Do vegans need iodine supplements, and how much?

The objection

“Iodine comes from milk and fish. Take them away and study after study finds vegans with the lowest iodine of any diet group, in countries like Britain and Ireland that don’t iodise their salt. Iodine runs the thyroid, and a shortfall in pregnancy can affect a baby’s developing brain, so this is a deficiency the diet itself creates, and the pill needed to fix it is an admission that the diet falls short on its own.”

The answer

On the numbers, the objection is right. Urinary iodine is the standard measure of a population’s iodine status [5], and a 2023 systematic review and meta-analysis of studies published from 2020 to 2022 (Eveleigh et al.) found that vegans had the lowest median in all five studies that measured it, at 12.2 to 44.0 µg per litre, as well as the lowest intake in 8 of the 11 study cohorts [1]. Pooled together, the gap in urinary iodine just missed statistical significance, though vegans came out lowest in every study, and the gap in intake was clear [1]. An earlier review (Eveleigh et al., 2020) found the same pattern with one twist: in five studies, some vegans who ate seaweed took in close to or more than the tolerable maximum [2]. In a 2019 study in Oslo, 18% of vegans had habitual intakes above the 600 µg upper limit, and every one of them ate seaweed (Groufh-Jacobsen et al., 2020) [3].

The chart comes from one of the five studies, NuEva (Dawczynski et al., 2022), which recruited 258 German adults in 2018, each on their diet for at least a year, and measured the iodine in their 24-hour urine. The vegans’ median was 21.5 µg per litre, against 42 for vegetarians, 52 for flexitarians and 53 for omnivores [4]. These are mostly cross-sectional studies of volunteers, many of them small, and none of the five corrected the urine for how dilute it was [1]. A single sample also swings with the last few meals, so urinary iodine can judge a group but not a person, and UK endocrinologists advise that a dietary history is the better guide for an individual (Taylor et al., 2026) [5][6].

What the objection leaves out is that people who eat meat and dairy are often short too. None of the diet groups in the 2023 meta-analysis, omnivores included, had a median urinary iodine in the WHO’s optimal range of 100 to 200 µg per litre [1], and in NuEva every group fell below the WHO cut-off of 100 [4]. In the UK’s National Diet and Nutrition Survey for 2019 to 2023, published in 2025, the median was 82 µg per litre in women aged 16 to 49, 95 in girls aged 11 to 18 and 89 in adults aged 19 to 64, all under the 100 that marks adequate intake for a population, and between 2013 and 2023 it fell by 29% in girls and 25% in adults [5]. In Ireland, girls aged 11 to 18 had a median of 77 µg per litre in national surveys from 2017 to 2020 (Kane et al., 2025) [7].

Milk and dairy are the main source of iodine for most people in the UK, and how much iodine milk carries depends partly on how the cows are farmed [11]. The 2026 review by UK endocrinologists says Britain’s past iodine sufficiency was “achieved entirely by accident”, after iodine disinfectants in dairies and iodine-supplemented cattle feed raised the iodine in milk in the 1940s and 1950s, with no government programme behind it [6]. Milk still supplies over half the iodine eaten by Irish schoolgirls, and in some regions farmers routinely clean cows’ teats with iodine disinfectants and give the animals mineral supplements, which is why milk often carries more iodine in winter, when herds are housed and fed concentrates [7]. Norway requires iodine to be added to cattle feed [3]. Some of the iodine in a glass of milk, in other words, began as a supplement in the cow’s feed.

Take the milk away without replacing its iodine and the numbers fall short. A 2024 review by researchers at the University of Surrey and Queen’s University Belfast calculated that the EAT-Lancet planetary health diet would supply 128 µg of iodine a day, 85% of the adult recommendation of 150 µg, but only 54 µg if its milk were swapped for unfortified plant drinks, and concluded that strict plant-based eaters such as vegans cannot reach the recommended intake from food alone and rely on supplements (Nicol et al., 2024) [19].

Where the law puts iodine into everyday food, the gap narrows. In the meta-analysis, vegans’ intake was not significantly lower than omnivores’ in countries with mandatory salt iodisation, though only three of the nine studies of intake came from such countries [1]. The UK has no mandatory iodine fortification of any food [1] and Ireland has no iodised salt or fortification programme [7], which leaves plant milks to do the work, and many still don’t: in UK supermarkets in December 2024, 48% of non-organic plant milks were fortified with iodine, up from 28% in 2020 but well behind the 88% fortified with calcium [8], and organic ones cannot be fortified at all [9]. A fortified one does nearly the job of cow’s milk. In the December 2020 survey, fortified plant milks carried 13 to 45 µg of iodine per 100 ml, with a median of 24.8 against 30 for cow’s milk, a difference that was not statistically significant, and a 200 ml glass gave about 50 µg (Nicol et al., 2023) [9].

What a low reading does to an adult’s thyroid is less certain. When the Oslo team took blood from its 205 volunteers, all of them vegans, vegetarians or pescatarians, thyroid hormones were in the normal range in every group and 3% showed signs of a mildly underactive thyroid, with no difference between diets. Vegans who took no iodine supplement, though, had higher thyroglobulin than vegans who did, 27 against 16 µg per litre, and thyroglobulin is a marker that rises when iodine runs short and the thyroid has to work harder (Henjum et al., 2023) [10][11]. Soya and cabbage-family vegetables, often blamed for slowing the thyroid, made little measurable difference in the research. A 2019 meta-analysis of randomised trials of soya, drawing on 18 papers, found no significant change in thyroid hormones and a very modest rise of 0.248 mIU per litre in TSH, the signal that drives the thyroid, whose clinical significance was unclear (Otun et al., 2019) [23]. A 2024 systematic review of 123 laboratory, animal and human studies concluded that brassica vegetables in the daily diet do not harm thyroid function, particularly when iodine intake is adequate (Galanty et al., 2024) [24]. The clearest risk is in pregnancy, when a low intake may affect the baby’s brain [11].

What is the best vegan source of iodine?

The most reliable vegan sources are a plant milk fortified with iodine and a supplement of potassium iodide or potassium iodate giving no more than 150 µg a day, the two options the British Dietetic Association names [11]. Those are adult amounts. Children need less iodine [25] and tolerate less, with a safe upper limit that the European Food Safety Authority sets at 200 µg a day at ages one to three, against 600 µg for adults [27], so ask a GP, health visitor or dietitian before giving a child an iodine supplement, and never give a child a seaweed or kelp supplement [11]. The NHS puts an adult’s need at 140 µg a day and suggests that strict vegans consider fortified foods or a supplement, taking advice from a healthcare professional before starting one [12], and the 2026 endocrinologists’ review advises a 150 µg supplement for vegans and anyone on a dairy-free diet [6]. In the 2019 Oslo study, vegans who took an iodine supplement or ate seaweed had a median urinary iodine of 59 µg per litre, against 31 for those who did neither, though both groups stayed below 100 [3]. Iodised salt is not widely sold in the UK, and because everyone is advised to eat less salt, the BDA says it should not be relied on for iodine [11], while sea salt, rock salt and pink Himalayan salt contain almost none [6].

The bodies that set daily figures don’t agree on them, least of all for pregnancy:

Who sets it	Adults	Pregnancy	Breastfeeding
UK, NHS [12][1]	140 µg	not set	not set
EFSA, via the BDA [11]	150 µg	200 µg	200 µg
WHO [1]	150 µg	250 µg	250 µg

Children on vegan diets can fall short too. In a 2023 Czech study of children aged up to 18, 31 of 75 vegan children had a single urine sample below 100 µg per litre, the study’s cut-off for deficiency, against 31 of 91 vegetarian and 10 of 52 omnivore children, and the vegan group held both the lowest and the highest readings, 5.99 and 991.8 µg per litre, though every group’s median stayed above 100 (Světnička et al., 2023) [25]. A 2025 German modelling study of children aged one to three found that swapping cow’s milk for unfortified plant drinks cut their iodine intake to around half, that ordinary fortified drinks still left it lower, and that only drinks made for children kept it up (Kersting et al., 2025) [26].

How much iodine do you need in pregnancy and breastfeeding?

Needs rise to 200 µg a day in pregnancy and while breastfeeding on the European figures the BDA uses, and the WHO sets 250 µg [11][1]. Iodine matters before conception as well, because the thyroid builds up stores that help it through early pregnancy [11]. The BDA says a pregnancy multivitamin containing iodine should give no more than 150 µg, with the rest coming from food, that anyone with thyroid disease or a long-term low intake should check with their GP before taking extra iodine, and that adults should keep their total intake below about 600 µg a day [11]. Check the label, because the BDA says many, but not all, pregnancy multivitamins contain iodine [11]. Countries disagree on whether everyone should supplement: Australia and New Zealand advise pregnant and breastfeeding women to take 150 µg a day [13][14], while the UK has no official recommendation to take an iodine supplement [11].

The evidence behind that split is mixed. A 2026 meta-analysis of eight prospective cohorts linked low iodine in pregnancy with modestly lower scores on children’s development tests, an association that studies of this kind cannot prove is cause and effect (Luo et al., 2026) [21]. A 2020 meta-analysis of studies in pregnant women with mild to moderate deficiency, defined as a median urinary iodine of 50 to 149 µg per litre, pooled two randomised trials and found no effect of supplements on children’s cognitive, language or motor scores, and concluded that there was not enough good-quality evidence to support the current recommendations to supplement in such areas (Dineva et al., 2020) [20]. Its authors called for trials in women with a median below 100, and vegan adults in the 2023 meta-analysis sat far lower, at 12.2 to 44.0 µg per litre [1], so a vegan planning a pregnancy has more reason than most to secure a reliable source before conceiving. Iodine is one of several nutrients to plan for in a vegan pregnancy.

Is seaweed a good source of iodine?

Seaweed is not a reliable source of iodine, because its content varies so widely that it can easily provide too much [11]. The BDA advises against eating brown seaweeds such as kelp more than once a week, especially in pregnancy, and against seaweed or kelp supplements altogether, since their iodine can differ widely from the label [11]. Nori tends to carry less than kelp, but its content still varies too much to rely on [11]. UK endocrinologists report cases of an overactive thyroid in people who drank kelp tea daily for several weeks, and typical amounts per serving show why seaweed is a gamble while ordinary plant foods carry very little [6][11]:

Serving	Typical iodine
Portion of fruit or vegetables (80 g)	3 µg [11]
Slice of bread, UK (36 g)	5 µg [11]
Sheet of nori	15 to 40 µg [6]
Glass of iodine-fortified plant milk, UK 2020 (200 ml)	about 50 µg [9]
Glass of cow’s milk (200 ml)	50 to 100 µg [11]
Prenatal multivitamin with potassium iodide	150 µg [6]
Cup of kelp tea	500 to 3,000 µg [6]
Kelp tablet or sea-moss capsule	1,000 to 10,000 µg [6]
Portion of kombu or kelp broth	2,000 to 10,000 µg [6]

The iodine in fruit, vegetables and grains also depends on how much was in the soil they grew in [12][14].

Can you have too much iodine?

Too much iodine can upset the thyroid, and the BDA’s rough guide is that adults should not go over about 600 µg a day in total [11]. The NHS says 0.5 mg (500 µg) a day or less from supplements is unlikely to cause harm [12]. Excess can push the thyroid either way, into underactivity or overactivity, depending on the gland [6], and people with thyroid disease, or who have been short of iodine for a long time, can be affected by amounts that are normal for everyone else, which is why the BDA tells them to see their GP first [11]. In pregnancy, too much iodine can harm the baby as well as too little, and a 2023 review advises avoiding excess (Nguyen, 2023) [22]. If you take medicine for a thyroid condition, speak to your GP before adding iodine, and don’t change or stop any medicine without their advice.