Everything You Need To Know About Iodine

It's probably not a micronutrient that's ever crossed your mind, but iodine plays key roles in your health, particularly the functioning of your thyroid. But what exactly is iodine? According to a 2012 paper published in Nutrients, iodine is a trace element found in Earth's upper crust. The highest concentrations of iodine can be found in the soil and waters of coastal areas. A few foods, such as seaweed and seafood, naturally contain iodine, while others are fortified. Although iodine deficiency in the United States is rare today, that wasn't the case before the 1920s.

In fact, even though iodine is today acknowledged as integral to human health, it wasn't even recognized as an element until the 1810s. In 1811, French chemist Bernard Courtois noticed a strange purple vapor while processing salts needed to make gunpowder. Two years later, another French chemist, Joseph Louis Gay-Lussac, published a paper identifying this substance as a new element: iodine. The link between iodine and thyroid health quickly became clear, and by 1852 experts recognized that goiters were caused by iodine deficiency. It wasn't until 1896, however, that scientists discovered that the thyroid gland stores this element. Despite these discoveries, the practice of iodizing salt to ensure adequate intake didn't begin until the 1920s. Switzerland and the United States were the first countries to begin fortifying salt. Today, 120 countries require the iodization of all food-grade salt, although in the United States, the practice is voluntary.

Iodine and the thyroid

The main job of iodine in the body is to assist with the production of thyroid hormones. As Endocrineweb explains, the thyroid is a butterfly-shaped gland located in front of the windpipe and below the larynx in the throat. If your metabolism is a factory where workers (the body's cells) turn food into energy, then the thyroid gland is the foreman, supervising and regulating everything. It does this by producing two hormones: T3 (triiodothyronine) and T4 (thyroxine). These hormones affect every cell in the body and dictate how quickly or slowly that cell uses energy. Iodine is an essential component of these hormones, and the thyroid extracts and stores this element. In addition to controlling how quickly cells use energy, thyroid hormones influence body temperature, heart rate, the speed at which food moves through the digestive tract, how muscles contract, and the rate at which dying or damaged cells are replaced (via the University of Michigan Health).

According to a 2014 paper published in the Annals of Pediatric Endocrinology & Metabolism, a healthy adult body contains about 15 to 20 milligrams of iodine, 70%-80% of which is stored in the thyroid. Most of the rest can be found in the breast tissue of lactating women and circulating in the blood as the result of the breakdown of T3 and T4.

How much iodine do we really need?

Because iodine is so important for our health, guidelines on how much individuals should be consuming each day have been established by the Food and Nutrition Board of the National Academies. Unlike many other micronutrients, the requirements for iodine don't vary between males and nonpregnant females. Adequate intake (AI) recommendations are used for infants, with those six months and younger needing 110 micrograms daily and those 7–12 months requiring 130 micrograms. Beginning at age one, iodine requirements are expressed as recommended dietary allowances (RDAs). Children age one through eight have an RDA of 90 micrograms. Tweens age 9–13 require 120 micrograms, while those 14 and older need 150 micrograms. Iodine needs increase substantially during pregnancy and lactation. Expectant mothers should aim for 220 micrograms, while breastfeeding women require 290 micrograms. This is because a woman's thyroid levels increase during pregnancy and lactation to support proper fetal development (via the Centers for Disease Control and Prevention).

But how much iodine are Americans actually getting? The good news is that, according to the National Institutes of Health (NIH), intake from food alone ranges from 141 micrograms to 296 micrograms, with an average of 216 micrograms. That number doesn't include the iodine individuals get from salting their food with iodized salt during cooking and before eating, so the actual average is likely much higher.

Food sources of iodine

As the Iodine Global Network explains, "Iodine ... is widely but unevenly distributed in the earth's environment. In many regions, leaching from glaciations, flooding, and erosion have depleted surface soils of iodide, and most iodide is found in the oceans." A liter of seawater contains about 50 micrograms of iodine. It's not surprising, then, that seaweed, saltwater fish, and seafood such as clams and oysters are the best natural sources of iodine. 

Because iodine is concentrated in mammary tissue, milk and products made from milk (such as cheese and yogurt) can contain significant amounts of iodine, but only if the cows' feed contained an adequate amount of it. Iodine solutions are also often used to clean a cow's udder before milking, and the residue from these solutions can also add to milk's iodine content. In fact, milk is a major source of iodine, especially for children, in many countries, including the United States, Canada, and the United Kingdom.

Most other foods and beverages are low in iodine, although it really depends on where they were grown or raised. Plant foods grown in iodine-depleted soils won't contain much iodine, and animal products won't contain much iodine unless the food they ate was rich in iodine. Because the iodine content of soil and ground water varies so much around the globe, it's impossible to know for sure how much iodine a particular food contains without testing it.

Iodized salt

According to a 2012 paper published in Nutrients, the first iodine-fortified salt became available in the United States in Michigan on May 1, 1924. Although Congress attempted to pass a bill in the 1940s to mandate that all salt be iodized, the measure failed. Today, you can buy (and manufacturers can use) both types of salt. Even so, an estimated 70%–76% of American households exclusively use iodized salt, and more than 90% of households have access to iodized salt. In approximately 120 countries, however, all food-grade salt must be iodized.

Initially, a lot of iodine was used to iodize salt — 100 milligrams of iodine per kilogram of salt — resulting in an average daily intake of 500 micrograms of iodine. But as the Iodine Global Network notes, today iodine is added at 15-80 micrograms per kilogram of salt. At this rate, 2 grams of salt contains roughly the RDA of iodine for an adult. Potassium iodide or potassium iodate are used to fortify salt, and today, about 50% of the salt sold in the United States is iodized. Manufacturers of processed foods don't have to disclose on the label whether their products contain iodized or non-iodized salt. Globally, iodizing salt has proven to be an easy and cheap way to ensure people in iodine-depleted areas get the iodine they need. The iodine in salt is very stable, and processes like boiling, baking, and canning reduce the iodine content of fortified salt by no more than 10%.

Other sources of iodine

There are other ways people get iodine besides iodized salt and the iodine found in food. According to the Iodine Global Network, several additives contain iodine, including bread stabilizer, which helps maintain the structure and texture of bread. The red dye erythrosine contains iodine and is used extensively as a coloring agent for foods and medications. Some drugs such as amiodarone (used to treat heart arrythmias) also contain iodine. Other ways people ingest iodine include the use of iodine-containing water purification tablets and contrast dyes used during imaging procedures like CT scans.

Many dietary supplements also contain iodine. As the National Institutes of Health explains, the iodine in supplements is usually in the form of potassium iodide or sodium iodide. Many multivitamins contain iodine, usually at 100% of the RDA for adults (150 micrograms). Because iodine needs increase dramatically during pregnancy, many prenatal vitamins also contain this element. Supplements that contain kelp or kelp extracts will also contain iodine. It's also possible to get standalone iodine supplements, although many of these contain very high doses of iodine. 

While most Americans appear to get enough iodine and probably don't need to specifically seek out an iodine-containing supplement, pregnant and breastfeeding women are encouraged to take a supplement that includes at least 150 micrograms of iodine and to use iodized salt. Despite these recommendations, research indicates that only about 17.8% of pregnant women take an iodine-containing prenatal supplement.

Some substances interfere with the uptake of iodine

According to the Iodine Global Network (IGN), goitrogens are naturally occurring substances found in foods that can interfere with the thyroid's normal functioning. Glucosinolates are one common class of goitrogens. These compounds are found in cruciferous vegetables, including spinach, kale, cabbage, cauliflower, and broccoli. 

Cyanogenic glucosides are another class of goitrogens and can be found in foods such as cassava, lima beans, sorghum, and sweet potato. In both cases, these substances, once ingested, are metabolized into compounds that compete with iodine, meaning they're chemically similar enough to iodine that the thyroid gets "confused" and absorbs and stores them rather than the iodine it needs to make thyroid hormones. Soy and millet have flavonoids that can interfere with thyroid peroxidase, an enzyme needed to create thyroid hormones.

Just because goitrogens have the potential to affect the thyroid doesn't mean you should swear them off. Goitrogenic foods like kale, sweet potato, and soy offer loads of micronutrients. IGN notes that although there are more than 100 known goitrogens, they don't appear to have a clinically significant impact on thyroid function unless a person has a pre-existing iodine deficiency. In other words, goitrogens can make the effects of iodine deficiency worse, but they won't cause issues if you're getting enough iodine in your diet.

Who's at risk for iodine deficiency?

Before the introduction of iodized salt, iodine deficiency was most common in areas where the soil was depleted of iodine. As a 2012 paper published in Nutrients explains, these areas in the United States included the Great Lakes region, areas in the Appalachian Mountains, and the northwestern states. Collectively, these areas were known as the "goiter belt" because as many as 70% of children in these regions had goiters — a clear sign of iodine deficiency. Today, the countries with the lowest iodine consumption (and thus, the greatest risk of iodine deficiency) include Burundi, Cambodia, Estonia, Haiti, Lebanon, Madagascar, Mali, and Tajikistan (via the Iodine Global Network).

Besides access to iodine-rich foods and iodized salt, certain other factors can increase the risk for iodine deficiency. As the National Institutes of Health explains, people who don't eat dairy, eggs, or seafood (such as vegans or those with multiple food allergies) are more likely to be iodine deficient, because these foods are major sources of iodine. Individuals who don't get a lot of iodine from their diet and also eat large amounts of goitrogens are also at elevated risk for iodine deficiency. 

Because pregnancy and lactation substantially increase a woman's iodine needs, pregnant and lactating women are more likely than others to be consuming too little iodine. In fact, research suggests that many pregnant women don't meet the RDA for iodine, even though they may not show obvious signs of iodine deficiency.

Iodine deficiency and your thyroid

Given how important iodine is to the proper functioning of your thyroid, it makes sense that iodine deficiency would have major consequences for this gland. As the Cleveland Clinic explains, when you don't consume enough iodine, your thyroid doesn't have the raw materials it needs to make sufficient thyroid hormones. This leads to hypothyroidism (an underactive thyroid). Hypothyroidism leads to a general slowdown of your metabolism, which can cause fatigue, weight gain, and cold intolerance. Other symptoms of hypothyroidism include coarse, thinning hair; dry, scaly, or puffy skin; confusion; hoarseness; and infertility. It's important to note, however, that there are other reasons why an individual's thyroid may become underactive besides too little iodine.

When the body doesn't get the thyroid hormones it needs to function properly, feedback mechanisms signal to the thyroid to grow larger in an attempt to gather more iodine and make more thyroid hormones. This enlarged thyroid, known as a goiter, creates a visible lump on the neck and can lead to difficulty swallowing or even breathing. As with hypothyroidism, goiters can have many other underlying causes other than iodine deficiency.

Iodine deficiency is treated with supplemental iodine. In some cases, individuals will also be prescribed synthetic thyroid hormones. While the effects of iodine deficiency on the thyroid can usually be reversed, in some cases they may be permanent.

Iodine deficiency during pregnancy and early childhood

While not getting sufficient iodine can have negative side effects at any point in life, it's especially dangerous during pregnancy and early childhood. 

As the Cleveland Clinic explains, iodine deficiency during pregnancy increases the risk for miscarriage, stillbirth, and a variety of birth defects. Babies born to mothers who didn't get enough iodine while pregnant may have stunted growth, intellectual disabilities, and developmental delays. If iodine deficiency during pregnancy is severe, babies may be born with a form of irreversible brain damage known as cretinism or congenital iodine deficiency syndrome. Symptoms include short stature; physical disabilities, including the inability to hear or speak; cognitive disabilities; and rigid muscles. A 2010 paper published in Orphanet Journal of Rare Diseases notes that about 1 in every 2,000–4,000 babies are born with congenital hypothyroidism, although maternal iodine deficiency isn't always the cause.

The National Institutes of Health states that infants are particularly sensitive to the effects of iodine deficiency. Even mild iodine deficiency can cause significant changes in their thyroid hormone levels. If the breastmilk, formula, or baby food they eat doesn't contain enough iodine, they will be at greater risk of neurodevelopmental delays, a lower IQ, and attention deficit hyperactivity disorder (ADHD). In fact, according to the American Thyroid Association, iodine deficiency is the leading preventable cause of intellectual disabilities around the globe.

Risks of too much iodine

As with most micronutrients, it's possible to get too much of a good thing. According to a 2014 paper published in Nature Reviews Endocrinology, excess iodine can lead to either an underactive thyroid (hypothyroidism) or an overactive thyroid (hyperthyroidism). Consuming too much iodine leads to the Wolff–Chaikoff effect, a usually temporary reduction in thyroid hormone production. The Wolff–Chaikoff effect may not reverse in some individuals, however, leading to hypothyroidism. Those with a pre-existing autoimmune thyroid disease, thyroiditis (inflammation of the thyroid), or previous thyroid surgery are at increased risk for iodine-induced hypothyroidism. For others, the thyroid may use all that iodine to ramp up thyroid hormone production, leading to hyperthyroidism. This appears more likely in individuals who previously had iodine deficiency or goiter.

To ensure that individuals don't suffer the ill effects of excess iodine intake, the Food and Nutrition Board (FNB) has established tolerable upper intake limits (ULs) for iodine. Although no UL has been established for infants one year and younger, the FNB advises that iodine should only come from breastmilk, formula, or food (in other words, no supplementation). The UL for children age 1 to 3 is 200 micrograms, while those age 4 to 8 should get no more than 300 micrograms and tweens age 9 to 13 should get no more than 600 micrograms. Teens age 14 through 18 should get no more than 900 micrograms, and adults should not exceed 1,100 micrograms daily (via the Library of Medicine).

Iodine and fibrocystic breasts

Iodine may be helpful in the treatment of fibrocystic breasts. According to the Mayo Clinic, "fibrocystic breasts are composed of tissue that feels lumpy or ropelike in texture." Having fibrocystic breast tissue isn't uncommon, and many women's breast become more fibrocystic at certain points during their menstrual cycle or as they age. While the exact cause is unknown, sex hormones, particularly estrogen, likely play a role. Fibrocystic breast tissue contains fluid-filled cysts that may be small to quite large, scar-like fibrous tissue, and overgrowth of the cells lining the milk ducts. Although fibrocystic breast changes aren't considered a disease and don't increase your risk for breast cancer, they can produce unpleasant symptoms for some women. These include tender or painful breasts and green or dark brown non-bloody nipple discharge.

According to Mount Sinai Hospital, iodine supplementation may reduce the pain some women with fibrocystic breasts experience. This appears to be especially true for women who were low on iodine to begin with. Women treated with iodine reported few adverse effects. For example, in a study published in 2004 in The Breast Journal, researchers gave women between 0 and 6,000 micrograms of iodine daily for 6 months. Approximately 50% of those receiving the highest dosage reported a significant decrease in breast pain. It's important to note, however, that these doses are well above the tolerable upper intake limit (UL) for iodine.

Iodine for imaging

If you've ever needed imaging done (such as a CT scan), chances are good you were given a contrast dye to improve the detail of these images. Iodine is often used in contrast dyes. According to Radiopaedia, iodinated contrast media (ICM) are most commonly used to enhance images produced by X-rays, including regular X-rays (plain radiographs), CT scans, and procedures that image the blood vessels and movement of blood through the body. The elemental structure of iodine makes it particularly good at providing high contrast, which creates more detailed and more useful images. ICM is usually administered intravenously, but it can also be swallowed or delivered other ways depending on the needs of the particular test.

According to the Food and Drug Administration (FDA), ICM is generally considered very safe and well tolerated. Side effects may include flushing in the face, nausea or vomiting, mild itchiness, or skin rash, and in rare cases individuals may have allergic reactions. But because the thyroids of babies and young children are still developing and they're easily affected by fluctuations in hormone levels, the FDA recommends that follow-up screening to check thyroid hormone levels be done for anyone under the age of three who receives ICM. In rare instances, ICM may cause an underactive thyroid in this age group.

Radioactive iodine and thyroid cancer

According to the American Cancer Society, radioactive iodine (radioiodine) is an effective treatment for thyroid cancer. Because almost all iodine is taken up by the thyroid, radioiodine is a much more targeted approach than traditional radiation therapy. The radiation destroys the thyroid (including both healthy and cancerous cells), but causes few systemic effects. But radioactive iodine isn't the first choice for treating thyroid cancer. If thyroid cancer is small or hasn't spread to surrounding areas, surgery to remove the thyroid is usually the preferred treatment. If some thyroid tissue still remains after surgery, or if the cancer has spread to nearby areas such as the lymph nodes, radioactive iodine can be used to destroy these cancerous cells.

Before receiving radioactive iodine, your TSH levels need to be high because this will encourage thyroid cells to take up iodine. TSH levels can be manipulated by taking or not taking certain medications. Doctors also recommend individuals eat a very low-iodine diet for a week or two before receiving treatment to help increase the thyroid's "appetite" for iodine. Even though treatment with radioactive iodine is safer than other types of radiation therapy, it can still cause side effects. These include nausea and vomiting, neck tenderness, dry mouth, dry eyes, problems with the salivary glands, changes in the sense of taste, and (rarely) fertility issues. People give off radiation for a while after receiving radioiodine treatment, so special precautions must be taken to protect others from accidental radiation exposure.

Iodine is a powerful wound disinfectant

According to Wounds International, iodine has been used for almost 200 years as an antiseptic. It can kill bacteria, fungi, viruses, and protozoa, and is considered by some to be the most potent antiseptic available. It was used extensively to prevent wound infections during both the Civil War and World War I. Today, it can be found as over-the-counter preparations like Betadine as well as in hospitals, where it's used to disinfect patients' skin and doctors' hands before surgery. These products come in a variety of forms, including liquid solutions, creams, sprays, and iodine-saturated wound dressings. One major advantage of iodine is that germs don't appear to be able to build up a resistance to it like they can with other antimicrobial drugs.

But using iodine on wounds does carry some risks. Historically, iodine solutions often caused skin irritation and pain because they contained elemental (pure) iodine. Today, iodine wound care products contain iodine bound to a "carrier" molecule. This allows the iodine to be released in lower concentrations over an extended period of time once the product is applied. Even so, iodine can cause temporary skin staining and may actually slow the healing process for some wounds. Long-term use of these products may cause systemic side effects such as mild hyperthyroidism. Iodine products aren't recommended for pregnant or nursing women.