Iodine is an essential trace element in some foods and is added to food products such as salt. It is also available as a dietary supplement. Iodine is a crucial component of the thyroid hormones triiodothyronine (T3) and thyroxine (T3). These hormones regulate critical biochemical reactions such as the synthesis of proteins and enzymatic activities [1].
These hormones are also necessary for the proper development of the central nervous system and skeletal system of fetuses and infants. Getting sufficient iodine is essential for all people, especially infants and those who are pregnant.
The thyroid-stimulating hormone (TSH) is primarily responsible for regulating thyroid function. Secreted by the pituitary gland, TSH controls the production of thyroid hormones and their secretions, thereby protecting individuals from hyperthyroidism or hypothyroidism [1]. Once TSH is secreted, iodine uptake increases while T3 and T3 are synthesized and released.
What is goiter, and how is it related to a lack of iodine?
When your diet lacks iodine, you are at increased risk of developing a goiter. When iodine levels are insufficient, the TSH becomes elevated, leading to thyroid gland enlargement or goiter. This thyroid gland enlargement is the body’s attempt to get more iodine from the blood and produce T3 and T4 hormones.
What are the physiological functions of iodine?
- Production of T3 and T4 hormones
- Immune response
- It benefits fibrocystic breast disease and mammary dysplasia [2].
Iodine-rich foods
The soil in different parts of the world holds varying amounts of iodine. Hence, the iodine content found in crops also varies.
In areas where the soil has insufficient iodine, governments have introduced policies that allow the iodine fortification of salt. These salt iodization programs have reduced the prevalence of iodine deficiency globally.
Iodine is found in sea vegetables and animal protein foods. It is also found to a lesser extent in fortified foods such as milk, cereals and loaves of bread.
Here are some excellent iodine-rich foods:
- Shellfish, fish (shrimp, oysters, canned tuna and cod)
- Iodized table salts
- Eggs
- Chicken
- Beef liver
- Dairy (yogurt, cheese and milk)
- Fortified infant formula
- Seaweed (kombu, kelp, nori and wakame)
Seaweed
Seaweed remains a good source of vitamins, minerals and antioxidants. It is one of the best sources of iodine. However, the amount of iodine in seaweeds varies by location and region or where it is found. There are at least three popular varieties of seaweed: kombu kelp, wakame and nori.
- Nori – this is a type of red seaweed that is commonly used in sushi rolls. The iodine content of this type of seaweed ranges from 16-43 mcg per gram, which amounts to approximately 11-29% of the RDA.
- Wakame – refers to brown seaweed with a slightly sweet flavor and is commonly used in miso soup. Its iodine content varies depending on where it is grown. For example, wakame grown in Asia contains higher amounts of iodine than in New Zealand and Australia.
- Kombu Kelp – refers to brown seaweed sold in the market as a fine powder or dried seaweed. Compared with other seaweed, kombu kelp has the highest amount of iodine. It is an essential ingredient for dashi, a Japanese soup stock.
Cod
Codfish has low amounts of fats but high amounts of iodine. It has been shown that fish low in fats are rich in iodine. For example, 85 grams of cod contains approximately 42% to 66% of the RDA for iodine.
However, similar to seaweeds, the amount of iodine in cod varies according to the region and area where it is grown and whether it is wild-caught or farm-raised.
Dairy
In western diets, dairy remains one of the essential sources of iodine. A cup of milk can contain approximately 59% to 112% of the RDA for iodine. Meanwhile, a cup of yogurt contains at least 50% of the RDA for iodine.
Apart from milk and yogurt, cheese also has high amounts of iodine. However, its iodine content varies according to the type of cheese. Cottage cheese has the best iodine source compared with other cheese types.
Iodized salt
One-quarter of a teaspoon of iodized salt has about 71 mcg of iodine, equivalent to 47% of the RDA.
Eggs
Eggs are one of the good iodine-rich foods. A large egg has approximately 16% of the RDA of iodine since it contains 24 mcg of iodine.
A whole egg is also an excellent source of vitamins, minerals and protein. However, most of these nutrients are found in the yolk of the egg.
Tuna
Tuna is a high-protein, low-calorie or low-fat and iodine-rich fish source. It is a good source of minerals and vitamins such as B vitamins, iron and potassium.
It is also an excellent source of omega-3 fatty acids, which have been shown to lower the risk of cardiovascular diseases.
Shrimp
Shrimp is another excellent source of iodine. It is also a protein-rich and low-calorie food. It is high in vitamin B12, phosphorus and selenium.
Like other marine organisms, shrimps absorb iodine that is naturally present in seawater. Three ounces of protein give at least 23% of the RDA for iodine or about 35 mcg.
Lima Beans
These beans are commonly grown in South America and are good magnesium, folate and fiber sources. Apart from preventing the risk of cardiovascular diseases, these beans are good sources of iodine for vegans.
However, the amount of iodine in Lima beans varies according to the amount in the soil, cultivation practices and the fertilizers used. On average, a cup of these beans provides 10% of the RDA for iodine or about 16 mcg of iodine.
Prunes
Like Lima beans, prunes are also good sources of iodine for vegans. Five dried prunes contain 9% of the RDA for iodine. Besides fiber, it is rich in iron, potassium and vitamins A and K.
What are the recommended amounts of iodine?
The Recommended Dietary Allowance (RDA) for iodine in women 19 years old and older and adult men is 220 mcg daily. However, the RDA for pregnant women is higher at 290 mcg daily.
The tolerable upper intake level for lactating and pregnant women is 1,100 mcg daily. This level, while very high, does not lead to adverse effects. A daily intake higher than 1,100 mcg could lead to adverse effects.
What are the signs of iodine deficiency?
Since iodine regulates metabolism, it is critical in the functioning and growth of cells. It helps the body convert food into energy. A deficiency of this essential element can prevent normal development and growth.
In children, deficiency in iodine leads to the following:
- Difficulties in talking, writing, reading, social skills and problem-solving
- Stunted growth
In pregnant women, iodine deficiency can lead to stillbirth or miscarriage. In adults, a deficiency of iodine (less than 10-20 mcg intake of iodine daily) can lead to pin adequate thyroid hormone production, leading to hypothyroidism.
When individuals have hypothyroidism, this disrupts normal metabolic functions, such as regulating body weight, body temperature and heart rate [2]. Goiter often develops during hypothyroidism. Other signs of this condition include the following:
- Dry skin and hair
- Sensitivity to cold
- Constipation
- Weakness
- Fatigue, lethargy
- Weight gain
People at risk of iodine deficiency include individuals who do not use supplements that contain iodine or do not use iodized salt. Pregnant women with poor iodine intake are also at risk of iodine deficiency.
In addition, vegans who do not consume animal food and people living in areas with deficient iodine levels in their soil, such as those in mountainous regions.
Take-home message
Iodine is an essential element necessary to promote optimal metabolism in the body—lack of iodine results in hypothyroidism leading to lethargy, poor cognitive performance, constipation and other symptoms. Getting iodine from the diet is critical in meeting the RDA for this nutrient in the body.
Excellent sources of iodine include fortified foods and iodized salt. Kinds of seafood such as seaweed, tuna, cod and shrimp have high amounts of iodine and contain other nutrients and essential vitamins.
Dairy products are another good source of iodine. For vegetarians, dried prunes and Lima beans are excellent sources of iodine.
[1] https://etj.bioscientifica.com/view/journals/etj/11/1/ETJ-21-0035.xml
[2] https://thyroidresearchjournal.biomedcentral.com/articles/10.1186/s13044-021-00116-y
