GUEST AUTHOR: Registered dietitian Juliette Kellow
Juliette is a registered dietitian and a member of the British Dietetic Association. She’s worked in the NHS and for the food industry, and is the former editor of magazines Slimming and Top Santé. She’s a consultant nutritionist for a number of food brands
Low iodine intakes are common in some population groups, including teenagers, women and mums-to be. Seaweed is one of the richest sources of this nutrient and while it’s not commonly consumed in the UK at the moment, more and more seaweeds – and products containing them – are becoming available. So could seaweed be a solution for boosting intakes? Do UK guidelines around eating it need to be reconsidered? Registered dietitian Juliette Kellow gives the lowdown…
In recent years, there’s been increasing awareness of the need to put iodine on the menu. Figures from the National Diet and Nutrition Survey (NDNS) show many teenagers and woman have very low intakes. Studies also reveal pregnant women are at risk of iodine deficiency, a major concern as this nutrient is vital for the development of a baby’s brain during pregnancy.
It’s a growing problem, too. The number of people with iodine intakes below the Lower Reference Nutrient Intake (LRNI) – the amount that meets the needs of just 2.5% of people – has risen across all age groups in the last decade (see Table 1)1 . More people than ever – from children at primary school to retired adults – have very low iodine intakes so are at risk of a deficiency.
Table 1: 10 year change in % below LRNI for iodine
Source: NDNS Years 1-2 (2008/09-2009/10 – pale colour) and Years 9-11 (2016/17-2018/19 – dark colour)1
Why so lean on iodine?
The growing popularity of vegan diets has recently put iodine in the spotlight. Dairy products (by fortification through sterilisation methods and fortified feeds) are the biggest supplier of iodine in the UK diet, providing around a third (32%) of our intake, with another 28% coming from seafood, eggs and meat1. So unsurprisingly, studies show vegan diets – which exclude these foods – can be lower in this nutrient.
However, as Table 1 confirms, low iodine intakes aren’t unique to the last few years when vegan and plant-based diets have gained a wider following. They’ve been a concern for many years in certain population groups. For example, in 2011 findings from a national survey looking at iodine status for the first time in more than 60 years highlighted potential low intakes in teenage girls. Other studies around this time, and even earlier, indicated signs of deficiency in pregnant women and those of child-bearing age.
The reality is, while vegan diets can make it harder to get enough iodine, the low intakes witnessed in the past decade or so are most likely to be the result of poor diets overall. In groups of people who are at greatest risk of iodine deficiency – teenagers and women, for example – low iodine intakes sit alongside low intakes of many other nutrients including iron, calcium, magnesium, potassium, selenium and zinc1. This suggests diets are generally lacking in nutrient-rich foods.
As dairy products are the main provider of iodine in UK diets, there are concerns that as the popularity of vegan diets grows – and more people switch from cows’ milk to plant alternatives – so too will the number of people suffering with low iodine intakes. However, there’s no current evidence to suggest milk consumption is dropping. While plant-based milks have enjoyed a recent surge in popularity, dairy intakes have remained fairly stable, suggesting plant-based milks are being added to diets containing dairy, rather than replacing them completely. Figures from DEFRA’s latest annual Family Food report show between 2013 and 2019, average intakes of milk and yogurt stayed at around 1.8 litres per person per week.
The amount of iodine provided by dairy has remained static, too – for example, milk provided 26% of the iodine in the diets of 11-18 year olds in both years 1-2 and years 9-11 of the NDNS1. Nevertheless, the British Dietetic Association is pushing for standardised iodine fortification of plant-based milks to a similar level seen in cows’ milk. It’s a positive step to help boost iodine intakes in those who don’t consume dairy… but many experts believe it’s unlikely to have a major impact on iodine intakes in the wider population who continue to consume dairy products.
Sea food…and eat it
Another way to boost iodine intakes is to eat more seafood in line with current health guidelines that recommend two 140g portions of sustainably sourced fish each week, one of which should be an oily fish such as mackerel or sardines. Many varieties of white fish and some shellfish such as pollock, cod, coley, haddock, whiting, sardines, sprats, cockles, crab, langoustine and mussels are naturally rich in iodine.
However, consumption of seafood in the UK is low. Adults of working age eat just one portion of fish and less than half a portion of oily fish a week; children aged 11-18 years have just over half a portion of fish a week and only eat one portion of oily fish every seven weeks1! It means seafood provides just one tenth of the iodine in our diet, despite being a good source of this nutrient.
A new catch of the day
But there’s another food from our oceans that could help to boost iodine intakes – and it’s suitable for those of us who want to add more plants to our diet. Cue seaweed! Although not commonly eaten in the UK, it’s becoming increasingly popular. Last December, Waitrose reported sales of seaweed had increased by 23% compared to 2019 and by 71% compared with 2018. Searches for ‘seaweed’ on the supermarket’s website increased by 141% in a year. Seaweed was set to be the must-have ingredient for 2021 said the supermarket!
The seaweed scene for iodine
It’s potentially good news as seaweed is packed with iodine. Nevertheless, the BDA offers a somewhat cautious approach stating: ‘Seaweed is a very concentrated source of iodine, but it can provide excessive amounts (particularly so in the case of brown seaweed such as kelp) and therefore eating seaweed more than once a week is not recommended, especially during pregnancy.’2
However, not all seaweed is created equal. Certainly, very high levels of iodine can be harmful to health, for example, causing thyroid problems. But blanket advice to avoid or limit seaweed may mean many of us potentially miss out on a very useful source of iodine, especially if we follow a plant-based diet or don’t consume dairy or seafood. The key is to know the iodine content of different varieties of seaweed and to consider the quantity and frequency with which they’re eaten.
Diving into the data on iodine levels
Finding the iodine content of different seaweeds can be a challenge as most products don’t include values on their packaging. Researchers Dr Emilie Combet and Maria Bouga from the University of Glasgow undertook an extensive survey of seaweed and seaweed-containing products in 2014 and found only one in 10 products provided iodine values. Fast forward to the current day and a scan of available products reveals iodine labelling remains scant, although this may change as awareness of this nutrient grows!
Fortunately, McCance & Widdowson’s The Composition of Foods provides average iodine values for three dried varieties of seaweed that are reasonably easy to find in the UK: nori (1,470mcg/100g), wakame (16,830mcg/100g) and kombu (440,670mcg/100g).
Meanwhile, Dr Emilie Combet and Maria Bouga have calculated the average iodine contents of different fresh and dried seaweeds (see Table 2). However, it’s important to bear in mind that, like many plant foods, the iodine content of seaweed can vary depending on where it grows. Plus, there’s the question of bioavailability. One in vitro study reports the iodine available for absorption from seaweed after the digestive process varies between 49-82% but this will depend on the food matrix, what else we eat with it, how we cook it and even our lifestyle – smoking, for example, inhibits iodine absorption13. Ultimately, a high iodine concentration in food doesn’t automatically mean high levels are absorbed by the body and it’s an area that needs more research.
Table 2: Iodine content of fresh and dried seaweed varieties12
|Average iodine content mcg/100g|
*Values differ from McCance & Widdowson’s The Composition of Foods
Nori enough…or too much iodine?
In the UK, the Reference Nutrient Intake (RNI) for iodine from 15 years onwards is 140mcg/day; for children, it varies from 50-140mcg depending on age. There are no recommended increases for pregnant or breastfeeding women. For food labelling purposes the Nutrient Reference Value for iodine is 150mcg. Similarly, the European Food Safety Authority (EFSA) sets a daily Adequate Intake for iodine at 150mcg for adults, increasing to 200mcg for pregnant and breastfeeding women.
Meanwhile, because high levels of iodine can be toxic, EFSA has set daily Tolerable Upper Intake Levels13 for iodine for different age groups as follows:
- 1-3 years – 200mcg
- 4-6 years – 250mcg
- 7-10 years – 300mcg
- 11-14 years – 450mcg
- 15-17 years – 500mcg
- Adults, including pregnant and breastfeeding women – 600mcg
Supplements shouldn’t contain more than 150mcg iodine a day and the BDA recommends avoiding seaweed or kelp supplements altogether, saying the amount of iodine in such supplements ‘can vary considerably from the value claimed on the label and can provide excessive quantities of iodine’2.
A kelping hand or hinderance for iodine?
Knowing the iodine content of different seaweed varieties a crucial factor in deciding whether seaweed is a potential help or hinderance. Compared with recommended upper limits for iodine, it’s clear from Table 2 that some varieties of fresh and dried seaweeds can greatly exceed these levels, even if consumed in small amounts. For example, just 1g dried kelp provides 1,327mcg iodine – more than double the recommended upper limit!
However, this isn’t the case for all seaweeds. One of the most common types available in the UK is dried nori12, often found in the form of snacks or sheets that can be used to make sushi or as a garnish for noodles, soups or salads. With a much lower iodine content than many other dried seaweed varieties, nori is actually a great choice for boosting intakes. For example, based on values presented in Table 2, a 2g sheet of dried nori contains 42mcg iodine – roughly a third of daily needs.
There are also lessons to be learnt from East Asian countries such as Japan, Korea and parts of China, where seaweed is a regular part of the diet. As a rule, these populations seem to strike a happy balance between gaining the benefits of enjoying seaweed without overdosing on iodine – most likely due to the type and amount of seaweed they eat. In Japanese diets, it seems wakame and nori are the most popular seaweeds used in cooking. According to one report, wakame, which is used to make salads and miso sops, accounts for 30% of seaweed consumed, and nori accounts for 45%.
The amounts eaten are relatively small, too. It’s estimated average intakes of seaweed per adult per day are 5.2g in China, and 8.5g in both South Korea18 and Japan.
There’s currently no data on seaweed consumption in European diets and within the UK, the only guidance on consumption is from the British Dietetic Association, which focusses largely on preventing excessive iodine intakes2. However, based on the main type and amount of seaweed consumed in East Asia, around 5g dried nori a day would seem to be a suitable serving, providing around 100mcg iodine a day or about 70% of daily needs. Dried wakame is much higher in iodine – just 1g provides 172mcg iodine so it would be advisable to stick to servings of no more than 1g a day.
Seaweed: Health hero or health hazard?
Seaweed doesn’t just come with iodine – depending on the species, it provides varying amounts of other nutrients, too, such as soluble fibre, protein, iron, magnesium, vitamin B12 (often in an inactive form), carotenoids and polyphenols. However, as only small amounts are usually eaten, the contribution to daily intakes for many nutrients is often quite small. Iodine is, of course, an exception – tiny amounts of seaweed can still deliver a large dose.
Studies have also looked at the impact seaweed itself may have on our overall health and wellbeing. Much of this has been based on Asian populations, with studies suggesting seaweed may reduce the risk of ischaemic heart disease and mortality from stroke, probably due to lowering blood pressure and lipids .
However, this needs to be balanced with the fact that some seaweeds can contain such high iodine levels that, in theory, they may do more damage than good. In reality though, studies – mainly from Japan – provide conflicting results making it difficult to draw any firm conclusions about whether iodine from seaweed or seaweed itself harms health20.
Another concern relates to seaweed containing heavy metals such as inorganic arsenic (the toxic carcinogenic form), mercury, cadmium and lead that are harmful to health20,21. It’s an area that’s currently being researched with funding from EFSA with a view to creating a global review of levels of these contaminants in seaweed and potentially setting maximum levels for seaweed in the future.
The conclusion from work so far suggests levels of mercury, cadmium and lead in seaweed pose a low health risk for the general population, however levels should be monitored. More research is needed to draw conclusions on arsenic. However, the UK Food Standards Agency advises against eating hijiki seaweed because it can contain high levels of inorganic arsenic. The researchers suggest for iodine, as well as setting maximum levels, the species of seaweed used in products should be identified on food labels to help reduce exposure to high levels.
What about products that contain seaweed?
The University of Glasgow research found out of 224 products available in the UK, just 23% were fresh or dried seaweed; the remainder were seaweed-containing products such as breads, sushi, pasta, soups, salads, snacks, condiments and supplements12. Many of these contain only small amounts of seaweed so may be useful for boosting iodine intakes. But a lack of iodine information on the nutrition panel means for most products it’s difficult to know the contribution they make.
It’s also important to check the overall nutrition and ingredients list. The sea is salty so seaweed is naturally higher in salt than many other vegetables, although levels in products based on 100% seaweed tend not to be too high. Indeed, the market for seaweed seasonings and flakes is growing. Such products often promote themselves as an alternative to salt – and can have a fraction of the sodium of regular table salt making them a good choice. But it remains important to check labels (although many niche products still fail to give any nutrition). Some seasonings are blended with other ingredients, such as herbs, spices, onions and garlic – and can include varying amounts of added salt.
Meanwhile, seaweed salt is available – effectively, it blends salt with a small amount of seaweed so isn’t a great way to add iodine to diets.
Finally, a few ‘seaweed’ products – such as crispy seaweed sold as an accompaniment to Chinese meals – contain no seaweed at all and instead are made from spring greens!
Table 3 looks at the salt content (and iodine content where available) of a selection of products currently available. It is not an exhaustive list, simply a snap-shot of products currently available in UK supermarkets. Serving sizes are based on single serve pack sizes or on-pack serving recommendations.
Table 3: Seaweed products available in UK supermarkets
|Product||Serving size||Iodine (mcg)||Salt (g)||Ingredients|
|The Cornish Seaweed Company, Cornish Seaweed Salt||1g||NV||0.73||Cornish Sea Salt, Dulse, Gutweed, Nori|
|Bart Sea Salt & Seaweed||1g||NV||0.94||Sea Salt, Organic Seaweed (7.5%)|
|Seaweed & Seaweed Seasonings|
|Seaspoon Seaweed Boost||2.5g||NV||0.13||Seaweed (100%) (Dulse, Sea Spaghetti, Sea Lettuce, Sea Greens)|
|Atlantic Kitchen Dulse Seaweed||10g||2,890||0.39||100% Dulse Seaweed|
|Atlantic Kitchen Sea Spaghetti Seaweed||12.5g||1,825||1.14||100% Sea Spaghetti Seaweed|
|Atlantic Kitchen Wakame Seaweed||10g||1,420||1.39||100% Wakame Seaweed|
|Mara Seaweed Shony Seaweed Seasoning||1g||2000||0.08||100% Organic Seaweed (Dulse, Kombu (Laminaria Digitata) Sugar Kelp)|
|The Cornish Seaweed Company Sea Salad Sea Flakes||5g||NV||0.34||Dulse, Sea Greens, Nori|
|The Cornish Seaweed Company Dried Sea Spaghetti||5g||NV||0.46||Sea Spaghetti|
|Seaspoon Seaweed Umami Mix||5g||NV||0.26||Seaweed (40%) (Dulse, Sea Spaghetti, Sea Lettuce, Sea Greens), Tomatoes, Mushrooms, Onions, Garlic, Salt, Pepper|
|Clearspring Japanese Green Nori Sprinkle||1g||NV||0.07||Dried Sea Vegetable (Ulva Pertusa)|
|Tao Kae Noi Tempura Seaweed Original||40g||NV||0.84||Wheat Flour (Contains Gluten) (35%), Seaweed (25%), Modified Tapioca Starch, Palm Oil, Egg White Powder, Salt, Sugar, Hydrolysed Soya Protein, White Pepper Powder, Flavour Enhancers, E631, E627|
|Abakus Foods Seaweed Crisps Sea Salted||18g||35||0.5||Tapioca (39%), Seaweed (Nori) (27%), Non-GMO Corn Oil, Rice, Sea Salt, Onion Powder|
|Abakus Foods Seaweed Crisps Salt & Vinegar||18g||34||0.4||Tapioca (37%), Seaweed (Nori) (26%), Non-GMO Corn Oil, Rice, Dried Vinegar, Sea Salt|
|Itsu Seaweed Thins Sweet Soy & Sea Salt||5g||92||0.14||Seaweed (Laver) (65%), Corn Oil, Sesame Oil, Sweet Soy Seasoning (Sugar, Salt, Corn Starch, Garlic Powder, Onion Powder, Soy Sauce Powder Soya Beans, Maltodextrin, Salt), Sea Salt|
|Itsu Crispy Seaweed Thins Wasabi||5g||92||0.22||Seaweed (Laver) (65%), Corn Oil, Sesame Oil, Sea Salt, Wasabi Powder (Salt, Glucose, Spice Extracts (contains Mustard), Yeast Extract Powder, Spinach Powder, Corn Oil, Wasabi Stem Powder)|
|Itsu Crispy Seaweed Thins Korean Bbq||5g||124||0.09||Seaweed (Laver) (65%), Corn Oil, Barbecue Seasoning (Dextrin, Sugar, Soya Bean Powder Soya Beans, Maltodextrin, Salt], Garlic Powder, Onion Powder, Tomato Powder, Salt, Paprika, Smoked Paprika, Black Pepper, Clove Powder, Acidity Regulator: Citric Acid), Sesame Oil|
|Selwyns Seaweed Snack Salt & Vinegar||4g||NV||0.03||Dried Seaweed (Porphyra), Rapeseed Oil, Salt, Rice Flour, Dried Vinegar, Sugar, Acidity Regulator: Citric Acid, Dried Balsamic Vinegar, Acidity Regulator: Malic Acid|
|Oceans Halo The Seaweed Snack Sea Salt||4g||NV||0.1||Organic Seaweed, Organic Sunflower Oil, Sea Salt|
|M&S Lightly Salted Seaweed||5g||NV||0.13||Seaweed (69%), Rapeseed Oil, Sea Salt (1%), Sesame Oil|
|Nagai Roasted Seaweed Sushi Nori||2.8g (1 piece)||NV||0.05||Dried Seaweed (Porphyra Yezoensis)|
|Yutaka Sushi Nori Half Cut||1.4g (1 piece)||NV||0.02||Seaweed (100%)|
|Clearspring Japanese Sushi Nori||2.4g (1 piece)||NV||0.03||Dried Sea Vegetable (Porphyra Yezoensis)|
|Blue Dragon Sushi Nori||2.2g (1 piece)||NV||0.03||Roasted Seaweed (100%)|
NV = No values
It’s clear more research is needed to assess levels of iodine and heavy metals in different varieties of seaweed – ideally with a view to setting national guidance levels on permitted maximum amounts. It’s also clear while blanket advice to limit seaweed is designed to safeguard against excessive iodine intakes, this may also detract from the fact some varieties of seaweed such as dried nori, which have a much lower iodine content, may be useful in helping to boost intakes and prevent deficiency, especially in those who follow vegan or predominantly plant-based diets that avoid dairy and seafood.
With the above points in mind, to help UK consumers gain benefits from adding seaweed to their diets, the following would be beneficial:
- Encourage manufacturers to undertake nutrition analysis for iodine and to provide this information on packaging for all seaweed and seaweed-containing products. This will help consumers avoid very high concentrations that may be linked to toxicity whilst also identifying safe and useful amounts of iodine to help boost intakes and reduce the risk of deficiency.
- Encourage manufacturers to provide greater detail on the species of seaweed used in products – for example, many products simply state ‘seaweed’ in the ingredients list making it impossible to get even a basic understanding of the potential iodine content. Similarly, some products just state ‘kelp’ but there are many different species, which can vary considerably in their iodine content12.
- Create national guidelines that give specific guidance on suitable levels of seaweed consumption for different varieties.
- Raise public awareness that it’s best to limit some (but not all) seaweed varieties such as kelp to no more than once a week, and if it’s eaten, to opt for very small servings. This will help reduce excessive iodine intakes, which may harm health. This advice is particularly important for pregnant women to follow.
- At the same time, raise awareness that enjoying small amounts of some types of seaweed can help to increase iodine intakes, making it easier to meet requirements and prevent iodine deficiency. A good option is around 5g dried nori, which provides about 70% of an adult’s daily iodine needs.
- Provide UK standardised macronutrient and micronutrient nutrition information for a wider range of seaweeds available in the UK.
- Encourage consumers to check the overall nutrition content of seaweed-containing products such as seaweed seasonings, snacks and soups, especially for salt.
- Raise awareness that supplements should contain no more than 150mcg iodine in a daily dose – and that kelp supplements are best avoided as they may contain excessive amounts of iodine despite what the label says. Pregnant women in particular should be advised to avoid kelp supplements.
- Prioritise the safeguarding of the environment as seaweed popularity grows and demand increases. Guidelines and regulations should be put in place to ensure seaweed is sustainably sourced.
 PHE/FSA (2020) National Diet and Nutrition Survey: Rolling Programme Years 9 to 11 (2016/2017 to 2018/2019).
 British Dietetic Association (2019) Iodine: Food Fact Sheet
 Eveleigh, E R et al (2020) Vegans, vegetarians, and omnivores: How does dietary choice influence iodine intake? A systematic review. Nutrients 2020, 12(6): 1606
 British Dietetic Association (2018) Iodine deficiency in the UK – dietetic implications
 Dineva, M et al (2020) Iodine status of consumers of milk-alternative drinks v. cows’ milk: data from the UK National Diet and Nutrition Survey. Br J Nutr 126,1
 Mintel Press Office (2019) Milking the vegan trend: a quarter (23%) of Brits use plant-based milk. 19 July 2019
 DEFRA (2020) National Statistics Family Food 2018/19. Published 29 October 2020.
 British Dietetic Association (2021) Iodine: The debate about fortification
 Mynutriweb (2021) Iodine: What action needs to be taken?
 NHS. Fish and shellfish
 Waitrose (2020) Seaweed set to be the superfood of 2021
 Bouga, M and Combet, E (2015) Emergence of Seaweed and Seaweed-containing Foods in the UK: Focus on Labelling, Iodine Content, Toxicity and Nutrition. Foods 2015, 4:240-253.
 European Food Safety Authority (2006) Tolerable Upper Intake Levels for Vitamins and Minerals. Accessed online 21 July 2021.
 Dominguez-Gonzalez, M R et al (2017) Evaluation of iodine bioavailability in seaweed using in vitro methods. J Agric Food Chem 2017, 65 (38):8435-8442
 Department of Health (1991) Dietary Reference Values for Food Energy and Nutrients for the United Kingdom, HMSO.
 European Food Safety Authority (2014) Scientific Opinion on Dietary Reference Values for Iodine. Published 7 April 2014.
 Zava, T T and Zava, D T (2011) Assessment of Japanese iodine intake based on seaweed consumption in Japan: A literature-based analysis.Thyroid Res 4: 14 doi
 Roleda, M Y et al (2018) Variations in polyphenol and heavy metal contents of wild-harvested and cultivated seaweed bulk biomass: Health risk assessment and implication for food applications. Food Control 95: 121-134
 The Ministry of Health, Labour and Welfare. The National Health and Nutrition Survey (NHNS) Japan, 2018. Summary.
 Cherry, P et al (2019) Risks and benefits of consuming edible seaweeds. Nut Rev 2019, 77(5):307-329. doi: 10.1093/nutrit/nuy066.
 Murai, U et al (2021) Impact of seaweed intake on health. Eur J Clin Nutr 75 (5): 877-889 doi: 10.1038/s41430-020-00761-w.
 Sa Monteiro, M et al (2019) Analysis and risk assessment of seaweed. EFSA J 2019, 17(Suppl 2):e170915. doi: 10.2903/j.efsa.2019.e170915.eCollection 2019 Sep.
 Food Standards Agency (2020) Importing fruit and vegetables. Updated 31 December 2020. https://www.food.gov.uk/business-guidance/importing-fruit-and-vegetables
Thank you for such a brilliant article on iodine.
It leaves me with the impression that deficiency in adults is more common in women in general. Is that correct?
Appart from smoking cigarettes are there any other substances (e.g. other nutrients like mineral, etc) that can hinder absorption of iodine from diet?
Thanks for your feedback and questions! Teenage girls and young women are the most likely to get insufficient iodine in their diets and studies particularly show iodine deficiency in pregnant women. The following link gives more detail on this: https://www.bda.uk.com/resource/iodine-deficiency-in-the-uk-dietetic-implications.html
Iodine deficiency in men in the UK isn’t an area that’s been researched very much. However, figures from The National Diet and Nutrition Survey do show that the percentage of people with iodine intakes below the LRNI has increased in both men and women across all age groups in the last decade. Currently 19% of 11-18 year old males (compared with 28% of females) and 8% of 19-64 year old men (vs 12% of women) have intakes below the LRNI. While this doesn’t translate into being iodine deficient, it does mean individuals are at a greater risk of becoming deficient.
In terms of your second question about substances that can hinder absorption, EFSA’s Tolerable Upper Intake Levels for Vitamins and Minerals provides detail on factors that affect iodine absorption. Details are on page 135-136 of the following link: https://www.efsa.europa.eu/sites/default/files/efsa_rep/blobserver_assets/ndatolerableuil.pdf
The report states that cooking reduces the iodine content of food, frying by 20%, grilling by 23% and boiling by 58%.
The report also indicates that dietary iodine absorption and incorporation is reduced by smoking, thiocyanates, isothiocyanates, nitrates, fluorides, calcium, magnesium and iron in food and water. Degradation of cyanoglycosides, glucosinolates and goitrins present in vegetables like brassica, crucifera, rape, cabbage, cauliflower, broccoli, Brussels sprouts, kale, kohlrabi, turnips, maize, lima beans, bamboo shoots, peanuts, walnuts, sweet potatoes, millets and cassava block the thyroidal uptake of iodine after ingestion, thereby decreasing the production of thyroid hormones. Soya flour also inhibits the absorption of iodide by interference with the enterohepatic circulation of thyroxine.
Practical information on thyroid disorders and diet is provided by the British Thyroid Foundation here: https://www.btf-thyroid.org/thyroid-and-diet-factsheet
There are also some useful Q&As here: https://www.ukiodine.org/faqs/