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} A product that declares its iodine can be judged; one that does not cannot. What sea moss is, why its iodine varies so much, and how to read a label are on the sea moss explainer.

What are the side effects of sea moss?

Three kinds. The first and best documented is thyroid disturbance from iodine. The thyroid needs about 150 µg a day to make its two hormones and defends itself against a surplus for a while, but a sustained excess 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 values returned to normal two weeks after stopping except TSH in the high-dose group.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 gelling soluble fibre; no trial has recorded its digestive effects, and any sudden increase in soluble fibre can bring bloating or loose stools at first. The third is accumulated metals. Seaweed concentrates what is in the water, and analyses find arsenic, cadmium and lead in every species at levels that vary hundreds to thousands of times between species; 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} Red seaweeds such as Chondrus crispus carry arsenic mainly in organic forms.

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 developing thyroid is sensitive to both too little and too much.⁴ The British Dietetic Association's position is that the extra should come from reliable sources, dairy, white fish, eggs and, where needed, a pregnancy supplement with a declared iodine figure, and that seaweed and kelp supplements should not be used as an iodine source because the dose is unknown. 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. Children's upper levels for iodine are a fraction of the adult 600 µg, so a serving that is a shrug for an adult can be several times a small child's limit; 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. Someone whose kidney function is reduced should treat seaweed as a food to ask the renal team about rather than a supplement to start.

Anyone on an anticoagulant. Purified carrageenan shows anticoagulant activity in laboratory work, which is where the "sea moss thins the blood" claim comes from; 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, and the reasoning matters more than the rule. Levothyroxine is thyroxine, the hormone itself. It is prescribed because the gland is no longer making enough, most often because of Hashimoto's, or because it has been removed or treated. Iodine is the raw material the gland uses to make hormone; giving more raw material to a gland that cannot make hormone, or is no longer there, does not make the medicine work better, and in Hashimoto's the review evidence is that a surplus can push the autoimmune process and the hypothyroidism further along.¹ Some people report a lift in energy in the first days of sea moss; it has not been studied, and it is not a reason to change a prescribed dose.

Two practical points follow. First, test before you supplement: iodine deficiency in the UK is real in some groups, but it is assessed by a clinician, not assumed from tiredness, and anyone on levothyroxine already has TSH monitored, which is the number that will move if iodine intake changes. Second, sea moss never replaces levothyroxine; stopping thyroid hormone for a seaweed does harm, slowly and then quickly. 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 600 µg a day is the upper level above which thyroid effects become likely in susceptible people.^{1, 4, 9} 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 at 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, and that is the direction in which long-term daily excess runs.²

Can sea moss affect your hormones?

Thyroid hormones, yes, through iodine, in either direction, as above. Anything else is unsupported. The claim that sea moss raises oestrogen circulates in forums without a single human measurement behind it; no trial has measured oestrogen, testosterone, fertility or libido on sea moss in anyone. Anyone taking it for those reasons should know that the one hormone it demonstrably reaches is the thyroid's.

Frequently asked questions

Why don't doctors recommend sea moss?

There is no human trial yet for a GP to recommend it on, and most sea moss products do not state their iodine content, which is why the British Dietetic Association advises against seaweed and kelp supplements as an iodine source. 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. PubMed 30891786.
2. Clark CD, et al. Effects of kelp supplementation on thyroid function in euthyroid subjects. Endocrine Practice, 2003. 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. PubMed 28231201.
4. EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for iodine. EFSA Journal, 2014. PubMed 42125360.
5. Gherbon A, et al. Transient hyperthyroidism following the ingestion of complementary medications containing kelp seaweed: a case report. Medicine, 2019. PubMed 31517826.
6. Rose M, et al. Arsenic in seaweed: forms, concentration and dietary exposure. Food and Chemical Toxicology, 2007. PubMed 17336439.
7. Desideri D, et al. Essential and toxic elements in seaweeds for human consumption. Journal of Toxicology and Environmental Health A, 2016. 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. PubMed 32803579.
9. NHS. Vitamins and minerals: iodine. 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. PubMed 34237393.
11. GOV.UK. Importing fruit and vegetables: Hijiki seaweed (Food Standards Agency advice not to eat hijiki). gov.uk.