Evidence explainerSea moss (Irish moss)Iodine (potassium iodide, kelp)

Sea Moss and Your Thyroid: The Iodine Number Nobody Puts on the Label

Sea moss gel and capsules are sold as a mineral-packed superfood. No human trial has tested them for any health outcome, and the real question is iodine: seaweed products range from a few micrograms to many times the adult daily limit, and labels rarely say which one you are holding.

By Kymata Labs EditorialPublished October 12, 2026Last reviewed October 12, 2026

Medical review pending. Written by Kymata Labs Editorial from the studies cited below; a credentialed clinician has not reviewed it yet. Educational only, not medical advice.

Key takeaways

  • We found no human trial of sea moss supplements for any health outcome, so the benefit claims are untested; we give it no evidence grade for benefits [5].
  • In 96 seaweed products tested in Norway, supplements ranged from 5 to 5,600 mcg of iodine per daily dose, and 54 products would exceed the Nordic adult upper limit of 600 mcg a day in one portion or dose [1].
  • Irish moss itself (Chondrus crispus) measured relatively low in iodine in one analysis, about 3.86 mg per kg dry weight, but blends and other seaweeds sold under the same name can be far higher [2][1].
  • Excess iodine can trigger hyperthyroidism, hypothyroidism or goiter in susceptible people, especially those with existing thyroid disease [3].
  • A 2026 test of 66 sea moss products sold online in the US detected arsenic and mercury in dried sea moss and recovered bacteria including Salmonella from some samples [6].

The evidence, graded

Pulled from our monograph evidence tables. Grades follow our evidence-grading policy.

The claim and what is actually known

Social videos sell sea moss as a thyroid-supporting superfood with nearly every mineral the body needs. The only graded human evidence in our sea moss monograph is a tiny trial of carrageenan, the main fibre in sea moss, in people with ulcerative colitis: 3 people on carrageenan capsules relapsed versus none on placebo [4]. That is not a trial of sea moss, and it points the wrong way [4].

NIH's LactMed database states that no medicinal value of sea moss has been proven, and that it contains iodine plus heavy metals such as arsenic, lead, cadmium and mercury from the water it grew in [5].

Iodine: the number that matters

The adult RDA for iodine is 150 mcg a day and the adult tolerable upper intake level (UL) is 1,100 mcg a day from all sources [7]. Seaweeds are among the richest iodine foods, and the amount varies greatly by species: commercial seaweeds in whole or sheet form have measured from 16 to 2,984 mcg per gram [7].

Irish moss is at the low end. A Spanish analysis of 30 red-seaweed samples found Irish moss averaged 3.86 mg of iodine per kg dry weight; the authors calculated that 4 g a day would supply about a quarter of a child's daily requirement and that an adult would need to eat about 286 g a day to pass the UL [2].

The problem is that you rarely know what is in the jar. In the Norwegian market survey, iodine per daily dose of seaweed supplements ranged from 5 to 5,600 mcg, the species and iodine content on labels were inadequate or inaccurate for several products, and 54 of 96 products would exceed the 600 mcg/day adult upper limit used in the Nordic recommendations, lower than the US UL of 1,100 mcg, with a single portion or dose [1][7]. Kelp-type species such as oarweed averaged 7,800 mcg per gram, roughly 2,000 times the Irish moss figure above [1][2]. Check the ingredient list: a product that adds other seaweeds, such as kelp or bladderwrack, cannot be assumed to match the low Irish moss figure.

  • Adult iodine RDA: 150 mcg/day; adult UL: 1,100 mcg/day from food and supplements combined [7].
  • Seaweed supplements in one market survey: 5 to 5,600 mcg per daily dose [1].
  • Irish moss in one analysis: 3.86 mg/kg dry weight (about 3.9 mcg per gram) [2].

Who is most at risk from too much iodine

High iodine intakes are tolerated by most healthy people, but in some people they can precipitate hyperthyroidism, hypothyroidism, goiter or thyroid autoimmunity [3]. People with existing thyroid disease, or who were previously iodine deficient, can react at intakes only slightly above need [3].

A published case describes a 39-year-old woman with multinodular goiter who developed iodine-induced hyperthyroidism after 4 weeks of a kelp-containing herbal tea given for her enlarged thyroid [8]. Excess iodine can also add to the effect of antithyroid medicines such as methimazole [7].

During breastfeeding, LactMed advises avoiding sea moss unless its iodine and heavy-metal levels have been shown to be low, because maternal seaweed iodine has suppressed thyroid function in breastfed infants [5]. For iodine needs in pregnancy, see iodine in prenatal vitamins.

Heavy metals and microbes in gels and dried sea moss

A Cornell team bought 66 sea moss products from major US online retailers, 53 dried and 13 gels [6]. Arsenic, cobalt and mercury were detected in dried sea moss; in gels, arsenic and cobalt were below detection limits and mercury was at trace levels [6]. Gels varied widely in acidity (pH 2.71 to 7.05), and isolates from some products included Salmonella, Cronobacter and Escherichia species [6]. The authors called for evidence-based guidance on processing, storage and handling [6].

Bottom line

There is no human evidence that sea moss improves thyroid function or anything else, and its iodine content is unpredictable from label to label [1][5]. We show no product on this page for that reason. If you have a thyroid condition, take thyroid medicine, or are pregnant or breastfeeding, a clinician can tell you whether any added iodine is appropriate. For other hidden iodine sources see sea salt, Himalayan salt and iodine and the iodine monograph; for the full safety profile see the sea moss monograph.

Quick answers

How much iodine is in sea moss?

It depends on the species and the product. Irish moss measured about 3.86 mg per kg dry weight in one analysis, while seaweed supplements in a market survey ranged from 5 to 5,600 mcg per daily dose [2][1].

Can sea moss cause thyroid problems?

Excess iodine from seaweed can trigger hyperthyroidism, hypothyroidism or goiter in susceptible people, and a case report describes hyperthyroidism after 4 weeks of kelp tea [3][8].

Is sea moss gel safer than capsules?

Not necessarily. A 2026 test of US products found mercury at trace levels in gels and wide variation in acidity and microbial quality [6].

Does sea moss help hypothyroidism?

No trial has tested it. The NIH LactMed entry states that no medicinal value of sea moss has been proven [5].

Keep reading

Full monographs

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References

  1. [1]Commercially available kelp and seaweed products - valuable iodine source or risk of excess intake?. Food & Nutrition Research, 2021. PMID 33889064
  2. [2]Risk assessment of iodine intake from the consumption of red seaweeds (Palmaria palmata and Chondrus crispus). Environmental Science and Pollution Research International, 2020. PMID 32803579
  3. [3]Excess iodine intake: sources, assessment, and effects on thyroid function. Annals of the New York Academy of Sciences, 2019. PMID 30891786
  4. [4]A randomized trial of the effects of the no-carrageenan diet on ulcerative colitis disease activity. Nutrition and Healthy Aging, 2017. PMID 28447072
  5. [5]Sea Moss (Drugs and Lactation Database, LactMed). National Institute of Child Health and Human Development, 2006. PMID 41730053
  6. [6]Microbial and Heavy Metal Analyses of Commercially Available Sea Moss and Sea Moss Products. Journal of Food Protection, 2026. PMID 42710593
  7. [7]Iodine: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2024.
  8. [8]Iodine-induced thyrotoxicosis after ingestion of kelp-containing tea. Journal of General Internal Medicine, 2006. PMID 16808731

Sea moss (Irish moss) and other dietary supplements are not intended to diagnose, treat, cure or prevent any disease, and these statements have not been evaluated by the FDA. Talk to your clinician or pharmacist before starting a supplement, especially if you take prescription medicines. This page has no product or affiliate links.

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