Digestion and Absorption of Iodine and Iodide: Know the Facts

Fortified table salt is a common source of iodine.
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Iodine is a trace element found in some foods and fortified salts. According to the National Institutes of Health (NIH), it's an essential part of the thyroid hormones thyroxine and triiodothyronine, which regulate vital processes such as protein synthesis and the development of the skeletal system.

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Iodine also helps the immune system. As researchers noted in the January 2021 issue of the International Journal of Molecular Sciences, molecular iodine "exerts multiple and complex actions on the organs that capture it, not including its effects as part of thyroid hormones."

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"As an oxidized component, [iodine] directly neutralizes free radicals, induces the expression of type II antioxidant enzymes, or inactivates proinflammatory pathways," they wrote.

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Iodine and Iodide

As described by the NIH, iodine can take several chemical forms, including iodide. "Iodine rarely occurs as the element, but rather as a salt," researchers explain. "For this reason, it is referred to as iodide and not iodine."

Iodine Consumption, Digestion and Absorption

Iodine is important for thyroid function in human growth, development and reproduction. The U.S. Department of Agriculture's Agriculture Research Service notes it can be obtained by eating seaweed, fish and other seafood, dairy products, eggs, dietary supplements and iodized salt.

The amount of iodine in common food sources can vary greatly, depending on factors such as the amount of iodine in the soil where the plants were grown, how much iodine was in the animals' feed, or the species of seaweed. Therefore, many countries have salt-iodization programs.

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Iodide (read: iodine) is quickly and efficiently absorbed through the walls of the digestive tract in the stomach and duodenum. After it's absorbed, most of it concentrates in the thyroid gland.

Iodine in Salt

Salt producers in the U.S. have been adding iodine to table salt since the 1920s, according to a July 2021 report in the ‌European Journal of Endocrinology‌, and 88 percent of households worldwide now use iodized salt — thereby helping to ensure adequate iodine consumption.

"However, most salt intake in the United States comes from processed foods, and food manufacturers almost always use non-iodized salt in these foods," researchers note. "Specialty salts, such as sea salt, kosher salt, Himalayan salt and fleur de sel, are not usually iodized [either]."

The Institute of Medicine of the National Academies sets the recommended dietary allowance for iodine at 150 micrograms per day for adults. Pregnant people need more — a daily intake of 250 micrograms, according to the World Health Organization.

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While the recommended daily intake of iodine for adults is 150 micrograms, the NIH notes that the so-called "tolerable upper limit" — the maximum safe dosage that's not likely to create negative effects on health — is twice that: 1,100 micrograms.

Safe Iodine Levels

Multivitamins and potassium iodide supplements can serve as reliable sources of iodine for those who need it. For example, iodine supplements are used to prevent goiter, help treat fibrocystic breast disease and reduce the risk of thyroid cancer after radiation exposure.

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Iodine deficiency during pregnancy can have a negative effect on fetal growth and development, possibly leading to brain and central nervous system damage. Low intakes of iodine can also cause hypothyroidism and goiter, which is an enlarged thyroid gland in the neck.

Iodine supplements are available in a variety of forms — including liquid solution — but you should speak to your doctor before using an iodine supplement because toxicity is a concern. Excess iodine intake can temporarily reduce thyroid hormone synthesis and may even induce hyperthyroidism or hypothyroidism, according to June 2020 research in Nutrients.

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Iodine consumption may also influence gut health, although more research is needed, especially in humans. For example, a March 2019 animal study in ‌Applied Microbiology and Biotechnology‌ found iodine treatment increased "bad" and decreased "good" bacteria in the gut microbiota of mice with obesity, but had the opposite effect on mice of a healthy weight.

Iodine Toxicity

Iodine might adversely interact with antithyroid drugs, lithium and warfarin, as noted by the Lima Memorial Health System in Ohio. In rare cases, overconsumption of iodine can lead to toxicity. High doses could potentially damage the thyroid gland and increase the risk of thyroid diseases.

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Moderate iodine toxicity is typically marked by elevated levels of thyroid stimulating hormone, or TSH, in the bloodstream. Moderate toxicity, which can occur at levels above the tolerable upper limit, may result in hypothyroidism, or insufficient production of thyroid hormones that regulate metabolism of food nutrients. Consumption of more than 1,700 micrograms of iodine per day may cause goiter, or enlargement of the thyroid gland, according to the Linus Pauling Institute at Oregon State College.

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Acute iodine toxicity is rare, and typically occurs at dosages of more than 1 gram of iodine, according to the National Library of Medicine. It can create symptoms such as diarrhea, nausea and vomiting.

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Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.

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