Sea Moss Side Effects: Who Should Avoid It and Why

Sea moss's side effects are the side effects of its iodine, and of whatever the sea it grew in held. Excess iodine can tip the thyroid either way, towards an overactive or an underactive gland, and does so most readily in people who already have thyroid disease or were once short of iodine; in healthy adults, kelp capsules raised TSH within four weeks.^{1, 2} So the people who should not add sea moss without asking first are those with Hashimoto's, Graves' disease or thyroid nodules, anyone taking levothyroxine, women who are pregnant or breastfeeding, children, anyone with kidney disease, and anyone on an anticoagulant. For everyone else, daily use is a dose question: the median UK seaweed product serving carries 585 µg of iodine and only one in ten products states a figure, against an adult reference intake of 150 µg and an upper level of 600 µg a day.^{3, 4} Why its iodine varies so much is on the sea moss explainer.

Sources checked on 30 October 2026.

What are the side effects of sea moss?

Three kinds. The first and best documented is thyroid disturbance: a sustained excess of iodine can produce hypothyroidism, hyperthyroidism, goitre or thyroid autoimmunity; the effects are usually mild and reversible, hyperthyroidism can occasionally be serious, and people with existing thyroid disease or a history of deficiency react at intakes only slightly above their need.¹ In 36 healthy adults given kelp capsules for four weeks, TSH rose on both the low and the high dose, T3 fell on the high dose, and two weeks after stopping all values were back at baseline except TSH in the high-dose group, which had fallen below it.² A published case describes transient hyperthyroidism in a 70-year-old woman with no previous thyroid disease after three months of kelp-containing tablets; she needed three months of antithyroid treatment after stopping them before her thyroid function returned to normal.⁵

The second is digestive: sea moss gel is mostly carrageenan, a soluble fibre, and no trial has recorded its digestive effects. The third is accumulated metals. Seaweed concentrates what is in the water: arsenic was found in all 31 UK retail samples in one analysis, and element levels differed by up to 10,000-fold between 14 species in another; the inorganic, carcinogenic form of arsenic was found only in hijiki, at 67 to 96 mg/kg across nine UK retail samples, which is why hijiki alone carries Food Standards Agency advice not to eat it.^{6, 7, 11} The other four varieties had less than 0.3 mg/kg of inorganic arsenic.⁶

Who should not take sea moss?

Anyone with a thyroid condition. Hashimoto's thyroiditis, Graves' disease, thyroid nodules, a goitre, a thyroid treated with radioiodine or surgery: in all of these the gland's handling of iodine is already abnormal, and the review evidence is that these are the people in whom a modest excess tips the balance.¹ Even a declared iodine figure should be agreed with the endocrinologist or GP.

Anyone on levothyroxine or liothyronine. Covered in full below; the short answer is not without the prescriber.

Pregnant and breastfeeding women. Iodine need rises to 200 µg a day in pregnancy and breastfeeding, and the baby's thyroid depends on the mother's supply.⁴ The extra is best met from sources with a known iodine content, which the NHS lists as milk and dairy, eggs, sea fish and shellfish, or a pregnancy supplement that states its figure; only one in ten UK seaweed products states one.^{3, 9} The same unknown applies to arsenic. Sea moss gel in pregnancy is therefore a food to leave out, and any supplement containing it is one to ask the midwife about.

Children. The European upper levels for children run from 200 µg a day at ages 1 to 3 to 500 µg at 15 to 17, so the UK median seaweed serving of 585 µg is nearly three times a toddler's limit;^{3, 12} the Spanish analysis found that even a low-iodine Irish moss at 4 g a day supplied a quarter of a child's recommended intake.⁸ Sea moss products are for adults; keep them out of reach.

Kidney disease. Iodine and any accumulated metals are cleared by the kidneys, so anyone with reduced kidney function should ask the renal team first.

Anyone on an anticoagulant. Carrageenan fractions from red seaweeds show anticoagulant activity in laboratory work, which is where the "sea moss thins the blood" claim comes from;¹³ none has been reported in the sources cited here from eating the seaweed, but the anticoagulant clinic should know about every supplement.

Can you take sea moss with levothyroxine?

Not without your prescriber. Levothyroxine is thyroxine, the hormone itself, prescribed because the gland no longer makes enough, most often because of Hashimoto's, or has been removed or treated. Iodine is the raw material for making hormone; more of it does not help a gland that cannot use it or is gone, and in Hashimoto's a surplus can push the autoimmune process further along.¹

Anyone on levothyroxine already has TSH monitored, the number that moves if iodine intake changes, and sea moss never replaces the medicine. The same prescriber-first rule applies to other supplements that reach the thyroid; ashwagandha with levothyroxine is the parallel case.

Is it OK to take sea moss every day?

If you know the iodine figure, you can answer it yourself. The adult reference intake is 150 µg a day, the NHS says adults need 140 µg and that 0.5 mg or less a day from supplements is unlikely to cause harm, and the European tolerable upper level for adults is 600 µg a day, though people with thyroid disease can react to intakes only slightly above need.^{1, 4, 9, 12} A declared 30 µg serving taken daily sits at 20% of the reference intake and leaves room for the iodine in milk, fish and eggs. A daily serving of an undeclared gel or powder is a daily unknown: at the UK median of 585 µg it would run just under the upper level every day, and a kelp-rich blend would pass it several times over.³ What happens if you take it every day depends on the iodine dose: the four-week kelp trial shows that at kelp-level iodine TSH can begin to rise within a month even in healthy adults; whether long-term use leads to thyroid disease was not tested.²

Can sea moss affect your hormones?

Thyroid hormones, yes, through iodine, in either direction, as above. No trial has measured oestrogen, testosterone, fertility or libido on sea moss in anyone, so the claim that it raises oestrogen has no human measurement behind it.

What we don't know

• Whether sea moss itself causes harm or benefit in people: no trial of the seaweed has been published; the thyroid evidence comes from kelp capsules and iodine.^{1, 2}
• How much iodine a soaked gel keeps: soaking and boiling leach some out, by an amount no study has measured for a gel.¹⁰
• Whether a month's TSH rise on kelp leads to thyroid disease over years: the trial's authors said longer studies are needed.²

Frequently asked questions

Why don't doctors recommend sea moss?

There is no human trial yet for a GP to recommend it on, and only one in ten UK seaweed products states its iodine content.³ A product that declares its iodine dose and lists its ingredients can be discussed with a GP like any other supplement; an undeclared gel cannot.

What should you not mix sea moss with?

Other iodine sources, which means kelp, seaweed-containing greens powders, iodine drops and any multivitamin with iodine, unless you have added the figures up; and thyroid medicines, unless your prescriber has agreed. Beyond those, none with food or common medicines is reported in the sources cited here.

What are the signs of too much iodine?

They are the signs of a thyroid out of range: a racing or irregular heartbeat, feeling hot and sweaty or cold and sluggish, unexplained weight change, tremor, anxiety or low mood, and swelling at the front of the neck. Any of these in someone eating seaweed regularly is a reason to stop and ask a GP for a thyroid function test.

Can I take sea moss if I have Hashimoto's?

Not without your endocrinologist or GP. Hashimoto's is the condition in which a surplus of iodine most reliably makes things worse, and most sea moss products cannot tell you how much iodine they contain.¹

Is sea moss gel safer than capsules or gummies?

Neither form is safer in itself; the iodine figure is what matters. A gel of unknown species and origin is an unknown dose, while a capsule or gummy that declares its iodine is a known one. Soaking leaches some iodine out, by an unmeasured amount.¹⁰

Sources

1. Farebrother J, Zimmermann MB, Andersson M. Excess iodine intake: sources, assessment, and effects on thyroid function. Annals of the New York Academy of Sciences, 2019. Narrative review; public or charity funding listed on PubMed. PubMed 30891786.
2. Clark CD, et al. Effects of kelp supplementation on thyroid function in euthyroid subjects. Endocrine Practice, 2003. Randomised double-blind placebo-controlled trial, 36 healthy adults, 4 weeks; NIH-funded. PubMed 14583417.
3. Bouga M, Combet E. Emergence of seaweed and seaweed-containing foods in the UK: focus on labeling, iodine content, toxicity and nutrition. Foods, 2015. Market survey, 224 UK seaweed products; no conflicts declared. PubMed 28231201.
4. EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for iodine. EFSA Journal, 2014. Agency opinion. PubMed 42125360.
5. Gherbon A, et al. Transient hyperthyroidism following the ingestion of complementary medications containing kelp seaweed: a case report. Medicine, 2019. Case report, one woman; no conflicts declared. PubMed 31517826.
6. Rose M, et al. Arsenic in seaweed: forms, concentration and dietary exposure. Food and Chemical Toxicology, 2007. Laboratory analysis, 31 UK retail samples of five varieties; public or charity funding listed on PubMed. PubMed 17336439.
7. Desideri D, et al. Essential and toxic elements in seaweeds for human consumption. Journal of Toxicology and Environmental Health A, 2016. Laboratory analysis, 14 seaweeds sold in Italy; public or charity funding listed on PubMed. PubMed 26817952.
8. Darias-Rosales J, et al. Risk assessment of iodine intake from the consumption of red seaweeds (Palmaria palmata and Chondrus crispus). Environmental Science and Pollution Research, 2020. Laboratory analysis, 30 retail samples; funding not stated on PubMed. PubMed 32803579.
9. NHS. Vitamins and minerals: iodine. Page last reviewed 3 August 2020 (review overdue). nhs.uk.
10. Correia H, et al. Seaweeds rehydration and boiling: impact on iodine, sodium, potassium, selenium, and total arsenic contents and health benefits for consumption. Food and Chemical Toxicology, 2021. Laboratory study, four seaweed species; funding not stated on PubMed. PubMed 34237393.
11. Food Standards Agency. Importing fruit and vegetables: hijiki seaweed. Guidance, published 25 June 2026. gov.uk.
12. Scientific Committee on Food. Tolerable upper intake levels for vitamins and minerals: iodine, 2006. Committee opinion. efsa.europa.eu.
13. Premarathna AD, et al. Immunomodulation by xylan and carrageenan-type polysaccharides from red seaweeds. International Journal of Biological Macromolecules, 2024. Laboratory study, four red seaweeds; no conflicts declared. PubMed 38232891.

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