis control. Iodine residue in the resulting milk supplies an estimated 30 to 40% of adult UK iodine intake, rising to around 50% in children who drink more milk per kg of body weight.

For a UK omnivore, the rest of the iodine comes from white fish (a 140 g portion of cod supplies around 320 mcg), eggs (around 25 mcg each), and trace amounts from bread and vegetables grown in iodine-replete soil. For a UK lacto-ovo vegetarian, dairy and eggs together supply roughly 100 mcg per day on a typical eating pattern, which is at the lower end of the RDA. For a UK vegan eating no fortified or supplementary iodine, intake commonly sits at 30 to 70 mcg per day, well below the RDA.

The US picture is different. Salt iodisation has been standard since 1924, and although it is technically voluntary, around 70% of US table salt is iodised and labelled as such. A US household using iodised salt picks up its iodine without thinking about it; the dairy contribution is supplementary rather than dominant. This is why UK and US vegan iodine guidance can read differently in places.

Plant iodine sources and their reliability

Source	Typical iodine per serving	Reliability
Nori (sushi sheet)	30 to 100 mcg per sheet (2.5 g)	Good (relatively consistent)
Wakame (miso soup)	40 to 90 mcg per gram	Moderate
Kombu (dashi base)	1,500 to 8,000 mcg per gram	Very high and variable; avoid daily
Hijiki	400 to 1,500 mcg per gram	High plus arsenic concerns; avoid
Iodised salt (1.5 g)	~115 mcg (US standard 76 mcg per gram)	High (where available)
Potassium iodide tablet	150 mcg standard dose	Very high
Cranberries (fresh)	~22 mcg per 30 g serving	Modest
Strawberries	~13 mcg per cup	Modest
Potato (with skin)	~60 mcg per medium potato	Variable by soil
White beans	~32 mcg per cup cooked	Variable
Cow milk	50 to 150 mcg per 250 ml	Variable seasonally (winter higher)
One large egg	~25 mcg	Moderate
Cheddar cheese	~15 mcg per 30 g	Moderate

Values for milk and eggs from UK Composition of Foods integrated dataset and the Bath et al. UK iodine analyses. Seaweed values from McKevith and Theobald (Nutr Bull 2005) and the FSA seaweed advisory. The bottom row of plant foods (cranberries, strawberries, potato) is included for completeness; none of these is a primary strategy because the values are modest and variable.

Why kelp and kombu supplements are not the answer

The intuitive vegan answer to iodine is to take a kelp supplement, because kelp marketing positions it as a natural high-iodine food. The problem is that natural iodine content of kelp varies by species, season, and harvest location by orders of magnitude. A single 500 mg kelp capsule has been reported to contain anywhere from 100 mcg to 16,000 mcg of iodine in published surveys. The tolerable upper intake is 600 mcg (UK SACN) or 1,100 mcg (US IOM), and the threshold for clinically meaningful thyroid disruption is around 600 to 1,800 mcg per day for sustained intake.

There are documented case reports of iodine-induced hyperthyroidism (Jod-Basedow phenomenon) and iodine-induced hypothyroidism in people taking kelp supplements daily. The thyroid auto-regulates to avoid overload (Wolff-Chaikoff effect) but the regulation can fail in people with pre-existing thyroid antibodies or in iodine-deficient populations transitioning to high intake. Potassium iodide at the 150 mcg RDA dose is far safer because it delivers a known, modest amount.

Pregnancy and breastfeeding

Adequate maternal iodine is necessary for fetal brain development. Maternal thyroxine crosses the placenta in early pregnancy before the fetal thyroid begins functioning, and inadequate maternal iodine produces hypothyroxinaemia that has been associated with lower offspring IQ in the ALSPAC cohort (Bath SC et al., Lancet 2013) and other observational studies. The published guidance is unambiguous: pregnant women, including vegans, need 200 mcg iodine per day from a combination of food and supplement.

Standard UK and US prenatal vitamins now usually include 150 mcg of iodine; check the label because some still do not. A vegan prenatal supplement that includes iodine (Pregnacare Vegan, Vega Pregnancy, Mama Bird, and others) is the cleanest single-product answer. Breastfeeding vegans need 290 mcg per day, primarily because iodine is concentrated in breast milk and the infant has rapidly increasing thyroid demand. Continue the prenatal vitamin (or its equivalent) through lactation.

Goitrogens, briefly

Some plants contain compounds that interfere with iodine uptake or thyroid hormone synthesis. The main ones in plant-heavy diets are soy (isoflavones), brassicas (cabbage, broccoli, kale, Brussels sprouts), and millet. The effect is small and clinically meaningful only when iodine intake is already low or when consumption is extreme. The published evidence does not support advising vegans to avoid soy or brassicas. The simpler and more effective intervention is to make sure iodine intake is adequate; once that is in place, normal consumption of soy and brassicas is fine. Cooking reduces goitrogenic activity in brassicas substantially.

Existing thyroid condition. If you have an autoimmune thyroid disease (Hashimoto's, Graves'), do not start a high-dose iodine supplement without endocrinology input. Even modest changes in iodine intake can destabilise treatment. The 150 mcg RDA dose is normally fine but discuss with your specialist.

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Frequently asked questions about iodine

Why is iodine a vegan issue in particular?

In the UK, dairy is the single largest contributor to dietary iodine intake, supplying around 30 to 40% of intake in adults and around 50% in children. The iodine in milk comes from cows' iodised feed and iodine-based teat disinfectants used in udder care. Removing dairy removes the main iodine source for a typical UK eater. In the US, table salt is mandatorily iodised and supplies most of the intake; the vegan vs vegetarian iodine gap is therefore narrower in the US but wider in the UK and most of the EU where salt iodisation is not mandatory.

How much iodine do I need?

The UK SACN reference nutrient intake is 140 mcg per day for adults. The US Institute of Medicine and the European Food Safety Authority both recommend 150 mcg per day for non-pregnant adults. Pregnancy increases the requirement to 200 mcg (UK), 220 mcg (US), or 200 mcg (WHO). Breastfeeding raises it further to 290 mcg per day. Tolerable upper intake from supplements is 600 mcg per day (UK SACN) or 1,100 mcg (US IOM). Too much iodine can be as harmful as too little, particularly for the thyroid.

Is seaweed a reliable iodine source?

Seaweed contains iodine but content varies wildly by species and source, which is its biggest drawback as a daily strategy. Nori (the seaweed in sushi rolls) is the most predictable, supplying around 30 to 100 mcg of iodine per sheet, well within a usable daily range. Wakame (in miso soup) supplies around 65 mcg per gram. Kombu and kelp are extreme outliers, sometimes containing 1,500 to 8,000 mcg per gram, which can easily exceed the tolerable upper intake from a single serving and induce iodine-driven hyperthyroidism. Avoid daily kelp supplements or kombu-heavy diets. Nori a few times a week, or a small piece of wakame in miso soup, is the safer route.

Should vegans supplement iodine?

The Vegan Society and the British Dietetic Association both recommend that vegans who do not eat seaweed regularly should take a 150 mcg iodine supplement daily, preferably as potassium iodide. The dose is at the RDA, not above it, to minimise the risk of hyperthyroidism. Pregnant and breastfeeding vegans should aim for 200 to 290 mcg daily. Multivitamins often include iodine; check the label and aim for the RDA range, not megadoses. Avoid kelp supplements, which deliver erratic and often excessive iodine.

What are the symptoms of iodine deficiency?

Mild iodine deficiency in adults usually does not produce symptoms but can be detected by raised TSH on a thyroid blood panel and elevated thyroid volume on ultrasound. Moderate deficiency causes goitre (visible thyroid enlargement). Severe deficiency causes hypothyroidism with fatigue, weight gain, cold intolerance, dry skin, and cognitive slowing. The clinically most important consequence of iodine deficiency is during pregnancy: maternal hypothyroxinaemia in early gestation reduces neurodevelopmental outcomes in offspring (lower IQ, motor and reading impairments). This is why iodine status in pregnancy is treated as a public health priority.

Is the UK actually iodine deficient?

Yes, on the WHO classification. The UK has been classified as mildly iodine deficient by the Iodine Global Network since at least 2011, based on median urinary iodine concentrations in school-age girls below the 100 mcg/L threshold. The position has been reaffirmed in subsequent surveys (Bath SC et al., Lancet 2013). The UK does not iodise salt despite repeated calls from public health groups including SACN and the Royal College of Obstetricians and Gynaecologists. For vegans this matters because the main UK iodine vehicle (dairy) is unavailable; for pregnant vegans the case for deliberate iodine supplementation is strong.

Sources cited. NIH ODS Iodine fact sheet; Bath SC et al. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC), Lancet 2013; 382: 331-337; BDA Iodine food fact sheet; Iodine Global Network country score card; Vegan Society iodine guidance; SACN Statement on Iodine 2014. All values as of May 2026.