Thyroid hormones play an important role in a wide range of bodily functions, including metabolism, bone health, immune response, and development of the central nervous system (CNS).
Iodine helps convert thyroid stimulating hormone (TSH) to triiodothyronine (T3) and thyroxine (T4). This conversion is important for the thyroid to function properly.
An iodine imbalance can lead to an overactive or underactive thyroid.
Around 70 to 80 percent of iodine is found in the thyroid gland in the neck. The rest is in the blood, the muscles, the ovaries, and other parts of the body.
Iodine deficiency is rare in Western nations because salt is iodized. However, an estimated 2 billion people worldwide remain at risk for iodine deficiency, and about 300 million people worldwide suffer from thyroid gland dysfunction.
Contents of this article:
Here are some key points about iodine. More detail is in the main article.
- Iodine is an important nutrient for thyroid functioning.
- Too much iodine or too little iodine can lead to symptoms of hyperthyroidism or hypothyroidism.
- Good sources of iodine are seaweed and iodized salt.
- Iodine deficiency is rare in western countries, and additional iodine should only be taken with a doctor's supervision.
Recommended intake and sources
Iodine is beneficial for health, and a deficiency of iodine can create health problems.
The recommended intake of iodine from the age of 14 years is 150 micrograms (mcg) for both males and females. During pregnancy, it is 220 mcg, rising to 290 mcg while breastfeeding.
Food is the best source of iodine.
The amount of iodine in a food depends on how much iodine there is at the source of production.
The amount of iodine in the soil where crops are grown, or where an animal is raised for meat will affect the amount of iodine in the food. Produce from the sea is a good source of iodine.
Levels of iodine in food vary according to where it comes from. In fruit and vegetables, it can range from 10 mcg per kilogram of dry weight to 1 gram.
Because of this variation, the iodine content in foods is often approximate.
Here are some good sources of iodine:
- Seaweed: 1 gram (g) of whole or sheet seaweed contains from 16 to 2,984 mcg of iodine
- Iodized salt: A quarter teaspoon, or 1.5 g, contains 71 mcg, or 47 percent of daily value (DV)
- Baked cod: A 3-ounce piece contains 99 mcg, or 65 percent of DV
- Reduced-fat milk: 1 cup contains 56 mcg, or 37 percent of DV
- White, enriched bread: 2 slices contain 45 mcg, or 30 percent of DV
- Egg: one large egg contains 24 mcg, or 16 percent of DV
- Cheddar cheese: 1 ounce contains 12 mcg, or 8 percent of DV
Multivitamins often contain iodine in the form of potassium iodide or sodium iodide. Supplements containing kelp are a good source of iodine, but supplements should be taken with a doctor's permission.
Some foods contain "goitrogens." These compounds can block the thyroid from absorbing iodine. Examples are turnips, cassava, soy, broccoli, cabbage, and other cruciferous vegetables. Cruciferous vegetables are also good sources of iodine.
However, cooking can inactivate these compounds, and the foods that contain them are nutritious in many ways, so avoiding them is not advised in the United States (U.S.).
Seaweed is a packed natural source of iodine.
Iodine deficiency during pregnancy and early childhood can lead to developmental problems, but this is rare in the U.S.
A low intake of iodine can lead to an increased output of thyroid stimulating hormone (TSH).
If TSH increases, the thyroid takes in more iodine from the blood, to balance the need for iodine. This could happen if daily iodine levels fall below 100 mcg per day.
If iodine intake falls below 10 to 20 mcg per day, the thyroid cannot get enough iodine, and hypothyroidism can occur, with its hallmark sign of goiter, a visible swelling in the neck area.
If a woman has this condition while she is pregnant, it can have a severe impact on the mental, physical, and neurological development of the fetus.
In children, iodine deficiency can cause poor cognition. Left untreated, it can lead to intellectual disability.
Apart from goiter, hypothyroidism can have the following symptoms:
- fatigue, depression, and forgetfulness
- hair weakness and hair loss
- dry skin
- weight gain
- cold intolerance
A large goiter can lead to difficulties with swallowing or breathing.
Globally, iodine deficiency is said to be the most preventable cause of damage or developmental delay in the brain.
Potential health risks
An iodine deficiency can lead to thyroid problems.
The maximum recommended intake of iodine is 1,100 mcg per day for all adults, and 200 to 300 mcg per day for children up to the age of 8 years.
Iodine has a complex impact on the thyroid. Both too much and too little iodine can lead to goiter and other thyroid problems. For this reason, it is important to speak to a doctor before taking iodine supplements.
Excessive iodine consumption can lead to similar symptoms as iodine deficiency, including thyroid dysfunction and goiter.
Too much iodine may lead to thyroiditis and thyroid papillary cancer.
At very high levels, iodine poisoning can cause:
- burning of the mouth, throat, and stomach
- abdominal pain, nausea, vomiting, and diarrhea
- weak pulse
However, such cases are rare.
Iodine supplements can interact with some drugs.
- medications for hyperthyroidism, as additional iodine can lead to hypothyroidism
- angiotensin-converting enzyme (ACE) inhibitors, because supplemental potassium iodine can increase the risk of hyperkalemia, or high blood levels of potassium
- potassium-sparing diuretics, because hyperkalemia can result from the additional potassium iodide
- warfarin and other blood thinners, as iodine may reduce their effectiveness
- amiodarone, because taking iodine supplements with this drug can cause dangerously high levels of iodine in the body
Other uses of iodine
Other uses of iodine include the following:
- inflammation of the mouth due to chemotherapy or radiotherapy can benefit from an iodine mouthwash
- an iodine douche may help relieve symptoms of vaginitis
- following exposure to radiation, potassium iodine can reduce the amount of radioactive iodine that builds up in the thyroid
- sodium iodide, or tincture of iodine, can be used for cleaning wounds
Where possible, it is best to get iodine from the diet, unless supplements are prescribed by a doctor or healthcare professional.
Consuming a diet that contains a variety of nutrients is more healthful than concentrating on individual nutrients as the key to good health.