Kelp and Seaweed Supplements: Iodine Content, Heavy Metals and Thyroid Risks
What research and thyroid and food-safety bodies say about kelp and seaweed supplements, including very variable iodine content, thyroid problems, arsenic in some seaweeds and who should ask first.
Content last updated . This page has not been medically reviewed.
The main concern in plain words
Kelp is a seaweed that MedlinePlus calls a rich source of iodine. [1] Supplements containing kelp or other seaweed are sold. [2] The first thing to know is that the iodine in these products can be very low or very high, and the label may not tell you which. A Norwegian laboratory study tested 96 seaweed products, including 14 supplements, and found 5 to 5,600 micrograms (mcg) of iodine per daily dose in the supplements. [3] The upper level for adults is 1,100 mcg a day in the US and Canada. [2] [4]This page gives information only. If you have thyroid disease, are pregnant or breastfeeding, or take thyroid medicine, ask a doctor or pharmacist before using any kelp product.
How we label evidence here: laboratory measurement means someone analyzed products in a lab; case report is one patient and cannot show how common a problem is; regulator or professional statement is advice from a named body in a named country.
What the sources found
| Question | What the source found | Type of evidence |
|---|---|---|
| How much iodine is in kelp and seaweed products? | In 96 products from the Norwegian market, supplements gave 5 to 5,600 mcg per daily dose; 54 products would exceed the upper level in one portion. [3] | Laboratory measurement, one country, 2021 |
| Is the label reliable? | The same study found labelling of seaweed species and iodine content was inadequate or inaccurate for several products. [3] | Laboratory measurement |
| Does iodine vary within seaweed? | NIH ODS reports whole or sheet seaweeds from 16 to 2,984 mcg of iodine per gram. [2] | Government fact sheet |
| Can kelp tablets affect the thyroid? | A 70-year-old woman with no earlier thyroid disease developed temporary overactive thyroid about 3 months after taking kelp tablets. It resolved after she stopped and was treated. [5] | Case report |
| Is there a benefit above the recommended level? | The American Thyroid Association said there is no known thyroid benefit of routine daily iodine above the US recommended daily allowance. [6] | Professional statement, 2013 |
What the American Thyroid Association said
In June 2013 the ATA advised against iodine and kelp supplements containing more than 500 mcg of iodine daily. It said intake above 1,100 mcg a day may cause thyroid dysfunction and that many iodine, potassium iodide and kelp supplements contain amounts up to several thousand times higher than the tolerable upper limits. [6] This is a US statement from 2013. We did not find a newer ATA statement in the pages we read.NIH ODS adds that high iodine can cause both an underactive and an overactive thyroid, and that people with autoimmune thyroid disease or iodine deficiency may react to amounts that are usually safe for others. [2]
Arsenic and other seaweed contaminants
Arsenic is one contaminant that has been measured in seaweed. In a UK survey of 31 retail seaweed samples, arsenic was found in every sample. Inorganic arsenic, the more harmful form, was found only in the hijiki samples, and the UK Food Standards Agency advised consumers to avoid eating hijiki. [7] A 2024 survey of 25 seaweeds from five origins found that inorganic arsenic gave a low risk except in Hizikia fusiforme samples. [8]For other metals such as lead, cadmium and mercury in kelp supplements: Proper evidence not found in the sources we read. We searched PubMed for kelp and seaweed supplement heavy-metal analyses and did not find a usable study. For product testing in general, see Quality testing and certifications and Heavy metals in mineral supplements.
Sea moss
Sea moss is sold as a seaweed-type product. Proper evidence not found: we searched PubMed for sea moss and Irish moss iodine content and for human trials and found nothing we could use. Because it is seaweed, the general seaweed cautions above are the safe assumption, but we cannot give an iodine figure.Regulation
The US FDA says it is not authorized to approve dietary supplements for safety and effectiveness before they are marketed. [9] That is a statement about US supplements; rules differ in other countries.Who should ask first
- Thyroid disease or thyroid medicine. NIH ODS says high iodine plus antithyroid medicines can add up and cause an underactive thyroid. [2] Tell your doctor or pharmacist about any seaweed product, and see Medication interactions.
- Pregnancy and breastfeeding. The ATA statement names pregnancy and lactation among the situations where it advised against kelp and iodine supplements above 500 mcg. [6] See Iodine in pregnancy and breastfeeding.
- Children and older adults. The ATA said infants and older adults are among those more susceptible to excess iodine. [6]
- Kidney and liver conditions, and before surgery. The sources we read did not cover kelp in these situations. Ask your clinician, and see Supplements before surgery.
- Hair, weight loss and thyroid "support" claims. We found no trial evidence for these uses in the sources we read.
Seaweed as a food is covered on Foods high in iodine, and the thyroid itself on Iodine and the thyroid.
Frequently asked questions
How much iodine is in a kelp supplement?
It varies enormously. In one 2021 study of seaweed supplements, iodine ranged from 5 to 5,600 mcg per daily dose, and labelling was sometimes inaccurate. [3]
Can kelp supplements cause thyroid problems?
They can in some people. A case report describes temporary overactive thyroid after kelp tablets, [5] and the ATA warned that excess iodine may cause thyroid dysfunction. [6]
Is kelp safe in pregnancy?
The ATA advised against kelp and iodine supplements above 500 mcg of iodine daily during pregnancy and lactation. [6] Ask your midwife or doctor before using any kelp product.
Is hijiki safe to eat?
The UK Food Standards Agency advised consumers to avoid hijiki because it contains inorganic arsenic. [7] A 2024 survey of 25 seaweeds found inorganic arsenic a low risk except in one species, Hizikia fusiforme. [8]
Is sea moss a good iodine source?
We did not find a usable source with iodine measurements for sea moss, so we cannot say. Proper evidence not found.
References
- MedlinePlus Medical Encyclopedia, Iodine in diet (US; review date 1/21/2025). Read 2026-10-23.
- NIH Office of Dietary Supplements, Iodine: Fact Sheet for Health Professionals (US). The ODS site blocked direct download, so this was read from an Internet Archive copy captured 2 January 2026. 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 (abstract read). Read 2026-10-23.
- Health Canada, Dietary reference intakes tables: reference values for elements (page dated 2025-11-19; read through a web-fetch tool that summarizes pages). 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 (abstract read). Read 2026-10-23.
- American Thyroid Association, ATA Statement on the Potential Risks of Excess Iodine Ingestion and Exposure (US; statement of June 2013). Read 2026-10-23.
- Rose M et al., Arsenic in seaweed: forms, concentration and dietary exposure, Food Chem Toxicol 2007 (UK retail survey; abstract read). Read 2026-10-23.
- Yu Y et al., Survey of arsenic content in edible seaweeds and their health risk assessment, Food Chem Toxicol 2024 (abstract read). Read 2026-10-23.
- US FDA, Information for Consumers on Using Dietary Supplements. Read 2026-10-23.