The Environmental Journey of Iodine: From Ocean to Soil to Table

Iodine's natural cycle runs from ocean water to atmosphere to soil, which is why coastal regions tend to have iodine-richer soil than inland areas. We trace that cycle.

The Ocean: Iodine's Reservoir

Most of the Earth's iodine is in the ocean, dissolved at very low concentrations. Seaweeds and other marine life take it up and concentrate it, which is why kelp, kombu and other seaweeds can be so rich in iodine. Fish and shellfish carry iodine too, from the water and the food they eat.

Into the Air

Iodine doesn't stay in the sea. At the ocean surface, sunlight, sea spray and marine organisms release iodine compounds into the atmosphere. Seaweeds exposed at low tide release some of this iodine directly. Once in the air, iodine compounds travel with weather systems over land.

Onto the Land

Rain and snow carry atmospheric iodine back down to the ground. That's why areas close to the coast tend to receive more iodine than places far inland. Soil holds iodine to different degrees depending on its organic matter and clay content. Over thousands of years, events like glaciation, heavy erosion and repeated flooding stripped iodine from the topsoil in some regions, leaving them naturally low.

Regions With Naturally Low Iodine

Historically low-iodine areas include mountain ranges such as the Alps, Himalayas and Andes, large river floodplains, and glaciated regions such as the Great Lakes area of North America. These were the places where iodine shortfalls were most common before fortification. See A Short History of Iodized Salt in the U.S.

Into the Food Chain

Plants absorb iodine from soil and water, so the iodine in vegetables and grains reflects where they were grown. Animals that eat those plants, plus any iodine added to their feed, pass iodine into milk, eggs and meat. That's how two servings of the same food can carry very different amounts of iodine depending on the region and farm.

Onto the Plate

Because soil iodine is so uneven, today's diets depend less on local soil and more on:

• Ocean foods: seafood and seaweed.
• Dairy and eggs: influenced by animal feed and farm practices.
• Iodized salt: a deliberate way to add iodine to the food supply. See How Fortification Programs Work.

For a list of foods and their approximate iodine content, see our iodine-rich foods list.

Why This Matters

The iodine cycle explains why intake varies so much from place to place and person to person, and why people pay attention to where their iodine comes from. Food is the starting point; some people also use a supplement for a consistent, measured amount, such as our Lugol's Solution.

Frequently Asked Questions

Does organic produce have more iodine?
Not necessarily. The iodine in produce depends mostly on the soil where it was grown, not on whether it was grown organically.

Can I get enough of this nutrient from food alone?
Diet is the primary source for most nutrients, iodine included, and dietary sources vary widely by region and food choices. A healthcare provider or registered dietitian is the right resource for questions about your specific intake.

How does soil-driven variability affect supplementation decisions?
See Daily Iodine Intake, Food Sources, and Supplementation, Explained for how the two compare.

Shop Lugol's Solution — the traditional, trusted way people have supported their iodine intake for over 200 years.

Ryan Whitfield covers the chemistry and wellness-routine side of iodine supplementation for the Iodine Journal.

This article is for general educational purposes only and has not been evaluated by the FDA. It is not intended to diagnose, treat, cure, or prevent any disease, and is not a substitute for advice from a qualified healthcare provider — especially if you are pregnant, nursing, taking thyroid medication, or managing a thyroid condition.