Which Fish Have the Most Iodine – And Why the Levels Vary?

You've probably heard that fish is a good source of iodine, but there's a catch: the actual amount can vary a lot. Since iodine is important for thyroid health, but also because too much iodine can be harmful, it's worth learning what influences the iodine content of fish. Once you know that, you can make smarter choices at the supermarket and in the kitchen!

Before we unpack what affects the iodine levels in fish, a quick note on terminology: In foods, iodine is usually present as iodide (I⁻), the form your body absorbs and uses to make thyroid hormones. But nutrition guidelines and food databases simply call it iodine, so that's the term we'll stick with here. [1]

With that cleared up, let's look at some of the main factors that affect how much iodine you might expect to get from fish.

Saltwater vs. Freshwater Fish

Where a fish lived is one of the biggest factors influencing its iodine content. Marine fish generally have higher levels of iodine than freshwater fish, simply because seawater contains more iodine than rivers and lakes [2].

Now you might think that's because seawater is a good source of iodine—but it isn't [3]. There's simply more iodine in seawater than in freshwater, and when you add to the equation one of the world's greatest accumulators of iodine, algae, the result is a marine food chain where plenty of iodine is available.

Some types of algae have been found to concentrate up to 30,000 times the iodine content of the surrounding sea water, highlighting just how efficient they are at storing this important mineral [3].

Diet of the Fish

Another important factor influencing the iodine content of fish is their diet. Species that feed on iodine-rich algae and plankton accumulate more iodine in their bodies, as do predatory fish that feed on smaller iodine-rich fish. In fact, studies have found that when freshwater fish are fed marine algae, their iodine content increases significantly [4-6].

One of these studies took it a step further by looking at whether the increased iodine levels in those fish could transfer to humans. Fourteen healthy volunteers ate algae-fed fish, and the next morning their iodine levels (measured in urine) were much higher than on other days [6]. This shows that feeding fish iodine-rich algae not only boosts the iodine content of the fish but also effectively delivers this essential nutrient to those who eat the fish.

Differences Between Species

So the diet of the fish is important, but the problem is you can only guess what kind of diet the fish at the supermarket have had. What you don't need to guess is what species you're buying—and that matters, too, when it comes to iodine levels.

Different fish species naturally contain varying amounts of iodine, so simply choosing one type over another can impact how much of this essential nutrient you're getting. For example, haddock and cod are well known for their high iodine contents, whereas other commonly eaten species, like salmon and tuna, generally contain considerably less.

That said, iodine levels among individuals within the same species are known to vary greatly, so any table listing the iodine contents of different types of fish should be used as an approximate guide only.

Still, having an idea of how much iodine different species of fish contain on average can be useful, which is why we've put together the following table, which you can use as a rough reference.

Notes:: Freshwater fish are marked with an asterisk (*); the rest are marine fish. Unless otherwise noted, all the data are for to wild-caught fish. The table above was created by healwithfood.org using data adapted from M. Sprague et al. and Public Health England [7, 8]. To keep the data comparable, only values based on raw fish samples made up of the edible flesh portion were included. The figures shown represent averages derived from multiple samples, and have been rounded to whole numbers.

Cooking Methods

Cooking can affect iodine levels too, mostly by changing the water content of the fish. One study found that cod had about 15–20% more iodine after pan-frying or oven-baking, simply because water loss concentrated what was left. Steaming, on the other hand, reduced iodine by about 10–20% [9].

Another study across several fish species showed a similar trend: baking and frying usually didn't reduce the total iodine in a fillet and sometimes even bumped it up slightly, while boiling tended to cause some losses [10].

That being said, steaming and boiling are generally considered very healthy ways of cooking, so don't think you should avoid them. In fact, it seems that if you use the cooking liquid—like when you make a fish soup or stew—you can actually hold on to that iodine [9].

Skinned vs Unskinned Fish

If you've been skipping the skin when preparing fish, you might want to rethink your habit. Turns out, fish skin is a particularly good source of iodine—it can contain up to 20 times the iodine found in the muscle.

What's also interesting is that the concentrations appear to gradually decrease as you move inward toward the backbone, with the darker muscle tissue, located just beneath the skin, tending to be richer in iodine than the lighter, white muscle closer to the center of the fillet. [7]

To put this knowledge into practice, look for recipes that call for pan-searing, grilling or broiling unskinned fillets, as these cooking methods typically make the skin crispy and easy to chew. Or, develop a taste for fish like sardines, which are usually sold and eaten with the skin on!

The Takeaway

Fish can be a great source of iodine, but the amounts can also vary greatly. If you're trying to increase your iodine intake naturally, it helps to understand the various factors that affect those levels.

However, it's equally important to acknowledge that the picture is complex, and even samples of the same species, prepared exactly the same way, can contain very different levels of iodine.

Therefore, instead of obsessing over data, the best strategy for a healthy person is probably to eat a variety of fish and seafood, prepared in different ways. That way you'll not only boost your chances of getting enough iodine in the long term but also benefit from a mix of nutrients that different fish have to offer.

References

1. NIH Office of Dietary Supplements (2022). Iodine Fact Sheet for Health Professionals. Last accessed on August 22, 2025.
2. British Geological Survey (2003). Department for International Development: The geochemistry of iodine and its application to environmental strategies for reducing the risks from iodine deficiency disorders (IDD). DfID KAR Project R7411. Commissioned Report CR/03/057N.
3. P. Smyth (2021). Iodine, Seaweed, and the Thyroid. European Thyroid Journal.
4. L. Valente et al (2015). Iodine enrichment of rainbow trout flesh by dietary supplementation with the red seaweed Gracilaria vermiculophylla. Aquaculture / ScienceDirect.
5. R. Ribeiro et al (2015). Dietary macroalgae is a natural and effective tool to fortify gilthead seabream fillets with iodine: Effects on growth, sensory quality and nutritional value. Aquaculture / ScienceDirect.
6. S. Schmid et al (2003). Marine algae as natural source of iodine in the feeding of freshwater fish: A new possibility to improve iodine supply of man. Revue de Médecine Vétérinaire / CABI Digital Library.
7. M. Sprague et al (2022). Iodine Content of Wild and Farmed Seafood and Its Estimated Contribution to UK Dietary Iodine Intake—Supplement. Nutrients, 14(1), 195.
8. Public Health England (2021). Composition of foods integrated dataset (CoFID).
9. L. Dahl et al (2020). Iodine and Mercury Content in Raw, Boiled, Pan-Fried, and Oven-Baked Atlantic Cod (Gadus morhua). Foods, 9(11), 1652.
10. Cin, N. A. (2024). The iodine content of raw, boiled, pan-fried, and oven-baked fish samples using ICP-MS: analysis and reliability of measurements. International Journal of Food Science & Technology, 59(5).