Iodine in foods

Which foods contain iodine?

According to the NIH, seaweed (kelp, nori, kombu, wakame) is among the best food sources of iodine; fish and other seafood and eggs are also good sources. Iodine is also found in breast milk and infant formulas. Iodized salt also provides iodine. The source of iodine in foods is the soil; the NIH reports that the amount of iodine in the world’s soils varies and that this affects the iodine of plant products. ^[1] The public page of TEMD (Society of Endocrinology and Metabolism of Türkiye) says that the soils of Türkiye are not rich in iodine and lists saltwater fish, shellfish, cow’s milk, cheese, yogurt, soy milk, eggs, ice cream and soy sauce as also containing iodine. ^[5]

The table below is selected from the NIH’s Table 2. Values are for US servings (1 oz ≈ 28 g; 3 oz ≈ 85 g; 1 cup ≈ 240 mL) and are approximate, used as a guide for estimation; the Daily Value (DV) is 150 µg. ^[1]

* The bread rows are for US breads that use potassium or calcium iodate as a dough conditioner; the same bread without iodate provides about 1 µg. ^[1] For the iodine content of a product in Türkiye, the label and manufacturer data are decisive; the table does not replace them.

While TEMD’s public page lists soy milk and soy sauce among iodine-containing foods, in the NIH’s US table 1 cup of soy beverage provides 3 µg and 1 tablespoon of soy sauce 0 µg of iodine; the two sources are based on different contexts and criteria. ^{[1] [5]}

How much iodine is in iodized table salt in Türkiye?

What does the communiqué say?

The Turkish Food Codex Salt Communiqué (2013/48; Official Gazette 16 August 2013) makes it mandatory to add potassium iodate to table salt at 25–40 mg/kg; the upper limit set for iodine may differ by +3 mg/kg. For people who should not consume iodine, non-iodized table salt may be produced. Adding iodine to salt for the food industry is not mandatory. ^[2] The Ministry of Health’s salt publication states that for non-iodized table salt the package size cannot exceed 250 g and its label must carry the words “non-iodized table salt” in a color contrasting with the package; that iodized salt must be named “Iodized Table Salt,” carry an expiry date and include the information “should be kept in a cool, dry and light-free place with the lid closed.” ^[3]

How many µg of iodine per gram?

The communiqué text writes the amount as “mg/kg potassium iodate”; it is not clear from the text whether this figure is the weight of iodine or the weight of iodate. The Ministry of Health’s publication, on the other hand, says there is about 70 µg of iodine in 1 g of iodized salt. ^[3] These three values do not match: if 25–40 mg/kg is read as iodate (about 59% of the chemical weight of potassium iodate is iodine; this proportion is a calculation), it means about 15–24 µg of iodine per gram, and if read as iodine, 25–40 µg per gram; the 70 µg in the publication is above these. Because of this uncertainty, a daily µg iodine calculation is not made from salt. The NIH’s US data report that according to the label, there are 45 µg of iodine per gram of iodized salt in the US and that the average of measured samples was 47.5–50.7 µg/g; this is not a reference for Türkiye. ^[1]

The TEMD guideline assumes that the daily iodine intake of a pregnant woman consuming standard iodized salt in Türkiye averages 100–150 µg. ^[4] According to the guideline, potassium iodide (KI) cannot maintain its stability in regions with large differences in humidity and heat; with the new codex its use in Türkiye was ended from 2009, and only potassium iodate (25–40 mg/kg) is used. In the 2007 study, 28% of salt samples were iodized with KI and 45% with KIO₃; of the KIO₃ salts, 60.6% complied with the codex, 11.3% were iodized above the codex and 28.1% were iodized below the codex and/or poorly stored. ^[4]

Storage and cooking loss

The Ministry of Health’s publication recommends adding salt to food after cooking to minimize iodine loss during cooking, and storing iodized salt in a cool, dry, light-free place and in dark-colored glass containers to prevent storage loss. ^[3] In the study by Rana and Raghuvanshi with six recipes specific to India (boiling, roasting, shallow frying, deep frying, pressure cooking and microwave) using iodized salt (potassium iodate), iodine loss was between 6.58% and 51.08%; the least loss was seen in shallow frying with 1 minute 15 seconds of cooking, and the most in pressure cooking with 26 minutes of cooking. ^[6] These results are not a measurement for Türkiye’s cuisine; they belong to Indian recipes and a single salt product.

Is using non-iodized salt a risk?

The NIH reports that specialty salts such as sea, kosher, Himalayan salt and fleur de sel are generally not iodized, that non-iodized sea salt provides 0 µg per quarter teaspoon, and that salt in processed foods in the US is almost always non-iodized. ^[1] The TEMD guideline does not recommend the use of rock salt and gourmet salts that are unrefined, of unclear content or with added additives; TEMD’s public page adds that it has observed that the frequency of non-iodized salt use without a physician’s recommendation has increased. ^{[4] [5]} Because iodine is not mandatory in salt for the food industry in Türkiye, the amount of iodine coming from processed foods depends on the product. ^[2]

Are seaweed and kelp supplements safe?

The NIH reports that the iodine concentration of commercial seaweeds ranges from 16 µg/g to 2,984 µg/g; in the table, 2 tablespoons (5 g) of nori is 116 µg. ^[1] If the NIH’s two extreme values are applied to 5 g of product (simple multiplication), an iodine amount between 80 µg and 14,920 µg results; that is, depending on the product, a single serving may fall far below the daily reference or exceed the adult upper limit of 1,100 µg. The NIH also states that there are iodine supplements containing kelp and that the Supplement Facts label shows whether iodine is present and its amount. ^[1]

The American Thyroid Association 2013 statement reports that many iodine, potassium iodide and kelp supplements can contain iodine at several thousand times the daily tolerable upper limit, that it recommends avoiding iodine or kelp supplements exceeding 500 µg per day in children and adults, in pregnancy and in breastfeeding, and that pregnant and breastfeeding women, older adults, infants and people with pre-existing thyroid disease are more sensitive to the adverse effects of excess iodine. The same statement also notes that, in the US, it recommends a multivitamin containing 150 µg of iodine per day (as potassium iodide) before pregnancy, during pregnancy and during breastfeeding; that is, the warning is given together with a low-dose support recommendation. ^[8]

Why does iodine in dairy products vary?

Milk and dairy products contain iodine, but the amount is not constant. In tank milk samples collected every three months for a year from 32 farms in Switzerland and Northwest Italy, median milk iodine was found to be 87 µg/L (range 5–371 µg/L); according to the study’s title, the main determinants of milk iodine are farm type, season and teat disinfection. ^[7] This study is not a measurement for milk in Türkiye; it shows that the iodine content of milk cannot be summarized with a single value.

This guide does not recommend a daily or weekly personal dose. For pregnant and breastfeeding women, people with thyroid disease, and people using kelp or high-dose iodine, the iodine decision is made with the treating physician.