Iodine is a trace mineral required for human life. Humans require iodine for proper physical and mental development. It impacts cell respiration, metabolism of energy and nutrients, functioning of nerves and muscles, differentiation of the fetus, growth and repair of tissues, and the condition of skin, hair, teeth, and nails. Iodine is also needed for the production of thyroid hormones. The thyroid (a small gland in the front of the neck), which contains 80% of the body's iodine pool, converts iodine into the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones are released into the bloodstream, controlling the body's metabolism.
As established by the National Research Council's Food and Nutrition Board, the revised 1989 Recommended Dietary Allowance (RDA) for iodine is 40 mcg for infants, increasing to 150 mcg for adults and children age 11 and older. The RDA for pregnant and lactating women increases to 175 and 200 mcg respectively. Harrison's Principles of Internal Medicine reports that average U.S. iodine daily intake ranges from approximately 0.5–1.0 mg. According to the Merck Manual of Diagnosis and Therapy, less than 20 mcg per day of iodide results in iodine deficiency; iodide intake 20 times greater than the daily requirement (2 mg) results in chronic iodine toxicity.
Iodine is available from a variety of food sources, drugs, and most commercial vitamin preparations. Some seafood and sea vegetables provide good sources of dietary iodine, including: canned sardines, canned tuna, clams, cod, haddock, halibut, herring, lobster, oyster, perch, salmon, sea bass, and shrimp. Dulse, kelp , and seaweed are also sources of dietary iodine. If grown in iodine-rich soil, foods including asparagus, green peppers, lettuce, lima beans, mushrooms, pineapple, raisins, spinach, summer squash, Swiss chard, turnip greens, and whole wheat bread may provide good sources of dietary iodine. Animal products can also provide a source of iodine, especially if the animals are fed iodine-enriched foods or salt: beef, beef liver, butter, cheddar cheese, cottage cheese, cream, eggs, lamb, milk, and pork. Some foods such as breads may contain iodine additives.
Another source of dietary iodine is iodized salt. Iodized table salt was introduced in the United States in 1924 and significantly reduced the incidence of iodine deficiency. Providing iodized salt licks for livestock adds iodine to animal products. In some parts of the world, iodized oil supplements and water iodination provide other means of iodine supplementation. Many countries, however, still have insufficient iodine supplementation programs.
Iodine has several medical applications. Typically, in conjunction with drug therapy, iodine may be used to treat goiter (an enlargement of the thyroid gland), symptoms of hypothyroidism (diminished production of thyroid hormone), and hyperthyroidism (increased production of the thyroid gland). It may also be used as an expectorant in cough medications. Applications of iodine to conditions including arteriosclerosis, arthritis, and angina pectoris have also been noted. Iodine tinctures (dilute mixtures of alcohol and iodine) or Betadine are used as antiseptics to kill bacteria in skin cuts. Atomidine (a product containing iodine trichloride and other unlisted ingredients) is also sold as an antiseptic. Atomidine taken orally in minute cyclic doses is also recommended as a glandular stimulant and purifier.
Some research has shown that oral iodine supplements have antifibrotic and anti-inflammatory effects. Commonly reported studies have also suggested that iodine deficiency may be a factor in fibrocystic breast disease (FBD), a catch-all term that describes general, often normal, lumpiness of the breast. Clinical trials on women diagnosed with FBD found that, even in women showing normal thyroid function, thyroid hormone supplementation produced results including decreased breast pain and decreased breast nodules. Some early research also correlated higher incidence of breast, endometrial, and ovarian cancers with hypothyroidism and/or iodine deficiency. However, others have noted that low levels of selenium , which is more classically associated with cancer , were also present in the women in these studies.
Iodine is used in several compounds for a variety of medical testing. For example, it may be used in x-rays of the gallbladder or kidneys or in cardiac imaging. It is used as a diagnostic tool to examine the thyroid gland's output. A common test measures thyroid radioactive iodine uptake (RAIU). Trace amounts of radioactive iodine (I123 or i131) are used to test thyroid function. Together with blood tests, examining how much iodine is taken up by the thyroid gland helps physicians diagnose hypothyroid conditions (when the thyroid takes up too little iodine) and hyperthyroid conditions (when it takes up too much). Radioactive iodine therapy is also used for treating thyroid disease and cancer. Radioactive iodine can cross the placenta, causing severe dysfunction and damage to the fetus's thyroid gland. Current Medical Diagnosis and Treatment 2000 notes that nursing mothers should discontinue nursing for a period of time after receiving test or treatment doses of radioactive iodine. One study published in the Journal of the American Medical Association (JAMA) in May 2000 reported radiation exposure to family members of non-pregnant, non-nursing outpatients from I131 treatment to be well below limits mandated by U.S. Nuclear Regulatory Commission (NRC) guidelines. Medical professionals may also prescribe low iodine diets in combination with radioactive iodine tests or treatments.
Too much or too little iodine intake results in a wide spectrum of disorders that are addressed by adjusting iodine intake. Too much iodine can result in toxicity.
Iodine deficiency disorders (IDDs) are preventable, but not curable, by ensuring adequate iodine intake. Only a small amount of iodine is required over the human life span. The body, however, does not store iodine for long periods, so the intake must be regular. Too little iodine intake can result in cold feet, fatigue , insomnia, problems with skin, nails, and hair, and weight gain. Goiter can result from iodine deficiency. Certain substances called goitrogens can also induce goiter by interfering with thyroid functioning. Some foods have goitrogenic tendencies, as do certain drugs, for example, thiourea, sulfonamides, and antipyrine. As listed by Prescriptions for Nutritional Healing and other sources, foods containing substances that can prevent the utilization of iodine when eaten in large quantities include Brussels sprouts, cabbage, cauliflower, kale, millet, mustard, peaches, peanuts, pears, pine nuts, soybeans, and turnips. Limiting consumption of these foods may be recommended for persons with an underactive thyroid.
Iodine deficiency can also result in serious irreversible disorders and, as of May 2000, is considered a major global health problem by organizations such as the World Health Organization (WHO) and the United Nations Children's Fund (UNICEF). According to the International Council on Control of Iodine Deficiency Disorders (ICCIDD), IDDs are the most common cause of preventable brain damage and mental retardation worldwide. IDD results in cretinism (a form of stunted growth) and problems in movement, speech, and hearing. A pregnant woman with an iodine deficiency risks miscarriage, stillbirth, and mental retardation of her baby. As of 1999, the WHO called IDD a significant public health problem in 130 countries. The ICCIDD reported 1.6 billion people worldwide at risk for IDDs, and 50 million children suffering from some degree of IDD. Although not common, iodine deficiency is on the rise in the United States.
In 2002, the United Nation's Children's Fund announced a pledge to eliminate iodine deficiency in the world by 2005, citing the problem as a major cause of psychiatric and learning disabilities.
Excess iodine is typically excreted, and output can be measured in the urine. Regular excessive iodine intake is needed for toxicity. Excess iodine, when used as a supplement or in drug therapy, may reduce thyroid function. Although more commonly associated with iodine deficiency, goiter can also result from too much iodine due to thyroid hyperactivity. Additionally, high amounts of iodine from sources such as overuse of iodized salt, vitamins, cough medications, kelp tablets, or from medical testing can cause effects including rapid pulse, nervousness, headaches, fatigue, a brassy taste in the mouth, excessive salivation, gastric irritation, and hypothyroidism. Acne can appear or become worse. Some iodine-sensitive individuals may have an allergic reaction to iodine, often a skin rash. A physician may recommend that high iodine foods be removed from the diet of those who are iodine-sensitive. Similar side effects have also been observed in some women participating in studies on iodine and diagnosed FBD. Radioactive iodine has been implicated in producing thyroid dysfunction and thyroid cancer.
The Merck Manual of Diagnosis and Therapy. 17th ed. Edited by Mark H. Beers and Robert Berkow. Whitehouse Station, N.J.: Merck Research Laboratories, 1999.
National Research Council. Recommended Dietary Allowances. 10th ed. Washington, D.C.: National Academy Press, 1989.
"In Case you Haven't Heard." Mental Health Weekly (July 1, 2002): 8.
International Council for Control of Iodine deficiency Disorders (ICCIDD). Prof. Jack Ling. Director, ICEC. 1501 Canal Street, Suite 1304, New Orleans, LA 70112. (504)584–3542 Fax: (504)585–4090. [email protected] tulane.edu. <http://www.people.virginia.edu/~jtd/iccidd/>.
U.S. Fund for UNICEF. 333 East 38th Street NY, NY 10016. [email protected] <http://www.unicefusa.org/issues99/sep99/learn.html>.
HealthWorld Online. <http://www.healthy.net>.
Teresa G. Odle
Iodine (I) is a non-metallic element that the body needs in very small (trace) amounts in order to remain healthy. It can only be acquired through diet. Deficiencies of iodine are a serious health problem in some parts of the world.
Iodine is essential to the formation of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). Thyroid hormones regulate many basic metabolic processes. Solutions containing iodine can be used on the skin as a disinfectant because iodine kills bacteria. It can also be used to purify water contaminated with bacteria. In medical settings, iodine is used in diagnostic radioisotope scanning and it has other industrial uses.
The thyroid gland is a located in the front of the neck just below the Adam’s apple. It is part of a complex, tightly-controlled feedback cycle that regulates basic aspects of metabolism, such as how fast the body burns calories, growth rate, and body temperature.
Under stimulation by thyroid stimulating hormone (TSH) produced by the pituitary gland, the thyroid produces two hormones, triiodothyronine (T3) and thyroxine (T4). The formation of one
|Age||Recommended Dietary Allowance (mcg)||Tolerable upper intake level (mcg)|
|Children 0–6 mos||110||Not established|
|Children 7–12 mos||130||Not established|
|Children 1–3 yrs||90||200|
|Children 4–8 yrs||90||300|
|Children 9–13 yrs||120||600|
|Adolescents 14–18 yrs||150||900|
|Pregnant women 18<yrs||220||900|
|Pregnant women 19<yrs||220||1,100|
|Breastfeeding women 18<yrs||290||900|
|Breastfeeding women 19<yrs||290||1,100|
|Seaweed, dried, 1 oz||up to 18,000|
|Kelp, ¼cup wet (amount is highly variable)||415+|
|Salt, iodized, 1 tsp||400|
|Haddock, 3 oz||104-145|
|Cod, 3 oz||99|
|Salt, iodized, 1 g||77|
|Milk, 1 cup||55-60|
|Turkey breast, cooked, 3 oz||34|
|Cottage cheese, ½cup||25-75|
|Shrimp, 3 oz||21-37|
|Egg, 1 large||18-29|
|Processed fish sticks, 1 piece||17|
|Tuna, canned, 3 oz||17|
|Ground beef, cooked, 3 oz||8|
|mcg = microgram|
molecule of T3 requires three molecules of iodine, while formation of T4 requires four molecules of iodine. The body contains between 20 and 30 mg of iodine, 60% of which is stored in the thyroid. The remainder is found in the blood, muscles, and ovaries. Thyroid hormones are broken down in the liver and some of the iodine is recycled. The rest is lost to the body in urine.
Iodine is found in soil and in the ocean. The amount of iodine varies widely by location. In mountainous regions where heavy rain and snow cause erosion or in low-lying regions where regular flooding occurs, the soil is especially deficient in iodine. The mountains of the Himalayas, Andes, and Alps are all iodine-poor as is the Ganges river valley. The International Council for the Control of Iodine Deficiency Disorders (ICCIDD) estimates that 38% of the world’s population, or about 2.2 billion people, live in areas where they are unlikely to get enough iodine without supplementation.
Iodine deficiency disorders (IDDs) create serious health problem. In the early 1900s, iodine deficiency
Hormone —A chemical messenger that is produced by one type of cell and travels through the bloodstream to change the metabolism of a different type of cell.
Mineral —An inorganic substance found in the earth that is necessary in small quantities for the body to maintain a health. Examples: zinc, copper, iron.
Pituitary gland —A small gland at the base of the brain that produces many regulating hormones.
was common in interior regions of the United States and Canada, as well as many other non-coastal regions of the world. In the 1920s, the United States began a voluntary program of adding iodine (in the form of potassium iodide) to salt. Salt was chosen because all races, cultures, and economic classes use it, its consumption is not seasonal, and it is inexpensive. Adding 77 mcg of iodine per gram of salt costs about $0.04 per year per person in the United States. About 50% of table salt sold in the United States contains iodine. It is labeled ‘iodized salt.’ All table salt sold in Canada is iodized. In most other countries iodine is added at lower concentrations ranging from 10–40 mcg/gram.
Normal iodine requirements
The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for vitamins and minerals . The DRIs consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97–98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.
The IOM has not set UL levels for iodine in children under one year old because of incomplete scientific information. RDAs for iodine are measured in micrograms (mcg). The following are the daily RDAs and IAs for iodine for healthy individuals. They are the same as the recommendations made by the World Health Organization (WHO).
- children birth-6 months: RDA 110 mcg; UL not available
- children 7-12 months: RDA 130 mcg; UL not available
- children 1-3 years: RDA 90 mcg; UL 200 mcg
- children 4-8 years: RDA 90 mcg: UL 300 mcg
- children 9-13 years: RDA 120 mcg; UL 600 mcg
- adolescents 14-18 years: RDA 150 mcg; UL 900 mcg
- adults 19 years and older: RDA 150 mcg; UL 1,100 mcg
- pregnant women under age 19: RDA 220 mcg; UL 900 mcg
- pregnant women age 19 and older: RDA 220 mcg; UL 1,100 mcg
- breastfeeding women under age 19: RDA 290 mcg; UL 900 mcg
- breastfeeding women age 19 and older: RDA 290 mcg; UL 1,100 mcg
Sources of iodine
Iodine must be acquired from diet. Marine plants and animals, such as cod, haddock, and kelp (seaweed), are an especially good source of iodine because they are able to concentrate the iodine found in sea-water. Freshwater fish are a less good source. Plants contain varying amounts of iodine depending on the soil in which they are grown.
In industrialized countries, feed for cattle, chickens, and other domestic animals is often fortified with iodine. Some of this iodine finds its way into animal products that humans eat— milk, eggs, and meat. In developing countries where feed is not enriched or cattle are raised on grass, these animal products do serve as a source of iodine.
Commercially processed foods are often made with iodized salt. The iodine content of salt changes very little during processing. Sometimes an iodine-containing stabilizer is added to commercial bread dough. This increases the iodine content of bread. The stabilizer is used less often now than it was in the twentieth century. However, for many people, commercially processed foods are their main source of iodine. Iodine is also found in most multivitamin tablets.
Iodine can be absorbed through the skin from iodine-based disinfectant solutions. Automobile exhaust puts some iodine into the air, and this can be absorbed through the lungs. Neither of these provide significant amounts of iodine for most people.
The following list gives the approximate iodine content for some common foods:
- kelp, 1/4 cup wet: 415 mcg or more. Amount is highly variable
- salt, iodized, 1 g: 77 mcg; 1 teaspoon: 400 mcg
- haddock, 3 ounces: 104-145 mcg
- cod, 3 ounces: 99 mcg
- shrimp, 3 ounces: 21-37 mcg
- processed fish sticks: 17 mcg per piece
- tuna, canned, 3 ounces: 17 mcg
- milk, 1 cup: 55-60 mcg
- cottage cheese, 1/2 cup: 25-75 mcg
- egg, 1 large: 18-29 mcg
- turkey breast, cooked, 3 ounces: 34 mcg
- ground beef, cooked, 3 ounces: 8 mcg
- seaweed, dried, 1 ounce: up to 18,000 mcg
Because of iodine supplementation, iodine deficiency is not a serious health problem in most industrialized countries, but it is in many developing countries. Internationally, about 2.2 billion people are at risk for IDDs. Women who do not get enough iodine have higher rates of infertility, miscarriages, pregnancy complications, and low birth weight babies than women who have adequate iodine intakes. However, iodine deficiency has its most damaging effects on the developing fetus.
Iodine deficiency is the leading cause of preventable mental retardation worldwide. Children born to iodine-deficient mothers have a condition called cretinism. Cretinism involves severe and permanent brain damage. These children have mental retardation and developmental disorders such as deafness, mutism, and inability to control muscle movements. Iodine deficiency in newborns and infants also results in abnormal brain development and retardation.
The most visible sign of iodine deficiency in children, adolescents, and adults is the development of a goiter. A goiter is a lump near the throat that signals an enlarged thyroid. When not enough iodine is available, the thyroid grows larger in a futile attempt to make more thyroid hormone. In adults hard lumps may form inside the goiter. When iodine deficiency is pronounced enough for a goiter to develop, memory and language skills decline. In children IQ may be affected. Some of these effects can be reversed in children, but not adults, by increasing iodine intake. In adults with goiter, increasing iodine intake may send the thyroid into overdrive, causing it to produce too much thyroid hormone, a serious condition called hyperthyroidism. Other conditions can also cause the thyroid gland to produce too much or too little hormone. A urine test is used to determine if an individual is iodine deficient, and blood tests can check for other thyroid function problems.
Pregnant and breastfeeding women must be especially careful to get enough iodine, since iodine deficiency has its greatest effect on the fetus and newborn. Vegans, who do not eat animal products and depend on soy for much of their protein , are at higher risk of iodine deficiency than the general population.
Amiodarone (Cordarone) a drug used to prevent irregular heart rhythms, contains enough iodine that it may affect thyroid function.
Some foods contain substances called goitrogens that interfere with the body’s ability to absorb or use iodine. These include broccoli, cabbage, cauliflower, and brussel sprouts. Other foods that contain goitrogens are canola oil, soybeans, turnips, peanuts, and cassava. These foods should not cause iodine deficiency unless they are tine mainstay of a very limited diet.
Selenium deficiency amplifies the effects of iodine deficiency. Vitamin A deficiency may amplify iodine deficiency.
Complications of iodine deficiency are discussed above. Iodine excess rarely is caused by diet, although an excess of thyroid hormones may result from other causes.
In developed countries, parents should have few concerns about their healthy children getting enough iodine, so long as they use iodized table salt.
Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Cooper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academy Press, 2001, pp. 162-177. http://books.nap.edu/books/0309072794/html/.
Lieberman, Shari and Nancy Bruning. The Real Vitamin and Mineral Book: The Definitive Guide to Designing Your Personal Supplement Program, 4th ed. New York: Avery, 2007.
Pressman, Alan H. and Sheila Buff. The Complete Idiot’s Guide to Vitamins and Minerals, 3rd ed. Indianapolis, IN: Alpha Books, 2007.
International Council for the Control of Iodine Deficiency Disorders. < http://indorgs.virginia.edu/iccidd> .
Linus Pauling Institute. Oregon State University, 571 Weniger Hall, Corvallis, OR 97331-6512. Telephone: (541) 717-5075. Fax: (541) 737-5077. Website: <http://lpi.oregonstate.edu>.
Higdon, Jane. ‘Iodine.’ Linus Pauling Institute-Oregon State University, April 11, 2003. <http://lpi.oregonstate.edu/infocenter/minerals/iodine> .
International Council for the Control of Iodine Deficiency Disorders. ‘Iodine Deficiency Disorder (IDD).’ August 18, 2005. < http://indorgs.virginia.edu/iccidd/aboutidd.htm>.
Lee, Stephanie L. ‘Iodine Deficiency.’ eMedicine.com, July 27, 2006. <http://www.emedicine.com/med/ topic.1187htm> .
Medline Plus. ‘Iodine.’ U. S. National Library of Medicine, November 1, 2006. <http://www.nlm.nih/gov/medlineplus/druginfo/natural/patient-iodine.html> .
Mercer, L. Preston. ‘Iodine.’ American Society for Nutrition, 2006. <http://jn.nutrition.org/nutinfo> .
Northwesternutrition ‘Nutrition Fact Sheet: Iodine.’ Northwestern University, September 21, 2006. <http://www.feinberg.northwestern.edu/nutrition/factsheets/iodine.html>.
Irish diet seeNorthern European diet .
IODINE. Iodine is an essential dietary element necessary for normal development and function of all vertebrates. Its sole physiological function is as a constituent of the thyroid hormones, thyroxine and triiodothyronine. It is removed from the blood by the thyroid gland for storage in organic form where it is found as iodinated amino acids in peptide linkage in thyroglobulin, a highmolecular weight protein.
Iodine is widely but usually sparsely distributed in nature, so that in vast areas of the world the supply in customary diets is marginal or insufficient. It has been estimated that over two billion persons are at risk of disorders attributable to iodine deficiency. Among these disorders are goiter, impaired intellectual function, growth retardation, reduced fecundity, lowered work capacity, increased rates of fetal loss and infant mortality, deafness, and in extreme instances a well-defined but somewhat varied constellation of physical findings collectively known as cretinism. Cretins are recognized by severe mental deficiency, disturbances in gait, impaired or absent hearing, and other neurological defects, but the signs and symptoms in these individuals may be subtle. These features merge with those of the less impaired members of the same community or nearby countryside where they may appear in lesser severity.
The iodine content of edible plants is largely dependent on the iodine content of the soil on which they are grown. The iodine content of foods of animal origin depends on the iodine in their food. Iodine is concentrated in milk, and is found in relatively high concentration in sea fish, who are at the upper levels of the food chain that contains algae. Some sea fish concentrate iodine from sea water. The only structure among the vertebrates that contains a significant amount of iodine is the thyroid gland.
Role of Iodine in Disease
For centuries the disorders arising from iodine deficiency have been recognized in well-defined regions. These have been called "goiter belts." Switzerland was included in the goiter belt until the iodine deficiency in that country was corrected in the first half of the twentieth century. Until recent years iodine deficiency was a recognized disorder in the United States, especially the Midwest and West, where goiter was commonplace. Iodine deficiency has been a major public health problem in the Andean region and eastward, in large areas of central and north Africa, in the Middle Eastern countries, in India, and in eastern and central Europe, and even today in localized regions of western Europe. Fortunately, remarkable headway has been made in elimination of iodine deficiency through various methods of supplementing diets.
Goiter is only one of the many consequences of iodine deficiency, and is relatively trivial when compared with the damaging effects of iodine deficiency on the nervous system. From the human point of view, it is more correct to speak of "endemic mental deficiency" than "endemic goiter."
Endemic thyroid disease has traditionally been considered a feature of iodine deficiency in the mountainous regions of the world. Endemic thyroid disease is found in regions of high elevation, but has also been common where glacial run-offs occur and in floodplains where there has been chronic leaching of the soil. Such geographic regions include the Gangetic plain and much of India and southeastern Asia, the Himalayan region, and central Africa, where the iodine deficiency disorders are frequent and severe; the coastal regions of western Europe are marginally iodine deficient. Endemic iodine deficiency can be detected almost anywhere with currently available sensitive techniques. In the United States until recently the mean intake of iodine was excessive, but recently has been rapidly falling into a normal range. The recent precipitous fall in iodine consumption in the United States has led to concern that iodine deficiency may again become a problem if the present rate of decline continues. The need for monitoring iodine intake is apparent. This is customarily done by measuring the iodine content of urine from a fair sample of the population under observation.
The optimal daily adult iodine intake is about 150 μ g/day, about half that for children and infants. This figure rises to about 200 μ g during pregnancy, but under normal circumstances there is wide latitude in intake because of the ability of the normal thyroid system to compensate for varying levels of supply. The thyroid and pituitary through a feedback relationship provide a highly efficient regulatory system. If iodine intake falls below about 50 μ g/day the pituitary gland becomes stimulated to increase its iodine uptake and hormone production, and, if the iodine supply exceeds needs, the pituitary shuts down appropriately.
Iodine is readily absorbed by the stomach and upper gastrointestinal tract. Iodine in chemical combination is released in the gut and absorbed; it may be rapidly taken up by the thyroid gland or excreted in the urine. Only a small fraction appears in the stool. Exceptions occur when iodine is in chemical combination with such drugs used as radio-contrast agents and amiodarone, the widely used cardiac medication.
Iodine Deficiency and Disease
Certain chemical agents found in some foods interfere with the uptake or utilization of iodine by the thyroid. Among these are the cyanoglycosides found in cassava (manioc), a component of millet, and a variety of chemical agents and some unidentified substances found in the effluent water from rock formations and in factory discharges. It must be stressed that the inhibitory effect of these substances may be bypassed if there is an ample supply of iodine in the diet, but their effect may be critical if the iodine intake is marginal or lower.
When marginal or low iodine intake is identified in a geographic regions such as a district or country, an effort should be made to correct the deficiency. A variety of techniques have been employed. These include distribution of iodine solution to school attendees, candies containing potassium iodide, addition of iodine to drinking water, and the use of canisters containing iodine that is slowly released into sources of drinking water. None of these methods has proved to be widely accepted. In addition, it should be stressed that the primary target for the prevention of neurological damage due to iodine deficiency is the pregnant and nursing mother.
Prevention of Iodine Deficiency
The most effective and widely employed method for correcting iodine deficiency is salt iodization. The technique is simple, inexpensive, and effective. Potassium iodate rather than iodide salts is used because it is more stable when mixed with salt. Nevertheless certain problems must be corrected. Unscrupulous traders may sharply increase the cost of iodized salt to the consumer. If improperly stored the iodine may sublime and be lost from the salt. If addition of iodine by the manufacturer is not done carefully the salt may be overiodinated. In certain cases, especially those in which people have nodular goiters resulting from prolonged iodine deficiency, thyrotoxicosis may result, which may be subtle in onset and chronic, with unwanted or disastrous results. Careful and continued monitoring of dietary supplementation by iodized salt must be done, as with all food additives.
Promotion of salt iodization, especially in areas of particular need in the developing world, has been a health priority of many public and private agencies, including the World Health Organization, UNICEF, the International Council for Prevention of the Iodine Deficiency Disorders, and others. One of the principal problems with programs of salt iodization is that governments tend to lose interest, and the programs lapse, leading to recurrence of the iodine deficiency disorders. Again, constant monitoring is the key to continued success.
Injections of heavily iodinated poppyseed and other oils have been tried in mass campaigns, first in New Guinea; these methods have since been widely employed elsewhere. These are the same oils that have been widely used as radio-contrast agents. The results have been impressive. The iodine is slowly released from the oil and may be effective for two or more years. The oral route has also been used to administer the oils, but effectiveness is less prolonged. The disadvantages of programs using iodinated oil are principally cost and the requirement for sterile needles and trained personnel, which may be difficult to obtain in remote regions. Iodine-induced thyrotoxicosis may occur after administration of iodinated oil.
A unique and successful method of iodine distribution has recently been introduced. This method can be used in regions where iodine can be drip-added to irrigation water. It has been used in the desert regions of western China with salutary human benefit, and with a highly satisfactory effect on livestock production. The problems with this method are the need for skilled personnel to add the iodine to the irrigation system at the right time and rate, and the fact that it is only feasible when it is possible to add iodine to irrigation water. A somewhat similar technique that has proved beneficial is adding iodine to a municipal water supply. As with other methods of iodine supplementation, skilled maintenance of the program is essential, and the subsequent appearance of thyrotoxicosis is unknown.
Iodine is thinly distributed in the earth's crust, and much of the human population lives in regions that have marginal or insufficient iodine. Mountainous regions, flood-plains, and regions where there has been extensive leaching of iodine from the soil may not provide sufficient iodine for human needs. The result is the appearance of iodine deficiency disorders, which include neurological damage, goiter, increased fetal and infant mortality, deafness, and diminished human energy and resulting economic underproductivity. Iodine deficiency is a major public health problem for a large fraction of the world's population.
Wherever marginal or insufficient iodine exists, implementation of iodine supplementation is required. This may be done by supplementing table salt with iodine, administration of iodinated oil by injection or orally, or addition of iodine to the drinking water. It is essential that a monitoring system be in place to ensure that the population is receiving an adequate iodine intake. Care must be exercised to avoid an excess of iodine, which might induce thyrotoxicosis.
See also Body Composition; Fluoride; International Agencies; Malnutrition; Nutrition; Nutrition Transition: Worldwide Diet Change; Salt; Sodium; Trace Elements.
Braverman, L. E., and R. D. Utiger, eds. Thyroid: A Fundamental and Clinical Text. 7th ed. Philadelphia: Lippincott, Williams, & Wilkins, 2000.
De Long, G. R., J. Robbins, and P. G. Condliffe, eds. Iodine and the Brain. New York: Plenum. 1989
De Long, et al. "Effect on infant mortality of iodination of irrigation water in a severely iodine-deficient area of China." Lancet 360 (1997).
Fernandez, R. L. A Simple Matter of Salt. Berkeley: University of California Press, 1990.
Gaitan, F., ed. Environmental Goitrogenesis. Boca Raton, Fla.:CRC Press, 1989.
Stanbury, John B., and John T. Dunn. "Iodine and the Iodine Deficiency Disorders." In Present Knowledge in Nutrition, 8th ed., edited by B. A. Bowman and R. M. Russell, p. 344. Washington, D.C.: ILSI Press, 2000.
Stanbury, J. B., et al. "Iodine-Induced Hyperthyroidism: Occurrence and Epidemiology." Thyroid 8 (1998).
World Health Organization. Assessment of Iodine Deficiency Disorders and Monitoring their Elimination. 2nd ed. World Health Organization, 2001.
John Stanbury John T. Dunn
Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Iodine is the heaviest of the commonly occurring halogens. The halogens are in Group 17 (VIIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Iodine's chemical properties are similar to the lighter halogens above it, fluorine, chlorine, and bromine. But its physical appearance is very different. It is a steel-gray solid that changes into beautiful purple vapors when heated.
Iodine was discovered in 1811 by French chemist Bernard Courtois (1777-1838). The element occurs primarily in seawater and in solids formed when seawater evaporates. Its single most important property may be the ability to kill germs. It is used in antiseptics, germicides (products that kill germs), and other medical applications. However, it has a great many other less common, but important, commercial applications.
Group 17 (VIIA)
Discovery and naming
One of Courtois' first jobs was to assist his father in making compounds of sodium and potassium from seaweed. Seaweed plants take sodium and potassium compounds out of seawater. The compounds become part of the growing seaweed.
Courtois and his father collected seaweed on the coasts of Normandy and Brittany in France. Then they burned it. Next, they soaked the seaweed ashes in water to dissolve the sodium and potassium compounds. Sulfuric acid was added to react with the unwanted seaweed chemicals. Finally, they allowed the water to evaporate, leaving the compounds behind. These compounds are white crystals, much like ordinary table salt. The compounds were sold to large industrial businesses for use in such products as table salt and baking soda.
One day in 1811, Courtois made a mistake. He added too much sulfuric acid to the mixture. He was amazed to see clouds of beautiful violet vapor rising from the mixture. He decided to study the new material. Eventually, he proved it was a new element. He named the element after its color. In Greek, the word iodes means "violet."
Iodine is one of the most striking and beautiful of all elements. As a solid, it is a heavy, grayish-black, metallic-looking material. When heated, it does not melt. Instead, it sublimes. Sublimation is the process by which a solid turns directly to a gas without first melting. The resulting iodine vapor has a violet color and a harsh odor. If a cold object, such as an iron bar, is placed in these vapors, iodine changes back to a solid. It forms attractive, delicate, metallic crystals.
Iodine dissolves only slightly in water. But it dissolves in many other liquids to give distinctive purple solutions. If heated under the proper conditions, it can be made to melt at 113.5°C (236.3°F) and to boil at 184°C (363°F). The density of the element is 4.98 grams per cubic centimeter.
Like the other halogens, iodine is an active element. However, it is less active than the three halogens above it in the periodic table. Its most common compounds are those of the alkali metals, sodium, and potassium. But it also forms compounds with other elements. It even forms compounds with the other halogens. Some examples are iodine monobromide (IBr), iodine monochloride (ICl), and iodine pentafluoride (IF5).
Occurrence in nature
Iodine is not very abundant in the Earth's crust. Its abundance is estimated to be about 0.3 to 0.5 parts per million. It ranks in the bottom third of the elements in terms of abundance. It is still more common than cadmium, silver, mercury, and gold. Its abundance in seawater is estimated to be even less, about 0.0003 parts per million.
Iodine tends to be concentrated in the Earth's crust in only a few places. These places were once covered by oceans. Over millions of years, the oceans evaporated. They left behind the chemical compounds that had been dissolved in them. The dry chemicals left behind were later buried by earth movements. Today, they exist underground as salt mines.
A mistake by Bernard Courtois led to clouds of beautiful violet vapor rising from a mixture on which he was working. It was iodine.
Iodine can also be collected from seawater, brackish water, brine, or sea kelp. Seawater is given different names depending on the amount of solids dissolved in it. Brackish water, for example, has a relatively low percentage of solids dissolved in water. The range that is sometimes given is 0.05 to 3 percent solids in the water. Brine has a higher percentage of dissolved solids. It may contain anywhere from 3 to 20 percent of solids dissolved in water.
Finally, sea kelp is a form of seaweed. As it grows, it takes iodine out of seawater. Over time, sea kelp has a much higher concentration of iodine than seawater. Sea kelp is harvested, dried, and burned to collect iodine. The process is not much different from the one used by Courtois in 1811.
Only one naturally occurring isotope of iodine is known, iodine-127. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
Approximately 30 radioactive isotopes of iodine have been made artificially. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
A number of iodine isotopes are used commercially. In medical applications, these isotopes are injected into the body or given to the patient through the mouth. The isotopes then travel through the body in the bloodstream. As they travel, they give off radiation. That radiation can be detected by using X-ray film. A medical specialist can tell how well the body is functioning by observing the pattern of radiation.
Iodine isotopes are used in many ways. Iodine-123 is used in studies of the brain, kidneys, and thyroid. Iodine-125 is used in studies of the pancreas, blood flow, thyroid, liver, take-up of minerals in bones, and loss of proteins in the body. And iodine-131 is used in studies of the liver, kidneys, blood flow, lungs, brain, pancreas, and thyroid.
Iodine and human health
T he amount of iodine in the human body is very small. To find out how much is in one's body, one's body weight is divided by 2,500,000. That number is the weight of iodine in the body. For normal people, the amount is about equal to the size of the head of a pin.
That tiny dot of iodine can mean the difference between good and bad health. People who do not have enough can develop serious health problems. At one time, the most common of those problems was a disease known as goiter. Goiter causes a large lump in the neck as the thyroid grows out of control. (It can grow as large as a grape-fruit.) A goiter tries to make thyroid hormones, but it does not receive enough iodine from the person's diet. So it keeps expanding, trying to do its job.
A lack of iodine can cause other problems too. For example, thyroid hormones are needed for normal brain development in an unborn child. They are also needed to continue that development after birth. People who do not include enough iodine in their diet do not develop normally. Today, experts say that low levels of iodine are the leading cause of mental retardation, deafness, mutism (the inability to speak), and paralysis. They also say less serious problems can be blamed on low iodine levels. These include lethargy drowsiness, clumsiness, and learning disabilities.
Low iodine levels can be easily corrected. In most developed countries today, companies that make table salt add a small amount of potassium iodide (KI) to their salt. The salt is labeled "iodized salt." People who use it get all the iodine they need for normal thyroid function.
But people who live in developing countries may not be able to get iodized salt. The World Health Organization (WHO) is trying to find ways of providing iodine to these people. The WHO estimates that 1.5 billion people live in areas where levels of iodine are low. Up to 20 million of these people may have mental disabilities because of a lack of iodine. The WHO has started a program to ensure that future generations in these regions get the iodine needed to develop and function normally.
The most common iodine isotope used is iodine-131. When iodine (of any kind) enters the body, it tends to go directly to the thyroid. The iodine is then used to make thyroid hormones. If radioactive iodine is used, a doctor can tell how well the thyroid gland is working. If a high level of radiation is given off, the gland may be overactive. If a low level of radiation is given off, the gland may be underactive. In either case, the person may need some treatment to help the thyroid gland work more normally.
When a mixture of substances containing iodine is heated, the iodine sublimes. It can then be collected and purified.
Uses and compounds
About two-thirds of all iodine and its compounds are used in sanitation systems or in making various antiseptics and drugs. Iodine is also used to make dyes, photographic film, and specialized soaps. It is used in some industries as a catalyst. A catalyst is a substance used to speed up or slow down a chemical reaction. The catalyst does not undergo any change itself during the reaction.
Iodine can have both favorable and unfavorable effects on living organisms. It tends to kill bacteria and other disease-causing organisms. In fact, this property leads to its use in sanitation systems and as an antiseptic. An antiseptic is a chemical that stops the growth of germs. Not so long ago, tincture of iodine was one of the most popular antiseptics. It was applied to cuts and wounds to prevent infection. Tincture is a solution made by dissolving some substance (such as iodine) in alcohol rather than in water. Today, tincture of iodine has been replaced by other antiseptics.
One reason that tincture of iodine is used less today is that it can also cause problems. In higher doses, iodine can irritate or burn the skin. It can also be quite poisonous if taken internally.
Iodine kills bacteria and other disease-causing organisms.
Iodine plays an important role in the health of plants and animals. It is needed to maintain good health and normal growth. In humans, iodine is used to make a group of important compounds known as thyroid hormones. These chemicals are produced in the thyroid gland at the base of the neck. These chemicals control many important bodily functions. A lack of thyroid hormones can result in the disorder known as goiter. Goiter causes a large lump in the neck as the thyroid grows out of control. Iodine is added to table salt today, so goiter is rarely seen in the United States.
Iodine is the heaviest member in a family of chemical elements called halogens. The halogen group includes fluorine, chlorine, bromine, astatine, and iodine. Halogens readily combine with other elements to form salts. At room temperature, iodine is a shiny, dark black, nonmetallic, crystalline solid. Good food sources of iodine are fish and shellfish from the sea, as well as other seaweeds. Milk and eggs, vegetables and fruit contain small amounts of iodine.
Pure iodine is never found in nature—it is always combined with other elements. When iodine is heated it sublimes (passes directly from a solid to a vapor, skipping the liquid state). The iodine vapor is violet colored and has an irritating odor like that of chlorine. The vapor rapidly condenses on a cold surface. Pure iodine is toxic (poisonous).
Iodine is only slightly soluble (capable of being dissolved) in water, but it dissolves easily in a potassium iodide solution. It is also soluble in alcohol, chloroform, and other organic substances. Tincture of iodine (iodine dissolved in alcohol) is commonly used to kill germs on cuts and scrapes.
Iodine compounds are found in seawater, soil, and rocks. Iodine is obtained as sodium iodate, an impurity in the sodium nitrate beds in the South American country of Chile (the world's largest source of commercial iodine). Other important sources of iodine are underground pockets of brine (saltwater) found in Michigan, California, and Louisiana.
Most plants and animals require small amounts of iodine for normal growth. In man and other mammals, iodine is concentrated in the thyroid gland (in the neck), where it helps the body make thyroxine and other bio-chemicals that are important in metabolism (the process by which cells provide energy for bodily functions). Without enough iodine, a person's growth may be stunted (halted or slowed), and he may develop a condition called goiter (in which the thyroid gland swells into a large lump).
Today these conditions have been mostly wiped out in the industrialized world by the introduction of table salt containing potassium iodide or sodium iodide.
Courtois Discovers Iodine
Iodine was discovered by French chemist Bernard Courtois (1777-1838) in 1811. Courtois was barely making a living in his family's business of manufacturing saltpeter, which was used to make gunpowder. Saltpeter was made from the ashes of seaweed, which were treated with acid to remove sulfur compounds. One day, Courtois accidentally added too much acid, producing clouds of vapor having an attractive violet color. When the vapor condensed on cold objects, it formed dark, shiny crystals.
Although Courtois investigated the properties of the new substance by combining it with several other elements, he did not have enough time or money to follow through on his discovery. He asked for help from two French chemist friends, Charles Bernard Desormes and Nicholas Clement, who completed the work and made the research public in 1813.
Later that year, Humphry Davy and Joseph Gay-Lussac, working independently, showed that iodine was a new element. This research was made public in 1813. Although Davy's and Gay-Lussac's research over-lapped, it was Gay-Lussac who gave the new element its name after the Greek word "iodes," meaning "purple." Gay-Lussac went on to study the substance and its compounds, such as hydrogen iodide, in great detail.
In 1831 Courtois received a prize from a French scientific institute for his work. Despite this small measure of fame, Courtois's saltpeter business declined. With the end of the Napoleonic Wars (1804-1815), the demand for gunpowder dropped, and Courtois's factory failed. Although he continued to produce and sell iodine, Courtois had little success, and he died in poverty.
In 1814 J. J. Colin made an important discovery in microanalytical chemistry when he showed that iodine reacted with starch to produce a blue color so bright that iodine could be detected in amounts as low as one part in 400,000. In 1819 Jean-Baptiste Dumas proved that sponges used for many years to treat goiter contained iodine. By 1820 iodine had been discovered in a seaweed called kelp, which had also been used as a goiter treatment. Kelp was one of mankind's earliest sources of iodine. In Japan, kelp is still harvested from the sea and dried under the sun to provide a raw material for iodine production. It is also considered a delicious food by the Japanese, so their diets do not need to be supplemented with iodized salt.
In the mid-1800s, French agricultural chemist Jean Boussingault first suggested that iodine compounds might be able to cure goiter. A young doctor had asked Boussingault to analyze samples of certain salts used by South American Indians to treat goiter. Boussingault found iodine in the salts and suggested the cure, but it was not until 1896 that this treatment was confirmed.
German chemist Eugen Baumann (1846-1896) discovered that the thyroid gland was rich in iodine, which had never been found before in animal tissue. Baumann also determined that the thyroid was the only tissue containing iodine. Just two years later, an Austrian psychiatrist named Julius Wagner von Jaurreg (1857-1940) established that goiter could be prevented by taking iodine tablets regularly. He also proposed that iodized salt be sold in areas where goiter was widespread; Austria and Switzerland later adopted this idea.
The modern use of iodine in the prevention of goiter was a result of studies by D. Marine in the United States. Marine used iodine to prevent goiter in schoolchildren in Akron, Ohio, where the disorder was common. The success of his experiments led to the adoption of this use of iodine in many regions of the world where goiter was a health problem.
Other Iodine Uses
Iodine's most important use is in the health sciences. One of iodine's radioactive isotopes, 1-131, is widely used in medical diagnosis as a radioactive tracer. It can also be used to treat thyroid cancer.
In addition to its use as a goiter treatment, iodine serves as an important antiseptic thanks to its germ-killing properties. Tincture of iodine was frequently used to disinfect open wounds. Because of the tincture's irritating sting, however, more complex iodine compounds have been developed for first-aid purposes. Iodine combined with cleaning agents is used in common sanitizers. Iodine is also used to sterilize drinking water.
Iodine has several major industrial uses. Silver iodide is the main light-sensitive substance in photographic film emulsions and photographic papers. Silver iodide is also used by weather scientists for "cloud seeding" in rain-making experiments. Other iodine-containing compounds are used as dyes, in engraving, as an indicator in analytic chemistry, and in special soaps and lubricants. Commercial bakeries add sodium iodate to certain kinds of flour to improve the quality of the bread. Some inorganic iodides are used in producing high-purity titanium and silicon metals.
iodine (ī´ədīn, –dĬn) [Gr.,=violet], nonmetallic chemical element; symbol I; at. no. 53; at. wt. 126.90447; m.p. 113.5°C; b.p. 184.35°C; sp. gr. 4.93 at 20°C; valence -1, +1, +3, +5, or +7. Iodine is a dark-gray to purple-black, lustrous, solid element with a rhombic crystalline structure. It is the least active of the halogens, which are found in Group 17 of the periodic table. It is normally diatomic, i.e., it has 2 iodine atoms in each molecule, in the solid, liquid, and vapor states. When heated it passes directly from the solid to the vapor state (sublimation), the vapor having an intense violet color and a characteristic irritating odor. Iodine is only slightly soluble in water but dissolves readily in a solution of sodium or potassium iodide. Tincture of iodine is a solution of iodine and potassium iodide in alcohol. Iodine also dissolves in carbon disulfide, carbon tetrachloride, and chloroform, giving a deep violet solution. Iodine forms many compounds. With hydrogen it forms hydrogen iodide, which in water solution becomes hydriodic acid. It forms compounds with certain nonmetals (e.g., carbon, nitrogen, phosphorus, and oxygen) and with most metals. Iodine is displaced from its compounds by the other halogens. The element is obtained from salt deposits, as from the saltpeter beds in Chile, where it occurs in small quantities as an iodate, and from the salt brines associated with some oil wells in California and Louisiana. It is also found as an iodide in the ash of certain seaweeds. Iodine may be prepared by displacement from its compounds with chlorine. Treating an iodide with manganese dioxide and sulfuric acid sublimes the iodine. Iodine is important in medical treatment; tincture of iodine and iodoform are widely used. Iodine is employed in the preparation of certain drugs and in the manufacture of some dyes. Silver iodide, a yellow salt, is used in photography; it is water insoluble and turns black when exposed to light. Starch turns deep blue (almost black) in the presence of a small amount of iodine; this reaction serves as a test for either starch or iodine. Iodine in small amounts is essential to human nutrition; in the thyroid gland it becomes a part of the iodine-containing hormones. Goiter, a swelling of the thyroid, is often a symptom of inadequate iodine in the diet. Iodine has only one stable isotope, iodine–127; it is the only isotope of iodine occurring in nature, although 24 iodine isotopes are known. Iodine–131 is a radioactive isotope with a half-life of 8 days. It is used medically to diagnose abnormalities of the thyroid gland. It is also a component of fallout produced by nuclear explosions. Iodine was discovered in 1811 by Bernard Courtois.
Iodine is the heaviest of the halogen family of elements, excluding the radioactive element astatine. It was discovered in 1811 by French chemist Bernard Courtois, who isolated the element from seaweed. The element is named for its color in the gas phase (the Greek word iodes means "violet").
At normal temperatures and pressures, iodine is a shiny, purplish-black or gray solid. Near room temperature, iodine sublimes (i.e., it does not melt to form a liquid but goes directly from the solid to the gas phase). It is found at a level of about 60 parts per billion (ppb) by weight in seawater but its concentration is enhanced in marine organisms.
As with the other halogens, iodine is a diatomic molecule. It is always found in nature in a combined state, often as iodide salts where it has a −1 oxidation number. Compounds in which iodine is found to have oxidation numbers of 7, 3, 5, and 1 are also well known. Iodine is prepared commercially by treatment of natural salt solutions (seawater or brines) with chlorine (a more reactive halogen), according to the reaction:
2 I− + Cl2 → I2 2Cl−
Iodine is necessary for the proper function of the thyroid gland in humans. Dietary deficiencies can be avoided by the occasional consumption of seafood or by using iodized salt, which combines common table salt (NaCl) with potassium iodide (KI). Iodine is a useful antiseptic, either as tincture of iodine (an alcohol solution of I2), or as an aqueous solution of providone iodine (Betadine).
see also Halogens; Inorganic Chemistry.
John Michael Nicovich
Lide, David R., ed. (2003). The CRC Handbook of Chemistry and Physics, 84th edition. Boca Raton, FL: CRC Press.
Winter, Mark. "Iodine." The University of Sheffield and WebElements Ltd., U.K. Available from <http://www.webelements.com>.
Iodine is a critically important component of thyroid hormones. There are four iodine atoms per molecule of l-thyroxine, and three per molecule of l-triiodothyronine. The highest content of iodine in food is found in fish, with lesser amounts occurring in eggs, milk, and meat. Fruits and vegetables contain little iodine. Without iodine supplementation, people in most inland areas of the world, particularly mountainous regions, have iodine deficiency. This was the case in the United States before iodinization of salt. When iodine deficiency prevails, goiter and hypothyroidism commonly occur, along with congenital cretinism—all preventable diseases. Iodine excess, usually a result of diagnostic medical procedures or medications, can produce either hypothyroidism or hyperthyroidism in patients with different types of underlying thyroid disease.
Martin I. Surks
(see also: Goiter; Hyperthyroidism; Hypothyroidism; Thyroid Disorders )
Medeiros-Netos, G. (1999). "Congenital and Iodine-Deficiency Goiters." In Atlas of Clinical Endocrinology, Vol. 1: Thyroid Diseases, ed. M. I. Surks. Philadelphia, PA: Current Medicine.
Iodine is plentifully supplied by sea foods and by vegetables grown in soil containing iodide. In areas where the soil is deficient in iodide, locally grown vegetables are also deficient, and hence goitre occurs in defined geographical regions, especially inland upland areas over limestone soil. Where deficiency is a problem, salt may be iodized to increase iodide intake.