Seaweed: iodine, the risk of too much, and what trials show

The thyroid uses iodine to make its hormones, and the adult RDA is 150 mcg a day, rising to 220 mcg in pregnancy and 290 mcg while breastfeeding NIH ODS. Seaweed is one of the best food sources; others are fish and seafood, eggs, dairy and iodized salt. In the ODS table, 2 tablespoons of flaked dried nori (5 grams) provide about 116 mcg, a quarter teaspoon of iodized salt about 78 mcg, and noniodized sea salt virtually noneNIH ODS.

People who eat little or no dairy, seafood or eggs, including many vegans, can fall short of iodine, and ODS notes that many pregnant women in the US may not get enough NIH ODS. Nutrition Facts labels do not have to list iodine that occurs naturally in a food, so seaweed snacks rarely tell you how much they containNIH ODS. If you want to see where your usual meals land against the RDA, Cronometer logs iodine for foods that have lab values, though its figures for seaweed are averages for the reason below.

In a study of 12 seaweed species sold in the US and elsewhere, iodine ranged from 16 mcg per gram in nori to over 8,165 mcg per gram in one sample of kelp granules sold as a salt substitute Teas 2004. Even the same species varied: in two Namibian kelps, sun-bleached blades held about 514 mcg per gram and freshly cut young blades about 6,571 mcg per gram. Iodine also dissolves into cooking water and can be lost in humid storage, and the authors judged that some Asian seaweed dishes may exceed the 1,100 mcg daily upper limitTeas 2004. ODS gives the range for commercially available whole or sheet seaweeds as 16 to 2,984 mcg per gram NIH ODS.

The adult upper limit for iodine from food and supplements is 1,100 mcg a day NIH ODS. ODS explains that high intakes can cause some of the same problems as deficiency, including goiter, raised TSH and an underactive thyroid, and can also cause iodine-induced hyperthyroidism and thyroiditis. People with autoimmune thyroid disease, or who were previously iodine deficient, may react to intakes considered safe for most peopleNIH ODS.

Kelp supplements are the easiest way to overshoot, because ODS lists them as a source of supplemental iodine alongside many multivitamins that already contain 150 mcg NIH ODS. A 2019 case report described a 70-year-old woman with no thyroid history who developed hyperthyroidism (fast heart rate, insomnia, weight loss) three months after starting kelp-containing slimming tablets; it resolved after she stopped them and had antithyroid treatment Gherbon 2019. The authors advised that kelp supplements be used under medical supervision with thyroid monitoring.

If you want seaweed, food forms such as nori or wakame are the sensible choice; online, iHerb lists seaweed snacks and dried sea vegetables. Check whether a product states its iodine content per serving.

In a UK study of 31 dried seaweed samples bought in shops and online, carried out to support advice to consumers, inorganic arsenic, the form linked to cancer, was found only in the nine samples of hijiki, at 67 to 96 mg per kg. Arame, kombu, nori and wakame were all below the detection limit of 0.3 mg per kg. Because hijiki could significantly raise arsenic exposure, the UK Food Standards Agency advised consumers to avoid eating it Rose 2007. Food Standards Australia New Zealand also classifies inorganic arsenic in hijiki as a medium or high public health risk in imported foodFSANZ. Hijiki is the dark, stringy seaweed often served in Japanese side dishes; other common seaweeds did not show the problem.