Is Sea Moss Safe? Iodine, Thyroid, and Heavy Metals (2026)

The iodine problem

Adults need 150 mcg of iodine a day (220 in pregnancy) and should stay under 1,100 mcg from all sources (NIH). Your thyroid needs iodine to make hormones, but too much can make it overactive or underactive (Leung 2014).

Sea moss takes up iodine from the sea, and the amount is hard to predict:

The two Irish moss figures come from different tests but are about 75 times apart, so a label number matters. The shop-sample study found 4 g a day of its Irish moss posed no health risk (Darias-Rosales 2020). Kelp and bladderwrack, brown seaweeds, hold the most; many sea moss blends add bladderwrack.

When seaweed has caused harm

Doctors have reported thyroid problems from seaweed, mostly kelp and other brown seaweeds:

• An overactive thyroid after drinking a tea made with kelp (Müssig 2006), and after taking pills containing kelp (Gherbon 2019).
• Thyroid problems in adults who drank a soy milk made with kombu, a kelp, and an underactive thyroid in newborns whose mothers drank it or ate seaweed soup (Crawford 2010).
• An underactive thyroid in a person who ate seaweed every day for their health (Unosawa 2024).

These cases were kelp, kombu or daily seaweed, not Irish moss; the risk is the iodine dose.

Who should ask a doctor first

• People with a thyroid condition (Hashimoto’s, Graves’, nodules) or who take thyroid medicine.
• People who are pregnant or breastfeeding. A baby’s thyroid is sensitive to iodine.
• Anyone who takes regular medicine, or who has a scan or treatment that uses iodine.
• Children and teens. Their iodine limits are lower (NIH: 300 mcg a day at ages 4 to 8, 900 at 14 to 18).

Heavy metals

Seaweed takes up arsenic, cadmium, lead and mercury from the water it grows in. Seaweed is an important source of these metals in the diets of people who eat it in Europe (EFSA 2023). Most arsenic in seaweed is a less harmful organic kind; the harmful kind is inorganic arsenic. Food agencies in the UK and elsewhere warn people not to eat hijiki, a brown seaweed high in inorganic arsenic (Yokoi 2012).

No study shows that eating sea moss clears metals out of your body. A good lab report lists lead, cadmium, mercury and inorganic arsenic for your batch. What to check on a label.

How much sea moss a day

No study has set a dose for sea moss, and no gel we found prints its iodine on the label (TrueSeaMoss gives a rough estimate in its FAQ). Start with the lowest serving on the label and do not stack products:

• Capsules: the label serving. Double Wood says 2 to 4 capsules a day and to take care above 4 because of iodine.
• Gel: the label’s lowest serving. The two gels we checked say 1 to 2 tablespoons and 2 tablespoons a day.
• Flakes: a quarter teaspoon if the brand lists about 888 mcg of iodine per teaspoon, as Maine Coast does.

Making gel from dried sea moss? Follow the bag’s directions for rinsing, soaking, blending and how long the gel keeps. Keep it in a sealed jar in the fridge, and throw it out if it smells sour or grows mold.

What to buy

Double Wood Irish Sea Moss is our pick. Its brand posts a lab report for its batch: lead was 0.355 mcg a serving, under the brand’s limit of 0.5 mcg, with total arsenic, cadmium and mercury also tested. It gives no iodine number, so take the label’s lowest serving, 2 capsules.

Want to know your iodine? Maine Coast lists 888 mcg of iodine in a 3 g teaspoon of its Irish moss flakes. A quarter teaspoon, about 222 mcg, is already more than the 150 mcg adults need a day.

Every product we compared: best sea moss.

What about carrageenan?

Sea moss contains carrageenan, a food thickener. Europe’s food safety agency found no cancer concern for food-grade carrageenan but set its safe daily intake as temporary pending more data (EFSA 2018).

Questions people ask