Kelp: The Food with the Highest Concentration of Iodine

November 13, 2025 •

3 min read

According to the National Institutes of Health, some species of seaweed, like kombu kelp, can contain thousands of micrograms of iodine in just a single sheet. This makes kelp, and other seaweeds, the food with the highest concentration of iodine by a significant margin.

Quick Summary

Kelp, a type of brown seaweed, has the highest concentration of iodine, with content varying by species and harvest location. Other reliable sources include dairy, eggs, and seafood.

Key Points

Kelp is the most potent iodine source: Certain species of dried seaweed, particularly kelp, contain extremely high concentrations of iodine, far exceeding other foods.
Iodine is essential for thyroid health: This trace mineral is crucial for the production of thyroid hormones, which regulate metabolism and vital bodily functions.
Diverse dietary sources are key: Reliable iodine can be found in seafood (cod, oysters), dairy products (milk, yogurt), and eggs.
Iodized salt is a common source: Fortified table salt is a significant source of iodine for many populations, though salt intake should be monitored.
Balance is vital to avoid health issues: Both iodine deficiency and excessive iodine intake can cause thyroid dysfunction, so moderation is important, especially with concentrated sources like kelp.
Risk factors exist for deficiency: Vegans, pregnant women, and people who avoid seafood and dairy are at a higher risk of iodine inadequacy.
Excessive intake can be harmful: Consuming too much iodine, especially through high-dose supplements, can lead to negative side effects and thyroid problems.

Kelp: The Undisputed King of Iodine Content

Kelp, a large, brown seaweed, is widely recognized as the food containing the highest concentration of iodine. This is because seaweed has a remarkable ability to absorb and concentrate iodine from the ocean, amassing it at levels far higher than any other food source. For example, the iodine content in a single gram of dried kombu kelp can be as high as 2,984 mcg, which is nearly 20 times the recommended daily intake for an adult. The specific iodine levels can vary widely based on the species, preparation method, and growing environment, so moderation is key when consuming this potent food.

Why Iodine is Crucial for Your Body

Iodine is a trace mineral that plays a vital role in human health, with its primary function being the synthesis of thyroid hormones. The thyroid gland uses iodine to produce hormones like thyroxine ($T_4$) and triiodothyronine ($T_3$), which regulate metabolism, energy production, body temperature, and nerve function. Adequate iodine intake is particularly critical during pregnancy and infancy for proper fetal and neurological development. A deficiency can lead to an enlarged thyroid gland, known as a goiter, and cause symptoms like fatigue, weight gain, and dry skin.

Other Top Dietary Sources of Iodine

While kelp is the most concentrated source, many other foods contribute to meeting your daily iodine needs. A balanced diet incorporating several of these options can help ensure sufficient intake without the risk of overconsumption associated with certain seaweeds.

Seafood: Marine fish and shellfish are excellent sources of iodine, as they absorb the mineral from the seawater they inhabit. Cod is particularly high, with a three-ounce serving providing a significant portion of the daily value. Oysters, shrimp, and scallops are also reliable sources.
Dairy Products: Milk, yogurt, and cheese contain iodine, though the concentration can vary based on the cow's diet and farming practices. Greek yogurt and nonfat milk are notably good sources.
Eggs: Eggs, specifically the yolk, provide a decent amount of iodine. A single large hard-boiled egg contains around 26 mcg.
Iodized Salt: The fortification of table salt with iodine has been a public health initiative to combat iodine deficiency. A quarter teaspoon of iodized salt contains approximately 78 mcg of iodine, making it an easy way to increase intake. However, the majority of salt used in processed foods is non-iodized, and overall salt consumption should be moderated.
Enriched Bread: Some commercial breads are made with iodate dough conditioners, which can significantly increase their iodine content. As with iodized salt, check the product label for confirmation.

Potential Risks of Excessive Iodine

While deficiency is a concern, consuming excessive amounts of iodine can also lead to thyroid problems, including goiter, hypothyroidism, and hyperthyroidism. The Tolerable Upper Intake Level for adults is 1,100 micrograms per day. Some individuals, such as those with pre-existing thyroid conditions, may be more susceptible to adverse effects from high iodine intake. Due to its extremely high concentration, certain seaweeds like kelp should be consumed in very small quantities and with caution. It is always advisable to consult a healthcare professional before taking high-dose iodine supplements.

A Comparison of Iodine Content in Foods


Food Source	Serving Size	Approximate Iodine (mcg)	Percent Daily Value (DV)*
Kombu Kelp	1 gram (dried)	up to 2,984	1,989%
Cod, baked	3 ounces	146	97%
Plain Greek Yogurt	3/4 cup	87	58%
Nonfat Milk	1 cup	84	56%
Iodized Salt	1/4 teaspoon	78	52%
Egg, hard-boiled	1 large	31	21%
Shrimp	3 ounces	13	9%

*Based on a 150 mcg Daily Value for adults.

Conclusion

To ensure optimal thyroid function and overall health, it is essential to consume a balanced diet with sufficient iodine. Kelp is a remarkably potent source, but due to its high concentration, care must be taken with serving sizes. For most people, a varied diet that includes seafood, dairy, and eggs is the most reliable way to meet the recommended daily intake. For those with dietary restrictions, particularly vegans, or those with specific health concerns, exploring fortified foods or supplements under medical guidance may be necessary. Prioritizing dietary sources over high-dose supplements helps maintain a safe and effective iodine level for your body.

For more detailed information on nutrient guidelines, consult the National Institutes of Health, Office of Dietary Supplements' fact sheets.

Frequently Asked Questions

Kelp, a type of brown seaweed, has the highest concentration of iodine, with some dried varieties providing thousands of micrograms in a single gram.

No, due to its extremely high concentration, consuming kelp excessively can lead to dangerously high iodine levels. It is best to use it sparingly or get iodine from more balanced sources.

Iodine deficiency can cause an enlarged thyroid gland (goiter), fatigue, weight gain, dry skin, and memory problems.

Yes, aside from seafood and seaweed, other sources of iodine include dairy products like milk and yogurt, eggs, and fortified items such as iodized salt and some breads.

People who are pregnant, vegan, or who consume very little seafood and dairy are at a higher risk for iodine deficiency. In some areas, iodine levels in soil are low, which can also affect the iodine content of local produce.

The Recommended Dietary Allowance (RDA) for iodine in most adults is 150 micrograms (mcg) per day.

While iodized salt is a convenient and effective source, it should not be the sole focus of intake. It's better to get iodine from a variety of nutritious foods while being mindful of overall salt consumption.