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How Seaweed and Kelp Affect Your Thyroid

A look at the pros and cons of seaweed and kelp for the thyroid.
How Seaweed and Kelp Affect Your Thyroid
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The health and wellness industry is always buzzing about the latest superfoods and natural remedies. Seaweed, particularly kelp, has been in the limelight in recent years. While seaweed has been a staple in some cuisines for centuries, it has emerged as a potential ally for individuals with Hashimoto’s disease and hypothyroidism.

Iodine is the raw material that the body uses to make thyroid hormones. Given that seaweed and kelp are high in iodine, it’s no surprise they land on the list of healthy foods recommended for thyroid patients. But are seaweed and kelp always good for your thyroid? In this article, we’ll take a look at this question.

First, let’s understand the basics. Hashimoto’s is an autoimmune disease where the body’s immune system attacks the thyroid gland, leading to an underactive thyroid or hypothyroidism. Symptoms can range from fatigue and weight gain to depression and hair loss. Hypothyroidism, on the other hand, refers to a condition where the thyroid gland doesn’t produce enough thyroid hormones, resulting in a slowdown of the body’s metabolism.

Seaweed, including kelp, is widely known for its rich iodine content. Iodine plays a crucial role in the production of thyroid hormones, and its deficiency can contribute to the manifestation and progression of hypothyroidism. Incorporating seaweed into one’s diet can provide a natural source of iodine, potentially supporting thyroid health. However, it is essential to approach seaweed consumption cautiously, as discussed in this article.

Understanding seaweed and kelp

While people often use “kelp” and “seaweed” interchangeably, they are different. Seaweed is an umbrella category that includes more than 10,000 different types of sea plants and vegetables. Seaweeds are typically categorized by three key colors:

  • Brown Seaweed: Includes Kelp, Kombu, Wakame, Arame, and Hijiki
  • Green Seaweed: Includes Sea Lettuce and Umibudo
  • Red Seaweed: Includes Nori, Dulse, and Irish Moss

Kelp is a type of brown seaweed and is often found in dense, fixed areas called kelp beds or kelp forests. It’s the most abundant type of brown seaweed.

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Seaweed’s nutritional benefits

Seaweed is highly nutritious and has many macronutrients, making it a popular food for health-minded people. Here are some primary health benefits of seaweed and kelp foods and supplements.

High in iodine: Seaweed is perhaps best known as a natural source of iodine, a crucial nutrient for thyroid function and the production of thyroid hormones that regulate metabolism. The iodine content in seaweed depends on the type, the season when it was harvested, and where it was grown. Including seaweed in the diet can help maintain a healthy thyroid and support overall well-being.

High in protein: Seaweeds contain protein, and for brown seaweeds like kelp, it varies between 5% and 20%. For red seaweeds, 1% to 45%, and green seaweeds from 3% to 30%.

High in 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 also has anti-obesity and prebiotic effects, which help people maintain a healthy weight (9-16).

High in vitamins and minerals: Seaweed contains iron, magnesium, vitamin A, vitamin C, vitamin D, and vitamin B12 (20-22). (23, 24). (23, 25-27).

Cautions for thyroid patients

Whether it’s sushi rolls, seaweed salad, sheets of dried nori, or capsule supplements, there’s no shortage of opportunities to increase your seaweed intake. But some risks come with seaweed and kelp.

Heavy metal concentration

Some types of seaweed have a high concentration of arsenic and other heavy metals like lead, mercury, and aluminum. Iodine and mercury can have a synergistic effect—affecting thyroid function and reducing the thyroid hormone T3. Heavy metal concentrations in seaweeds are generally below levels that are toxic for humans. However, the bioaccumulation of arsenic is a concern with continuous seaweed consumption.

Excessive salt intake

Eating 5g a day or more of dried seaweed can create a situation where sodium intake becomes excessively high,

Slowed thyroid function

Kelp supplements can cause decreased thyroid function (hypothyroidism) in some people due to the high iodine content.

Interaction with levothyroxine

Excess iodine from kelp supplements can alter the dosage requirements for levothyroxine, a thyroid hormone replacement medication, in ways that are not predictable. Some practitioners recommend that thyroid patients who are taking levothyroxine avoid seaweed and kelp supplementation entirely.

Overactive thyroid function

Even short-term supplementation with kelp or seaweed -- especially at levels of more than 150 micrograms of iodine per day -- can 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, watch for some of the signs of hyperthyroidism, including:

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

Next steps

While iodine is essential for thyroid function, it is crucial to maintain a balance. Excessive iodine intake can lead to complications, especially in individuals with Hashimoto’s disease or hypothyroidism. These individuals are typically more sensitive to changes in iodine levels. If iodine intake surpasses their tolerance, it may trigger or exacerbate hypothyroid symptoms.

Before incorporating seaweed into your diet, it is advisable to consult with a healthcare professional, specifically an endocrinologist or a registered dietitian who specializes in thyroid health. They can assess your iodine levels, evaluate potential risks, and recommend the appropriate dosage to support your thyroid health.

If you decide to include seaweed in your diet, choosing your sources carefully is essential. Organic, sustainably harvested seaweed products from reputable brands are ideal, as they are more likely to be free from harmful contaminants. Additionally, be mindful of the preparation methods. Seaweed can be enjoyed in various forms, such as dried, powdered, or as an ingredient in soups and salads. Experiment and find a preparation method that suits your taste buds.

A note from Paloma

Anyone with Hashimoto’s and hypothyroidism needs to be cautious about iodine intake, as excessive iodine can worsen your thyroid condition.

Specifically, Paloma recommends that:

  • you should consult a healthcare professional, such as a thyroid nutritionist, before incorporating seaweed or kelp into your diet
  • if you are consuming seaweed regularly and/or supplementing with kelp and seaweed, you should closely monitor your thyroid hormone levels and work with your practitioner to adjust your medication dosage as needed.

We also recommend that you learn more about the complexities of supplementing with iodine, in Paloma’s free Iodine Thyroid Guide online.

Paloma’s home test kit makes it easy to monitor your thyroid levels from the convenience of home. Paloma’s thyroid practitioners, including doctors and nutritionists, can help you manage your thyroid condition for optimal health and wellness. Consider becoming a Paloma member to take advantage of the full range of benefits of a hypothyroidism-focused medical practice.

Vedrana Högqvist Tabor, Ph.D., contributed to this article.


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Smyth PPA. Iodine, Seaweed, and the Thyroid. Eur Thyroid J. 2021 Apr;10(2):101-108. doi: 10.1159/000512971. Epub 2021 Jan 27. PMID: 33981614; PMCID: PMC8077470.

Gherbon A, Frandes M, Lungeanu D, Nicula M, Timar R. Transient Hyperthyroidism following the ingestion of complementary medications containing kelp seaweed: A case-report. Medicine (Baltimore). 2019 Sep;98(37):e17058. doi: 10.1097/MD.0000000000017058. PMID: 31517826; PMCID: PMC6750240.

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Trigo JP, Palmnäs-Bédard M, Juanola MV, Undeland I. Effects of whole seaweed consumption on humans: current evidence from randomized-controlled intervention trials, knowledge gaps, and limitations. Front Nutr. 2023 Jul 20;10:1226168. doi: 10.3389/fnut.2023.1226168. PMID: 37545570; PMCID: PMC10399747.

Aakre I, Tveito Evensen L, Kjellevold M, Dahl L, Henjum S, Alexander J, Madsen L, Markhus MW. Iodine Status and Thyroid Function in a Group of Seaweed Consumers in Norway. Nutrients. 2020 Nov 13;12(11):3483. doi: 10.3390/nu12113483. PMID: 33202773; PMCID: PMC7697291.

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Mary Shomon

Patient Advocate

Mary Shomon is an internationally-recognized writer, award-winning patient advocate, health coach, and activist, and the New York Times bestselling author of 15 books on health and wellness, including the Thyroid Diet Revolution and Living Well With Hypothyroidism. On social media, Mary empowers and informs a community of more than a quarter million patients who have thyroid and hormonal health challenges.

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