How do humans naturally get iodine? A comprehensive guide to dietary sources

October 25, 2025 •

4 min read

An estimated two billion people worldwide are affected by iodine deficiency, making it the leading preventable cause of intellectual and developmental disabilities. Understanding how do humans naturally get iodine through dietary sources is crucial for maintaining optimal health and preventing deficiency disorders.

Quick Summary

A sufficient iodine intake is vital for thyroid hormone production. Key sources include seafood and dairy, while iodized salt remains a global public health strategy. The amount of iodine in plant foods often depends on soil mineral content.

Key Points

Rich Marine Sources: The ocean is the richest natural source of iodine, with seaweed and seafood like cod and shellfish containing high concentrations due to absorption from seawater.
The Role of Iodized Salt: Fortified table salt is a major source of iodine for many populations, but most processed foods and sea salt are not iodized.
Dairy and Eggs as Key Contributors: The iodine content in milk and eggs is largely influenced by iodine supplementation in animal feed and disinfectants used in dairy farming.
Variable Iodine in Plant Foods: The amount of iodine in plants is inconsistent and depends on the mineral content of the soil, which is often depleted in inland regions.
Importance for Thyroid Health: Iodine is vital for the production of thyroid hormones, which regulate metabolism and are essential for proper growth and development, especially for a developing fetus and young children.
Potential for Deficiency: Individuals on vegan or dairy-free diets, pregnant women, and those living in iodine-poor inland regions are at a higher risk of deficiency without careful dietary planning or supplementation.

The Essential Role of Iodine

Iodine is a trace mineral that is a critical component of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). These hormones regulate numerous metabolic functions, including growth, energy production, and the development of the central nervous system. Because the human body cannot produce iodine on its own, it must be obtained through food. The body absorbs ingested iodide, and the thyroid gland concentrates it for hormone synthesis. A deficiency can lead to an enlarged thyroid gland, known as a goiter, and hypothyroidism, while severe deficiency during pregnancy can cause irreversible cognitive damage to a fetus.

The Oceanic Connection: Seafood and Seaweed

Perhaps the most reliable and naturally rich sources of iodine are found in the ocean. The concentration of iodine in seawater is high, and marine organisms absorb and concentrate this mineral, making them excellent dietary sources.

Seaweed

Seaweed, such as kelp, nori, and wakame, is exceptionally high in iodine. The concentration, however, varies dramatically depending on the species and the region where it grew. Kombu kelp, for example, can contain extremely high levels of iodine, so it is important to consume it in moderation. For example, dried nori, used in sushi, contains a significant amount of iodine per serving.

Fish and Shellfish

Fish and shellfish are also reliable sources of iodine. The amount can vary based on whether the animal was farm-raised or wild-caught and the specific region. Leaner fish tend to have higher concentrations of iodine. Good examples include:

Cod
Haddock
Oysters
Scallops
Shrimp
Tuna

Iodized Salt: A Public Health Success Story

In the early 20th century, iodine deficiency was widespread in many inland areas, leading to goiters and developmental problems. The practice of fortifying salt with iodine was introduced to combat this public health issue. This simple and inexpensive measure dramatically reduced the prevalence of iodine deficiency disorders in many countries.

How it Works

During production, potassium iodide or potassium iodate is added to table salt. The body easily absorbs this added iodine. While most table salt is iodized, it is important to remember that not all salt is. Processed foods, for example, often use non-iodized salt, and sea salt and kosher salt are typically not iodized. Therefore, relying solely on processed foods for your iodine intake is not a dependable strategy.

Dairy and Eggs: Fortified and Natural Sources

Milk and dairy products are significant sources of iodine in many diets. The iodine content is not inherent to the cow's milk itself but rather a result of modern farming practices.

Milk and Dairy

Factors influencing iodine levels in milk include:

Iodine-fortified cattle feed: Many countries supplement animal feed with iodine to improve animal health, which increases the iodine concentration in milk.
Iodine-based disinfectants: Iodine-containing disinfectants used to clean milking equipment and udders also contribute to the iodine levels in the final product.

Eggs

Eggs, specifically the yolk, are another good source of iodine. This is also due to the supplementation of iodine into chicken feed, which is then passed into the eggs. The iodine level in eggs can vary based on the chicken's diet.

Plant-Based Options and Soil Depletion

The iodine content of plant-based foods, such as fruits and vegetables, is highly dependent on the soil in which they were grown. Unfortunately, glacial leaching and repeated flooding have depleted much of the topsoil in many regions of its natural iodine content. This means that relying solely on terrestrial plant foods for iodine can be unreliable, especially for those living in inland areas far from the coast. However, some plant-based options can contribute to intake:

Fortified plant milks: Some plant-based milk alternatives, like soy and oat milk, are fortified with iodine. Always check the nutrition label.
Lima beans and prunes: These are cited as providing a small but notable amount of iodine.

The Challenge of Goitrogens

Some foods, known as goitrogens, can interfere with the body's utilization of iodine. These include cruciferous vegetables like broccoli, cauliflower, and cabbage. However, for people with adequate iodine intake, eating a reasonable amount of these foods is not typically a concern.

Comparison of Common Iodine Sources


Source	Serving Size	Approximate Iodine Content	Notes
Seaweed (Kelp)	1g dried	Up to 2,984 mcg	Highly variable, small amounts can exceed daily needs.
Cod	3 oz (85g) baked	99-158 mcg	Lean fish is a reliable source.
Iodized Salt	1/4 tsp	71-78 mcg	A key public health measure for intake.
Greek Yogurt	1 cup (245g)	75-116 mcg	Dependent on animal feed practices.
Milk	1 cup (244g)	56-85 mcg	Concentration varies by season and farming.
Eggs	1 large (50g)	24-31 mcg	Found primarily in the yolk.
Shrimp	3 oz (85g) cooked	13-35 mcg	Lower concentration than lean fish.

Conclusion

Maintaining an adequate iodine intake is essential for overall health, particularly for proper thyroid function. Humans naturally obtain iodine primarily from marine sources, such as seaweed, fish, and shellfish, due to the high concentration of the mineral in the ocean. For those who do not consume significant amounts of seafood, fortified foods like iodized salt and dairy products are crucial for preventing deficiency. While iodine levels in plant foods are inconsistent due to soil variations, certain options like fortified plant milks can help meet dietary needs. A varied diet including marine products and fortified staples ensures sufficient iodine for most people. Anyone following a restrictive diet or with concerns about their iodine status should consult a healthcare professional.

For more information on the health benefits of iodine and the risks of deficiency, consult the Office of Dietary Supplements at the National Institutes of Health.(https://ods.od.nih.gov/factsheets/Iodine-Consumer/)

Frequently Asked Questions

The best natural sources of iodine are marine products, with seaweed like kelp containing some of the highest concentrations. Other excellent sources include lean fish such as cod, haddock, and shellfish like oysters and scallops.

The iodine in milk and eggs is not naturally high but comes from iodine added to animal feed. In dairy farming, iodine-containing disinfectants also contribute to milk's iodine content.

Relying on vegetables for iodine is not reliable because their iodine content is dependent on the soil, which is often deficient in iodine, especially in inland areas. Marine and fortified products are more dependable sources.

No, not all salt contains iodine. While most table salt is fortified with it, specialty salts like sea salt and kosher salt typically do not contain added iodine. Processed foods also often use non-iodized salt.

Symptoms of iodine deficiency include a goiter (enlarged thyroid gland), fatigue, weight gain, cold sensitivity, and dry skin. It can also lead to learning difficulties and intellectual impairment, particularly in children.

Iodine is absorbed in the stomach and small intestine as the iodide ion. From there, it enters the bloodstream and is transported to the thyroid gland, which concentrates it to synthesize thyroid hormones.

People who do not use iodized salt, pregnant and breastfeeding women, vegans who do not consume iodine-fortified products or supplements, and individuals living in inland regions with iodine-poor soil are at a higher risk.