Seaweed, Iodine and the Thyroid
Can seaweed affect your thyroid? How much iodine seaweed products contain, what official upper limits say, what studies found, and who should ask a clinician first.
Content last updated . This page has not been medically reviewed.
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Short answer: yes, it can. Iodine is needed to make thyroid hormones, and seaweed can supply a lot of it. The amount in seaweed products varies enormously and is often not on the label, so seaweed can be a source of too much iodine. [1][2]
How the sources define the terms
- RDA (recommended dietary allowance): the daily amount that meets the needs of nearly all healthy people. Health Canada's 2025 table lists 150 mcg per day of iodine for adults. [3]
- Tolerable upper intake level (UL): the highest daily intake not expected to cause harm in most people. The same table lists 1,100 mcg per day for adults, and for pregnancy and lactation aged 19 to 50. [3] Two European product surveys cite a European adult upper level of 600 mcg per day. [4][5]
- American Thyroid Association (ATA) advice: it advises against iodine and kelp supplements containing more than 500 mcg of iodine daily, and says that in pregnancy and lactation recommended upper limits vary from 500 to 1,100 mcg daily. [6]
Different bodies set different numbers for different purposes. We could not read the NIH Office of Dietary Supplements iodine page (it returned an access error), so we do not quote it. These are official reference values, not advice for any one person; a clinician or pharmacist decides what is right for you.
What studies found
| Question | What the source found | Strength of evidence |
|---|---|---|
| How much iodine is in seaweed products? | In 96 Norwegian products, one portion of whole seaweed held 128 to 62,400 mcg, supplements held 5 to 5,600 mcg per daily dose, and 54 products would exceed the upper level in one portion or dose. The authors called such products unreliable iodine sources. [2] | Product survey, one country |
| Are labels reliable? | Of 224 UK seaweed-containing products, 10% stated iodine content. [4] | Product survey |
| Dutch products | Iodine ranged from under 0.1 to 7,910 mcg per gram of fresh weight; in a lab digestion model, 24 to 56% was bio-accessible; 25% of 80 regular seaweed consumers exceeded 600 mcg a day over the last month. [5] | Survey plus lab model |
| Regular seaweed eaters | In 44 Norwegian seaweed consumers the median urinary iodine was 1200 mcg/L, which the authors called excessive iodine status; TSH levels were within reference values. [7] | Small observational study |
| Thyroid harm | A case report: a 70-year-old woman with no previous thyroid disease developed temporary hyperthyroidism about 3 months after taking kelp tablets; thyroid function returned to normal after she stopped them and received treatment. [8] | Single case |
| Who is at risk? | A 2019 review says people with existing thyroid disease or past iodine deficiency may be more susceptible, sometimes at intakes only slightly above need. [1] | Review |
Frequently asked questions
Does kelp help the thyroid?
The ATA says there is no known thyroid benefit of routine daily iodine above the US RDA. [6] Kelp supplements can contain far more than that. [2]
How much iodine is too much?
Official upper limits differ: 1,100 mcg per day for adults in Health Canada's table, [3] 600 mcg in the European figure cited by two surveys. [4][5] Individual sensitivity varies. [1]
Can eating seaweed cause an overactive thyroid?
Excess iodine, including from certain seaweeds, may in some people lead to an overactive or underactive thyroid. [1]
Is dried seaweed lower in iodine than kelp supplements?
Not necessarily. One portion of some whole seaweed products contained more iodine than the highest supplement daily dose in the same survey. [2]
Are low-iodine seaweeds available?
Content varies widely by species and product. [2] We found no source that identifies a reliably low-iodine product, so check with a clinician.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Limits and uncertainty
The survey and consumer studies measure iodine and urine levels, not long-term thyroid disease. The lab digestion result suggests less than all the iodine is absorbed, but the authors say this needs confirmation in people. [5] A single case report cannot show how common problems are. We did not read studies of seaweed iodine in children, so we make no claim for them.
Seaweed is not only a risk: a UK review notes it could help with low dietary iodine, but also that some seaweeds carry risks from high iodine and from arsenic and heavy metals. [4] See Seaweed, Sea Moss and Kelp for contaminants and Iodine for the nutrient itself.
Who should ask a clinician before using seaweed products
- People with thyroid disease, or past iodine deficiency. [1]
- People taking antithyroid medicines: high-dose iodine may add to their effect and could cause an underactive thyroid. [9]
- People who are pregnant or breastfeeding: iodine is actively transported into breast milk, and some sources recommend against seaweed while breastfeeding. [10] See Pregnancy and Breastfeeding.
- Anyone with new symptoms. The case report patient had a fast heartbeat, trouble sleeping, anxiety and weight loss. [8] Call a clinician; call 911 for chest pain, fainting or trouble breathing.
Never stop or change a prescribed medicine on your own; ask the prescriber or pharmacist. More on thyroid symptoms: Thyroid Problems and Fatigue.
References
- Farebrother J, Zimmermann MB, Andersson M. Excess iodine intake: sources, assessment, and effects on thyroid function. Ann N Y Acad Sci. 2019;1446(1):44-65. PubMed 30891786. Read 2026-10-23.
- Aakre I, et al. Commercially available kelp and seaweed products - valuable iodine source or risk of excess intake? Food Nutr Res. 2021;65. PubMed 33889064. Read 2026-10-23.
- Health Canada. Dietary reference intakes tables: Reference values for elements. Page dated 2025-11-19. Read 2026-10-23.
- Bouga M, Combet E. Emergence of Seaweed and Seaweed-Containing Foods in the UK: Focus on Labeling, Iodine Content, Toxicity and Nutrition. Foods. 2015;4(2):240-253. PubMed 28231201. Read 2026-10-23.
- Melse-Boonstra A, et al. Content, bio-accessibility and intake of iodine from seaweed-containing food products available in The Netherlands. Eur J Nutr. 2025;65(1):7. PubMed 41351699. Read 2026-10-23.
- American Thyroid Association. ATA Statement on the Potential Risks of Excess Iodine Ingestion and Exposure. Published 2013, updated 2024. Read 2026-10-23.
- Aakre I, et al. Iodine Status and Thyroid Function in a Group of Seaweed Consumers in Norway. Nutrients. 2020;12(11):3483. PubMed 33202773. Read 2026-10-23.
- Gherbon A, et al. Transient Hyperthyroidism following the ingestion of complementary medications containing kelp seaweed: A case-report. Medicine (Baltimore). 2019;98(37):e17058. PubMed 31517826. Read 2026-10-23.
- MedlinePlus Medical Encyclopedia. Iodine in diet. Reviewed 2025-01-21. Read 2026-10-23.
- Drugs and Lactation Database (LactMed). Seaweed. National Institute of Child Health and Human Development; record dated 2024-10-15. PubMed 30000910. Read 2026-10-23.