How seaweed and kelp supplements affect your thyroid

Incorporating seaweed into the diet can be beneficial, as it contains nutrients including iodine (1). Seaweed is an umbrella term for different types of sea vegetables. Kelp is a popular type of brown seaweed.

But there are some risks that come with eating seaweed — the concentration of iodine and arsenic can be very high in certain types of seaweed (2, 3). Currently, law doesn’t require nutritional supplements to:

  • list iodine or arsenic content
  • indicate a safe portion size in order to prevent excess iodine and arsenic intake, which is of a concern for people with thyroid conditions (4)

Seaweed and kelp nutrition

The nutritional composition of brown, red, and green seaweeds can vary greatly between species, season, and the location where they grow (5).


The protein content of brown seaweeds varies between 5% and 20%, for red seaweeds 1% to 45%, and green seaweeds from 3% to 30%. Generally a five gram (g) portion of dried seaweed corresponds to 2% of protein for brown seaweed, 4.5% for red, and 3% for green. Seaweeds have a comparable amount of protein per gram to beef — typically beef has five times more protein per gram (6, 7).

Dietary fiber

Seaweed is high in dietary fiber. To achieve the currently recommended dose of 25 g of fiber per day, you would need to eat 35 g of brown seaweed, 47 g of red seaweed, or 41 g of green seaweed (8). Seaweed dietary fiber has anti-obesity and prebiotic effects, which helps people maintain a healthy weight (9–16).


Seaweed is low in fat, but the content varies between seasons. Fat content is highest during the winter and lowest during the summer (17–18).


Seaweed contains polyphenols — micronutrients that help with digestive health, particularly by protecting the cell wall against damage caused by molecules (19).

Vitamins and minerals

Seaweed contains iron and magnesium (20–22). Vitamin A and C concentrations typically vary a lot between seasons and different types of seaweeds (23, 24).

It’s also rich in vitamin D — and potentially in vitamin B12 — with 1g of dried seaweed exceeding the recommended daily intake of both vitamins (23, 25–27).

Seaweed is popular for its iodine content, however it can vary from 0.06 mg per 100g of powder to 625mg per 100g. Iodine content in seaweed depends on the type, season it was harvested, and place it was grown (28). Eating as little as 3g of dried seaweed per day may cause problems with thyroid function.

Arsenic and other heavy metals

Eating seaweed might expose people to heavy metals, including arsenic, lead, mercury, and aluminium (28).

Iodine and mercury can have a synergistic effect — affecting thyroid function and reducing the thyroid hormone T3 (29). Heavy metal concentrations in seaweeds are generally below levels that are toxic for humans. But bioaccumulation of arsenic is a concern with continuous seaweed consumption (30).


Eating small amounts of seaweed can be helpful to keep your sodium intake low. But a 5g portion of dried seaweed per day can exceed sodium quantities found in some high salt foods, like bacon (31).

The impact of seaweed and kelp on thyroid function

Even short-term kelp supplementation (a few weeks) can increase TSH levels. It can also cause temporary hyperthyroidism and thyrotoxicosis — excess levels of thyroid hormones in the bloodstream (32–35).

If you’re consuming kelp or seaweed daily and experience the following symptoms, it might be good to check in with your healthcare practitioner:

  • Excessive weight loss
  • Racing heart or irregular heartbeat
  • Excessive sweating
  • Shaky hands
  • Feeling extra anxious
  • Diarrhea
  • Nausea
  • Numbness in hands and feet

Seaweed supplementation is not recommended during pregnancy. Its iodine concentration can vary greatly, which poses a risk for a developing fetus (36, 37).

Inform your healthcare provider if you’re taking seaweed supplements, they might need to adjust the doses of other medication you may be taking.

How we write: our information is based on the results of peer reviewed studies using the National Library of Medicine platform. It is written by scientists and reviewed by external experts. If you believe we might have overseen crucial scientific information, please contact us at

Disclaimer: This information is not intended to mitigate, prevent, treat, cure or diagnose any disease or condition. If you want to change your treatment, lifestyle, your diet, include supplements in your diet or have concerns about your health, please consult your doctor before trying new approaches.


  1. MacArtain P, et al. Nutritional value of edible seaweed, 2007
  2. Holdt SL, et al. Bioactive compounds in seaweed: functional food applications and legislation, 2011
  3. Suleria HA, et al. Marine-based nutraceuticals: an innovative trend in the food and supplement industries, 2015
  4. Bouga M, et al. Emergence of seaweed and seaweed-containing foods in the UK: focus on labeling, iodine content, toxicity and nutrition, 2015
  5. European Commission. Novel food catalogue. Version1.1, 2018
  6. Greenwood DA, et al. Amino acid composition of fresh and cooked beef cuts, 1951
  7. Lourenço SO, et al. Amino acid composition, protein content and calculation of nitrogen-to-protein conversion factors for 19 tropical seaweeds, 2002
  8. EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for carbohydrates and dietary fibre, 2010
  9. Brownlee IA, et al. Alginate as a source of dietary fiber, 2005
  10. El Khoury D, et al. The role of alginates in regulation of food intake and glycemia: a gastroenterological perspective, 2015
  11. Chater PI, et al. Inhibitory activity of extracts of Hebridean brown seaweeds on lipase activity, 2016
  12. Michel C, et al. In vitro fermentation by human faecal bacteria of total and purified dietary fibres from brown seaweeds, 1996
  13. Ramnani P, et al. In vitro fermentation and prebiotic potential of novel low molecular weight polysaccharides derived from agar and alginate seaweeds, 2012
  14. Bai S, et al. Comparative study on the in vitro effects of Pseudomonas aeruginosa and seaweed alginates on human gut microbiota, 2017
  15. Bajury DM, et al. Prebiotic evaluation of red seaweed (Kappaphycus alvarezii) using in vitro colon model, 2017
  16. Fu X, et al. Structural characterization and in vitro fermentation of a novel polysaccharide from Sargassum thunbergii and its impact on gut microbiota, 2018
  17. Madden M, et al. Seasonality of selected nutritional constituents of edible Delmarva seaweeds, 2012
  18. Gressler V, et al. Biochemical composition of two red seaweed species grown on the Brazilian coast, 2011
  19. Heffernan N, et al. Profiling of the molecular weight and structural isomer abundance of macroalgae-derived phlorotannins, 2015
  20. Garcia-Casal MN, et al. Antioxidant capacity, polyphenol content and iron bioavailability from algae (Ulva sp., Sargassum sp. and Porphyra sp.) in human subjects, 2009
  21. Garcia-Casal MN, et al. High iron content and bioavailability in humans from four species of marine algae, 2007
  22. Nakamura E, et al. The in vitro digestibility and absorption of magnesium in some edible seaweeds, 2012
  23. Paiva L, et al. Edible Azorean macroalgae as source of rich nutrients with impact on human health, 2014
  24. Taboada C, et al. Composition, nutritional aspects and effect on serum parameters of marine algae Ulva rigida, 2010
  25. European Parliament, Council of the European Union. Regulation (EU) No 1169/2011 of the European Parliament and of the Council of 25 October 2011 on the provision of food information to consumers, amending Regulations (EC) No 1924/2006 and (EC) No 1925/2006 of the European Parliament and of the Council, and repealing Commission Directive 87/250/EEC, Council Directive 90/496/EEC, Commission Directive 1999/10/EC, Directive 2000/13/EC of the European Parliament and of the Council, Commission Directives 2002/67/EC and 2008/5/EC and Commission Regulation (EC) No 608/2004 (Text with EEA relevance), 2011
  26. Watanabe F, et al. Dried green and purple lavers (nori) contain substantial amounts of biologically active vitamin B12 but less of dietary iodine relative to other edible seaweeds, 1999
  27. Miyamoto E, et al. Characterization of vitamin B12 compounds from Korean purple laver (Porphyra sp.) products, 2009
  28. Desideri D, et al. Essential and toxic elements in seaweeds for human consumption, 2016
  29. Llop S, et al. Synergism between exposure to mercury and use of iodine supplements on thyroid hormones in pregnant women, 2015
  30. Cherry P, et al. Risks and benefits of consuming edible seaweeds, 2019
  31. Zava TT, et al. Assessment of Japanese iodine intake based on seaweed consumption in Japan: a literature-based analysis, 2011
  32. Clark CD, et al. Effects of kelp supplementation on thyroid function in euthyroid subjects, 2003
  33. Gherbon A, et al. Transient Hyperthyroidism following the ingestion of complementary medications containing kelp seaweed. A case-report, 2019
  34. Di Matola T, et al. Thyroid dysfunction following a kelp-containing marketed diet, 2014
  35. Müssig K, et al. Iodine-Induced Thyrotoxicosis After Ingestion of Kelp-Containing Tea, 2006
  36. Zimmermann M, et al. Iodine supplementation of pregnant women in Europe: a review and recommendations, 2004
  37. Bath SC, et al. Iodine concentration of milk-alternative drinks available in the UK in comparison with cows’ milk, 2017

Photo: Unsplash; Design: VLM Health



Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store