Do Soy Sauces Have Iodine? What You Need to Know for Your Health

November 10, 2025 •

5 min read

While soybeans themselves do not contain significant amounts of iodine, the presence of additives like kelp extract or iodized salt means that the question of whether do soy sauces have iodine is more complex than it appears. The actual iodine content can vary dramatically between different brands and types.

Quick Summary

The iodine content in soy sauce is highly variable and depends on manufacturing ingredients, not the soybean base. High-iodine additives like kelp or iodized salt are the primary sources, not the fermentation itself.

Key Points

Iodine is Not Inherent: Soy sauce does not naturally contain significant amounts of iodine; any content is from additives like iodized salt or kelp extract.
Additives Are Key: The primary sources of iodine in soy sauce are iodized salt used in the brine or seaweed-based flavorings, such as kelp extract.
High Sodium Content: All types of soy sauce, including reduced-sodium versions, are very high in sodium and should be consumed in moderation.
Goitrogenic Effects of Soy: Soy's isoflavones can act as goitrogens, potentially interfering with thyroid function and radioactive iodine therapy, regardless of the iodine content.
Check Labels for Additives: For those on low-iodine diets, check labels for added kelp or unspecified salt types; opting for a soy-free alternative may be the safest bet.
Most Brands Are Low-Iodine: Research has shown that many commercial soy sauces, particularly those made with non-iodized refined salt, are low in or free of iodine.

The Surprising Truth About Iodine in Soy Sauce

For many health-conscious individuals, understanding the nutritional composition of everyday condiments is crucial. Soy sauce, a staple in Asian cuisine, is often a point of inquiry, especially regarding its iodine content. The simple answer—that it depends entirely on the manufacturing process—is key to understanding why some soy sauces are virtually iodine-free while others can contain high amounts. The soybean itself, the main ingredient, is not a significant source of iodine. Therefore, any notable iodine in soy sauce is due to additives. These can include iodized salt used during fermentation or flavor-enhancing ingredients like seaweed extract, such as kelp, which is naturally rich in iodine.

Factors That Influence Iodine Content

Several factors play a role in determining how much iodine is in a bottle of soy sauce. The two most prominent are the type of salt used and the presence of seaweed-based additives. Some regional studies have shown that most soy sauces use non-iodized salt, resulting in a low iodine product. However, without clear labeling, consumers often have no way of knowing for certain. This is particularly relevant for those needing to follow a strict low-iodine diet, such as patients undergoing radioactive iodine therapy for thyroid conditions.

Type of Salt: The most significant source of iodine is salt, specifically iodized salt. If a manufacturer uses non-iodized salt for the brine, the iodine content remains very low. Conversely, a manufacturer using iodized salt will produce a high-iodine product. Unfortunately, many labels do not specify the salt type.
Seaweed Additives: To create a deeper umami flavor, some manufacturers add kelp or other seaweed extracts. Since seaweed is one of the most iodine-rich food sources on the planet, even small amounts can dramatically increase the iodine content of the final product. Brands with these additives are typically marketed for specific flavor profiles rather than as a health-food option.
Brewing Method: Naturally brewed soy sauces, which undergo a lengthy fermentation process, are typically safer for low-iodine diets, assuming non-iodized salt is used. Chemically produced soy sauces use a different process that also relies on salt, but the primary risk remains the salt source or any flavor-enhancing additives.

Soy's Effect on Thyroid Function (Beyond Iodine)

It is important to differentiate between iodine content and the potential goitrogenic effects of soy. Soybeans contain isoflavones, which are compounds known as goitrogens. For individuals with existing thyroid conditions, especially those with an inadequate iodine intake, high consumption of soy could potentially interfere with thyroid function. The American Thyroid Association, for example, recommends avoiding soy products on a low-iodine diet not just due to potential iodine but because of the goitrogenic effects that can interfere with radioactive iodine uptake. This means that even a certified low-iodine soy sauce might still be restricted in certain medical contexts.

How to Find Low-Iodine or Iodine-Free Soy Sauce

For those on a low-iodine diet, selecting the right soy sauce requires careful attention. The best practice is to look for products that explicitly state they are iodine-free or use non-iodized salt. Some manufacturers cater specifically to this market. Alternatively, seeking products without seaweed-based additives is crucial. When in doubt, a soy sauce substitute or a homemade version with known ingredients may be the safest route.

Comparison: Soy Sauce Varieties and Iodine Risk


Type of Soy Sauce	Common Characteristics	Iodine Content	Sodium Content	Risk for LID Patients	Considerations
Regular Soy Sauce	Typically made with soybeans, wheat, salt, and water. Fermented.	Varies, usually low unless iodized salt is used.	High (approx. 900 mg/tbsp).	Variable; check label for kelp/salt type.	Always check ingredients. High sodium.
Reduced-Sodium Soy Sauce	Less salt added, but still a source of sodium.	Varies, depends on salt source.	Moderate-High (approx. 500 mg/tbsp).	Variable; check label.	Better for sodium intake, but not guaranteed low iodine.
Tamari (Gluten-Free)	Made with little to no wheat. Fermented.	Varies, depends on salt source.	Moderate-High, often less than regular soy sauce.	Variable; check label.	Must still check for kelp or salt type.
Kelp-Infused Soy Sauce	Contains added kelp or seaweed extract for flavor.	High to very high.	High.	High risk; avoid.	Often explicitly marketed with kelp.
Homemade Substitute	Known ingredients, no fermentation.	Can be iodine-free by using non-iodized salt.	Controllable.	Very low risk.	Most reliable option for a strict LID.

Navigating Soy Sauce on a Low-Iodine Diet

For patients preparing for radioiodine therapy, most health professionals advise avoiding all soy products, including soy sauce, for a period of time. This is primarily due to the goitrogenic effects of soy and not just the iodine content. The isoflavones in soy can inhibit the thyroid's ability to absorb iodine, which could decrease the effectiveness of the therapy. Even if a product is certified low-iodine, the soy component itself can be problematic. The safest approach is to use a non-soy-based alternative during the dietary restriction period. However, for those not on a strict medical diet, choosing a soy sauce made without kelp extract or iodized salt poses very little risk from an iodine perspective.

Beyond Iodine: Sodium Content in Soy Sauce

Regardless of its iodine content, soy sauce is notoriously high in sodium. A single tablespoon can contain a significant portion of the daily recommended sodium intake. Excessive sodium intake is linked to increased blood pressure and other cardiovascular risks. While reduced-sodium versions are available, even these contain considerable amounts. For general health, it is wise to consume soy sauce in moderation or opt for lower-sodium varieties. Always check nutritional labels and consider that reduced-sodium options are not automatically low-iodine options unless specified.

Conclusion: Making an Informed Choice

In conclusion, the iodine content in soy sauce is not inherent to the fermented soybeans but comes from added ingredients, primarily iodized salt or seaweed extracts. For the average consumer, most standard soy sauces are low in iodine, but they remain high in sodium. For individuals following a strict low-iodine diet for medical reasons, avoiding soy sauce altogether or confirming the absence of high-iodine additives is the safest approach, as recommended by the American Thyroid Association. Always read labels carefully, and when in doubt, consider safer, alternative condiments.

For more information on iodine intake and thyroid health, consult the NIH Office of Dietary Supplements.

Frequently Asked Questions

For those on a strict, medically supervised low-iodine diet, especially before radioactive iodine therapy, it is safest to avoid soy sauce entirely. This is due to both potential iodine content from additives like kelp or iodized salt and the goitrogenic properties of soy itself, which can interfere with treatment effectiveness.

No, not all soy sauces contain iodine. The iodine content depends on the ingredients used by the manufacturer. Most soy sauces made with non-iodized salt are very low in iodine, but those with added seaweed extract (kelp) will contain high amounts.

Reduced-sodium soy sauce is formulated to contain less sodium, but it is not necessarily lower in iodine. The iodine content still depends on whether iodized salt or seaweed additives were used during production. Always check the ingredient list carefully.

The primary source of iodine in soy sauce comes from intentionally added ingredients, not the soybeans themselves. These additives are typically iodized salt or flavor enhancers derived from seaweed, like kelp extract.

Soy contains goitrogens, compounds that can interfere with the thyroid's ability to use iodine. For people with thyroid issues or those undergoing radioiodine therapy, high soy intake can potentially slow thyroid function and reduce treatment efficacy.

The best way to check is by reading the ingredient label. Look for kelp extract, seaweed, or unspecified salt. Without explicit labeling, it is difficult to be certain. Some specialty brands market their products as low-iodine for this reason.

Yes, there are several iodine-free substitutes. These include homemade sauces using low-sodium, non-iodized salt or commercially available coconut aminos, which offer a similar salty, savory flavor profile without soy or its associated goitrogens.