Which Seaweed Has the Highest Iodine Content?

November 19, 2025 •

4 min read

According to a 2021 study, the iodine content in kelp, specifically sugar kelp and oarweed, can reach levels of up to 440,670 mcg per 100g in some commercial products, significantly more than other types. This makes brown seaweeds, like kombu and other kelps, the definitive answer to which seaweed has the highest iodine content. The precise amount, however, can vary widely based on species, harvesting location, and processing methods.

Quick Summary

Kombu kelp and other brown seaweeds contain the most iodine, far exceeding the amounts found in red or green varieties like nori. The iodine content is highly variable depending on species and origin, and overconsumption can lead to thyroid issues. Always consume these high-iodine seaweeds in moderation.

Key Points

Kombu Kelp is Highest: The brown seaweed Kombu (Laminaria) has the highest iodine content, with some varieties exceeding 250,000 mcg per 100g.
Variable Levels: Iodine concentration varies significantly across seaweed species, harvesting location, and processing methods.
Moderation is Crucial: Due to the extremely high levels in some kelp, consuming large quantities can cause excessive iodine intake and potentially harm thyroid health.
Lower-Iodine Alternatives: Red and green seaweeds like nori have much lower iodine levels, making them safer for more frequent consumption.
Processing Reduces Iodine: Soaking or boiling high-iodine seaweeds, like kombu, can significantly reduce the mineral content.
Risk for Thyroid Issues: Individuals with pre-existing thyroid conditions, as well as pregnant and breastfeeding women, should be cautious with high-iodine seaweed consumption.
Inadequate Labeling: Many commercial seaweed products have inaccurate or non-existent iodine content labeling, making it difficult for consumers to track intake.

Kombu Kelp: The Undisputed Winner in Iodine Content

Kombu kelp, a type of large, brown seaweed, consistently ranks as the highest source of iodine among all edible seaweeds. Studies show that the iodine content in kombu can be exceptionally high, with some dried varieties exceeding 2,500 mg per kilogram (250,000 mcg per 100g). The high concentration is attributed to the presence of haloperoxidases in brown algae, which facilitates the uptake and storage of iodine from seawater. This exceptional iodine density is why kombu is often used sparingly, such as in making dashi soup stock, and is not typically consumed in large quantities.

Why Iodine Content Varies in Seaweed

Several factors contribute to the wide-ranging iodine content across different seaweed products:

Species: Brown seaweeds, including various types of kelp, are generally richer in iodine than red and green seaweeds like nori.
Harvesting Location: The iodine levels in the surrounding seawater can affect the amount accumulated by the seaweed. For example, studies found that wakame from Asia generally has more iodine than that from Australia or New Zealand.
Season and Age: The age of the seaweed and the season in which it is harvested can influence its mineral content. Younger, faster-growing parts of the kelp plant may have higher concentrations of iodine.
Processing: Simple preparation methods, such as soaking in freshwater or boiling, can significantly reduce the iodine content. Drying and pulverizing processes also affect the final concentration.

Comparison of Common Edible Seaweeds

This table compares the typical iodine content of several popular seaweed varieties, highlighting the dramatic difference in concentration.


Seaweed Type	Common Name	Typical Iodine Content (mcg per 100g dried)	Relative Iodine Level	Notes
Laminaria japonica	Kombu Kelp	2,000 - 10,000+	Very High	Often used for flavoring due to extreme potency.
Saccharina latissima	Sugar Kelp	23,800	Very High	Similar to kombu, should be used cautiously.
Undaria pinnatifida	Wakame	6,600 - 17,200	High	Used in miso soup and salads; requires moderation.
Alaria esculenta	Winged Kelp	1,510 - 4,500	Moderate-to-High	A less potent kelp, but still a significant source.
Sargassum fusiforme	Hijiki / Arame	400	Moderate	Contains inorganic arsenic; consumption advisories exist.
Palmaria palmata	Dulse	140 - 750	Low-to-Moderate	Flakes are a less potent alternative for seasoning.
Porphyra spp.	Nori	5 - 550	Low	Used in sushi rolls; low enough for regular consumption.

The Health Implications of High Iodine Intake

While iodine is essential for thyroid function, consuming excessive amounts can cause adverse health effects. Both too little and too much iodine can lead to thyroid dysfunction, including hyperthyroidism or hypothyroidism. The tolerable upper intake level (UL) for adults is set at 1,100 mcg per day in the US, but consuming even small quantities of kelp can easily exceed this limit.

For most people, a balanced and varied diet that includes moderate amounts of seaweed or other iodine sources like dairy, eggs, and seafood is sufficient. Vegans and those avoiding seafood may need to be more mindful of their intake and consider supplements if necessary, but should avoid high-dose kelp supplements.

Importance for Thyroid Health

The thyroid gland requires iodine to produce the hormones thyroxine (T4) and triiodothyronine (T3), which regulate metabolism, growth, and development. While seaweed can be an excellent natural source, its variable potency makes it a double-edged sword. Proper knowledge of a seaweed's origin and processing is crucial for safe consumption. Some manufacturers voluntarily label their products, but this is not always reliable.

Conclusion: Navigating Iodine from Seaweed

Kombu kelp stands out as the seaweed with the highest iodine content, but this does not necessarily make it the 'best' option for regular dietary intake. Its extreme potency makes moderation critical to avoid exceeding the tolerable upper intake level for iodine. For individuals seeking a consistent and safer iodine source from seaweed, lower-iodine varieties like nori or dulse are more suitable for daily consumption. The variability in iodine concentration due to species, origin, and processing makes it essential to be an informed consumer, particularly for those with pre-existing thyroid conditions or for pregnant and breastfeeding women. As seaweed popularity continues to grow, clear labeling and consumer education are becoming increasingly important for ensuring public health. The scientific community continues to explore iodine bioavailability from different seaweed types to provide more accurate dietary guidance. For reliable health information, always consult with a healthcare professional or visit official health resources like the National Institutes of Health.

How to Safely Consume High-Iodine Seaweeds

If you choose to consume high-iodine varieties like kombu, consider using it as a flavoring agent rather than a primary ingredient. Boiling the seaweed and discarding the broth can also help reduce the iodine concentration. For general seasoning or sushi, nori and dulse are safer options for more regular use. Always check product labels for warnings and iodine content, if available.

Author's Note: This article is for informational purposes only and does not constitute medical advice. Please consult a healthcare professional for personalized dietary recommendations, especially if you have thyroid concerns.

Frequently Asked Questions

Brown seaweeds, especially kombu kelp, sugar kelp, and oarweed, are extremely high in iodine and should be used with extreme caution or limited to very small quantities. Regular or heavy consumption is not recommended for most people.

Yes, it is very easy to consume an excessive amount of iodine from high-potency seaweed, especially from kelp varieties. Over-consumption can lead to thyroid dysfunction, including both hypo- and hyperthyroidism.

Nori, the red seaweed used for sushi, has a significantly lower iodine content than kombu and other kelps. While still a good source of iodine, its lower concentration makes it a much safer option for regular consumption.

Yes, studies have shown that cooking methods like boiling can substantially reduce the iodine content in seaweed, particularly if the cooking water is discarded.

Seaweed supplements are not always a reliable source of iodine. The content can be highly variable, often lacking reliable labeling, and some can contain excessively high doses that are unsafe.

The Recommended Dietary Allowance (RDA) for iodine is typically around 150 mcg per day for adults, although specific recommendations can vary by country and health status (e.g., higher for pregnant or lactating women).

Excessive iodine intake can disrupt thyroid function, leading to symptoms like goiter, elevated TSH levels, and thyroiditis. These risks are particularly relevant for those with pre-existing thyroid conditions.