iodine

1. (Symbol I)² A lustrous, grayish-black, corrosive, poisonous halogen element having radioactive isotopes, especially I 131, used as a medical tracer and in thyroid disease diagnosis and therapy. Iodine compounds are used as germicides, antiseptics, and dyes. Atomic number 53; atomic weight 126.9045; melting point 113.5°C; boiling point 184.35°C; specific gravity (solid, at 20°C) 4.93; valence 1, 3, 5, 7.
2. An antiseptic preparation containing iodine in solution, used to treat wounds.

[French iode, iodine (from Greek ioeidēs, violet-colored : ion, violet + -oeidēs, -oid) + –INE².]

A nonmetallic element, symbol I, atomic number 53, relative atomic mass 126.9045, the heaviest of the naturally occurring halogens. Under normal conditions iodine is a black, lustrous, volatile solid; it is named after its violet vapor. See also Halogen elements; Periodic table.

The chemistry of iodine, like that of the other halogens, is dominated by the facility with which the atom acquires an electron to form either the iodide ion I⁻ or a single covalent bond I, and by the formation, with more electronegative elements, of compounds in which the formal oxidation state of iodine is +1, +3, +5, or +7. Iodine is more electropositive than the other halogens, and its properties are modulated by: the relative weakness of covalent bonds between iodine and more electropositive elements; the large sizes of the iodine atom and iodide ion, which reduce lattice and solvation enthalpies for iodides while increasing the importance of van der Waals forces in iodine compounds; and the relative ease with which iodine is oxidized, Some properties of iodine are listed in the table. See also Astatine; Bromine; Chemical bonding; Chlorine.

Iodine occurs widely, although rarely in high concentration and never in elemental form. Despite the low concentration of iodine in sea water, certain species of seaweed can extract and accumulate the element. In the form of calcium iodate, iodine is found in the caliche beds in Chile. Iodine also occurs as iodide ion in some oil well brines in California, Michigan, and Japan.

The sole stable isotope of iodine is ¹²⁷I (53 protons, 74 neutrons). Of the 22 artificial isotopes (masses between 117 and 139), the most important is ¹³¹I, with a half-life of 8 days. It is widely used in radioactive tracer work and certain radiotherapy procedures.

Iodine exists as diatomic I₂ molecules in solid, liquid, and vapor phases, although at elevated temperatures (>200°C or 390°F) dissociation into atoms is appreciable. Short intermolecular I…I distances in the crystalline solid indicate strong inter-molecular van der Waals forces. Iodine is moderately soluble in nonpolar liquids, and the violet color of the solutions suggests that I₂ molecules are present, as in iodine vapor.

Although it is usually less vigorous in its reactions than the other halogens, iodine combines directly with most elements. Important exceptions are the noble gases, carbon, nitrogen, and some noble metals. The inorganic derivatives of iodine may be grouped into three classes of compounds: those with more electropositive elements, that is, iodides; those with other halogens; and those with oxygen. Organoiodine compounds fall into two categories: the iodides; and the derivatives in which iodine is in a formal positive oxidation state by virtue of bonding to another, more electronegative element. See also Grignard reaction; Halogenated hydrocarbon; Halogenation.

Iodine appears to be a trace element essential to animal and vegetable life. Iodide and iodate in sea water enter into the metabolic cycle of most marine flora and fauna, while in the higher mammals iodine is concentrated in the thyroid gland, being converted there to iodinated amino acids (chiefly thyroxine and iodotyrosines). They are stored in the thyroid as thyroglobulin, and thyroxine is apparently secreted by the gland. Iodine deficiency in mammals leads to goiter, a condition in which the thyroid gland becomes enlarged.

The bactericidal properties of iodine and its compounds bolster their major uses, whether for treatment of wounds or sterilization of drinking water. Also, iodine compounds are used to treat certain thyroid and heart conditions, as a dietary supplement (in the form of iodized salt), and for x-ray contrast media.

Major industrial uses are in photography, where silver iodide is a constituent of fast photographic film emulsions, and in the dye industry, where iodine-containing dyes are produced for food processing and for color photography. See also Dye.

An essential mineral, a trace element; the reference intake is about 140 μg per day. Iodine is required for synthesis of the thyroid hormones, which are iodo-tyrosine derivatives. A prolonged deficiency of iodine leads to goitre.

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.

A trace element present in fish, iodized salt, and vegetables grown in iodine-rich soils. Iodine is an essential ingredient of thyroid hormone, which helps to regulate growth, development, and metabolic rate. The Reference Nutrient Intake for adults is 140 micrograms each day. An excess of iodine can be poisonous; a deficit leads to an underactive thyroid gland.

A halogen element that is nonmetallic in nature; atomic weight is 126.91. As a nutritional element, iodine is vital to the production of thyroxin by the thyroid gland. In radioactive form, iodine is used as a diagnostic substance to determine the ability of the thyroid gland to take up iodine. In tincture form, iodine is used as a locally applied antiseptic, germicide, and disclosing solution.

Description

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.

General Use

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.

Precautions

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.

Side Effects

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.

Resources

Books

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.

Periodicals

"In Case you Haven't Heard." Mental Health Weekly (July 1, 2002): 8.

Organizations

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. ICEC@mailhost.tcs. tulane.edu. .

U.S. Fund for UNICEF. 333 East 38th Street NY, NY 10016. webmaster@unicefusa.org. .

World Health Organization (WHO). Avenue Appia 20 1211 Geneva 27, Switzerland. (+00–41–22)791–21–11. Fax: (+00–41–22)791–311. info@who.int. .

Other

HealthWorld Online. .

[Article by: Kathy Stolley; Teresa G. Odle]

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.

(SEE ALSO: Goiter; Hyperthyroidism; Hypothyroidism; Thyroid Disorders)

Bibliography

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.

— MARTIN I. SURKS

For more information on iodine, visit Britannica.com.

A trace element required for normal growth and development. It is a component of thyroid hormones that help regulate growth. development, and metabolic rate. Deficiency results in an underactive thyroid gland and a lower metabolic rate; excess may suppress the secretion of thyroid hormones. In the UK, the Reference Nutrient Intake for adults is 140 μg day⁻¹; in the USA, the Recommended Dietary Allowance is 150 μg.

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.

Summary

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.

Bibliography

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.

Hetzel, B. S. The Story of Iodine Deficiency. New York: Oxford University Press, 1989.

Hetzel, B. S., and C. S. Pandav. S.O.S. for a Billion. Bombay: Oxford University Press, 1996.

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

A chemical element, atomic number 53, atomic weight 126.904, symbol I. Iodine is essential in nutrition, being especially prevalent in the colloid of the thyroid gland. It is used in the treatment of hypothyroidism and as a topical antiseptic. Iodine is a frequent cause of poisoning. See also iodism.

• i.-125 — a radioisotope of iodine having a half-life of 60 days and a principal gamma-ray photon energy of 28 keV; used as a label in radioimmunoassays and other in vitro tests, and also for thyroid imaging. Symbol ¹²⁵I.
• ¹²³i.-metaiodobenzylguanidine — a radioisotope which concentrates in chromaffin cells; used in diagnostic scintigraphy, e.g. in cases of pheochromocytoma.
• i.-131 — a radioisotope of iodine having a half-life of 8.1 days and a principal gamma-ray photon energy of 364 keV; used in treatment of hyperthyroidism and carcinoma of the thyroid, in thyroid function testing, and in imaging of the thyroid gland and other organs. Symbol ¹³¹I.
• i. deficiency — may occur in all species under certain conditions; in dogs and cats, a factor in all-meat diets. See also goiter.
• i. contrast agents — iodine salts are opaque to x-rays; therefore they can be combined with other compounds and used as contrast media in diagnostic x-ray examinations.
• i. nutritional deficiency — is characterized by goiter, neonatal mortality and alopecia.
• i. poisoning — occurs usually due to accidental overdosing. It causes lacrimation, anorexia, coughing due to bronchopneumonia, and a heavy dandruff. Paradoxically, iodine excess may result in thyroid hyperplasia and goiter, especially in the young.
• protein-bound i. — a test of thyroid function. See also protein-bound iodine (PBI) test.
• radioactive i. — see iodine-125, iodine-131 (above).
• i. residues in milk — careless use of iodine-based teat dips results in unacceptable residues of iodine in milk.
• i. solution — contains 2% free iodine and 2.4% sodium iodide in an aqueous solution.
• i. solution (strong) — contains 5% free iodine and 10% potassium iodide in an aqueous solution.
• tamed i. — see iodophor.
• i. trapping — the selective absorption of iodine from the circulation by the thyroid gland.

For other uses, see Iodine (disambiguation).

53 tellurium ← iodine → xenon Br ↑ I ↓ At Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	iodine, I, 53
Element category	halogens
Group, Period, Block	17, 5, p
Appearance	violet-dark gray, lustrous
Standard atomic weight	126.90447(3)  g·mol−1
Electron configuration	[Kr] 4d10 5s2 5p5
Electrons per shell	2, 8, 18, 18, 7
Physical properties
Phase	solid
Density (near r.t.)	4.933  g·cm−3
Melting point	386.85 K (113.7 °C, 236.66 °F)
Boiling point	457.4 K (184.3 °C, 363.7 °F)
Triple point	386.65 K, 12.1×103 Pa
Critical point	819 K, 11.7 MPa
Heat of fusion	(I2) 15.52  kJ·mol−1
Heat of vaporization	(I2) 41.57  kJ·mol−1
Specific heat capacity	(25 °C) (I2) 54.44  J·mol−1·K−1
Vapor pressure (rhombic) P(Pa) 1 10 100 1 k 10 k 100 k at T(K) 260 282 309 342 381 457
Atomic properties
Crystal structure	orthorhombic
Oxidation states	7, 5, 3, 1, -1 (strongly acidic oxide)
Electronegativity	2.66 (Pauling scale)
Ionization energies	1st: 1008.4 kJ/mol
	2nd: 1845.9 kJ/mol
	3rd: 3180 kJ/mol
Atomic radius	140  pm
Covalent radius	139±3  pm
Van der Waals radius	198 pm
Miscellaneous
Magnetic ordering	diamagnetic[1]
Electrical resistivity	(0 °C) 1.3×107 Ω·m
Thermal conductivity	(300 K) 0.449  W·m−1·K−1
Bulk modulus	7.7  GPa
CAS registry number	7553-56-2
Most-stable isotopes
Main article: Isotopes of iodine iso NA half-life DM DE (MeV) DP 123I syn 13 h ε, γ 0.16 123Te 127I 100% 127I is stable with 74 neutrons 129I syn 15.7×106 y β− 0.194 129Xe 131I syn 8.02070 d β−, γ 0.971 131Xe
References

Iodine (pronounced /ˈaɪ.ədaɪn/, /ˈaɪ.ədɨn/, or in chemistry /ˈaɪ.ədiːn/; from Greek: ιώδης iodes "violet"), is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons.

Chemically, iodine is the second least reactive of the halogens, and the second most electropositive halogen; trailing behind astatine in both of these categories. However, the element does not occur in the free state in nature. As with all other halogens (members of Group XVII in the periodic table), when freed from its compounds iodine forms diatomic molecules (I₂).

Iodine and its compounds are primarily used in medicine, photography, and dyes. Although it is rare in the solar system and Earth's crust, the iodides are very soluble in water, and the element is concentrated in seawater. This mechanism helps to explain how the element came to be required in trace amounts by all animals and some plants, being the heaviest element commonly used by living organisms (only tungsten, used in enzymes by a few bacteria, is heavier).

Contents 1 Characteristics 2 Occurrence 3 Structure 4 Production 5 Isotopes 6 History 7 Applications 7.1 Disinfectant 7.2 Staining 7.3 Radiocontrast agent 7.4 Radioiodine 8 Iodine compounds 8.1 Organic compounds 9 Chemistry 9.1 Organic synthesis 9.2 Clandestine synthetic chemical use 10 Biological role 10.1 Iodine and breast cancer 10.2 Iodine and immunity 10.3 Human dietary intake 10.4 Deficiency 11 Radioiodine in biology 11.1 Radioiodine and the thyroid 11.2 Radioiodine and the kidney 12 Precautions and toxicity of elemental iodine 12.1 Toxicity of iodide ion 13 See also 14 References 15 External links

Characteristics

Iodine under standard conditions is a shiny grey solid. It can be seen apparently sublimating at standard temperatures into a violet-pink gas that has an irritating odor. This halogen forms compounds with many elements, but is less reactive than the other members of its Group VII (halogens) and has some metallic light reflectance.

Elemental iodine dissolves easily in chloroform and carbon tetrachloride. The solubility of elemental iodine in water can be vastly increased by the addition of potassium iodide. The molecular iodine reacts reversibly with the negative ion, creating the triiodide anion, I₃⁻, which dissolves well in water. This is also the formulation of some types of medicinal (antiseptic) iodine, although tincture of iodine classically dissolves the element in alcohol. The deep blue color of starch-iodine complexes is produced only by the free element.

Students who have seen the classroom demonstration in which iodine crystals are gently heated in a test tube to violet vapor may gain the impression that liquid iodine does not exist at atmospheric pressure. This misconception arises because the vapor produced has such a deep colour that the liquid appears not to form. In fact, if iodine crystals are heated carefully to just above their melting point of 113.7 °C, the crystals melt into a liquid which is present under a dense blanket of the vapor.

When iodine is encapsulated into carbon nanotubes it forms atomic chains, whose structure depends on the nanotube diameter.^[2]

Occurrence

Iodomethane

Iodine naturally occurs in the environment chiefly as a dissolved iodide in seawater, although it is also found in some minerals and soils.^[3] This element also exists in small amounts in the mineral caliche, found in Chile, between the Andes and the sea. A type of seaweed, kelp, tends to be high in iodine as well.

Organoiodine compounds are produced by marine life forms, the most notable being iodomethane (commonly called methyl iodide). The total iodomethane that is produced by the marine environment, by microbial activitiy in rice paddies and by the burning of biological material is estimated to be 214 kilotonnes.^[4] The volatile iodomethane is broken up by oxidation reactions in the atmosphere and a global iodine cycle is established.^[3][4] Although the element is actually quite rare, kelp and certain plants and other algae have some ability to concentrate iodine, which helps introduce the element into the food chain.

Structure

Structure of solid iodine

Iodine crystallizes in the orthorombic space group Cmca No 64, Pearson symbol oS8, the same as black phosphorus. In the solid state, I₂ molecules are still represented by a short I-I bond of 270pm.

Production

From the several places in which iodine occures in nature only two are used as source for iodine: the caliche, found in Chile and the iodine containing brines of gas and oil fields, especially in Japan and the United States.

The caliche, found in Chile contains sodium nitrate, which is the main product of the mining activities and small amounts of sodium iodate and sodium iodide. During leaching and production of pure sodium nitrate the sodium iodate and iodide is extracted.^[5] The high concentration of iodine in the caliche and the extensive mining made Chile the largest producer of iodine in 2007.

Iodine output in 2005

Most other producers use natural occurring brine for the production of iodine. The Japanese Minami Kanto gas field east of Tokyo and the American Anadarko Basin gas field in northwest Oklahoma are the two largest sources for iodine from brine. The brine has a temperature of over 60°C due to the depth of the source. The brine is first purified and acidified using sulfuric acid, then the iodide present is oxidized to iodine with chlorine. An iodine solution is produced, but is dilute and must be concentrated. Air is blown into the solution, causing the iodine to evaporate, then it is passed into an absorbing tower containing acid where sulfur dioxide is added to reduce the iodine. The hydrogen iodide (HI) is reacted with chlorine to precipitate the iodine. After filtering and purification the iodine is packed.^[5][6]

2 HI + Cl₂ → I₂↑ + 2 HCl

I₂ + 2 H₂O + SO₂ → 2 HI + H₂SO₄

2 HI + Cl₂ → I₂↓ + 2 HCl

The production of iodine from seawater via electrolysis is not used due to the sufficient abundance of iodine-rich brine. Another source of iodine was kelp, used in the 18th and 19th centuries, but no longer economically viable.

Commercial samples often contain a large amount of impurities; they may be removed by sublimation. The element may also be prepared in an ultrapure form through the reaction of potassium iodide with copper(II) sulfate, which gives copper(II) iodide initially. That decomposes spontaneously to copper(I) iodide and iodine:

Cu²⁺ + 2 I⁻ → CuI₂

2 CuI₂ → 2 CuI + I₂

There are also other methods of isolating this element in the laboratory, for example the method used to isolate other halogens: oxidation of the iodide in hydroiodic acid (often made in situ with an iodide and sulfuric acid) by manganese dioxide (see below in Descriptive chemistry).

Isotopes

Main article: isotopes of iodine

There are 37 isotopes of iodine, but only one, ¹²⁷I, is stable.

In many ways, ¹²⁹I is similar to ³⁶Cl. It is a soluble halogen, fairly non-reactive, exists mainly as a non-sorbing anion, and is produced by cosmogenic, thermonuclear, and in-situ reactions. In hydrologic studies, ¹²⁹I concentrations are usually reported as the ratio of ¹²⁹I to total I (which is virtually all ¹²⁷I). As is the case with ³⁶Cl/Cl, ¹²⁹I/I ratios in nature are quite small, 10⁻¹⁴ to 10⁻¹⁰ (peak thermonuclear ¹²⁹I/I during the 1960s and 1970s reached about 10⁻⁷). ¹²⁹I differs from ³⁶Cl in that its halflife is longer (15.7 vs. 0.301 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I⁻ and IO₃⁻) which have different chemical behaviors. This makes it fairly easy for ¹²⁹I to enter the biosphere as it becomes incorporated into vegetation, soil, milk, animal tissue, etc.

Excesses of stable ¹²⁹Xe in meteorites have been shown to result from decay of "primordial" iodine-129 produced newly by the supernovas which created the dust and gas from which the solar system formed. ¹²⁹I was the first extinct radionuclide to be identified as present in the early solar system. Its decay is the basis of the I-Xe Iodine-xenon radiometric dating scheme, which covers the first 85 million years of solar system evolution.

Effects of various radioiodine isotopes in biology are discussed below.

History

Iodine was discovered by Bernard Courtois in 1811.^[7][8] He was born to a manufacturer of saltpeter (a vital part of gunpowder). At the time of the Napoleonic Wars, France was at war and saltpeter was in great demand. Saltpeter produced from French niter beds required sodium carbonate, which could be isolated from seaweed washed up on the coasts of Normandy and Brittany. To isolate the sodium carbonate, seaweed was burned and the ash then washed with water. The remaining waste was destroyed by adding sulfuric acid. One day Courtois added too much sulfuric acid and a cloud of purple vapor rose. Courtois noted that the vapor crystallized on cold surfaces making dark crystals. Courtois suspected that this was a new element but lacked the money to pursue his observations.

However he gave samples to his friends, Charles Bernard Desormes (1777–1862) and Nicolas Clément (1779–1841), to continue research. He also gave some of the substance to Joseph Louis Gay-Lussac (1778–1850), a well-known chemist at that time, and to physicist André-Marie Ampère (1775–1836). On 29 November 1813, Dersormes and Clément made public Courtois’s discovery. They described the substance to a meeting of the Imperial Institute of France. On December 6, Gay-Lussac announced that the new substance was either an element or a compound of oxygen.^[9][10][11] Ampère had given some of his sample to Humphry Davy (1778–1829). Davy did some experiments on the substance and noted its similarity to chlorine.^[12] Davy sent a letter dated December 10 to the Royal Society of London stating that he had identified a new element.^[13] A large argument erupted between Davy and Gay-Lussac over who identified iodine first but both scientists acknowledged Courtois as the first to isolate the chemical element.

Applications

Disinfectant

Elemental iodine is used as a disinfectant in various forms. The iodine exists as the element, or as the water soluble triiodide anion generated in situ by adding iodide to poorly-soluble iodine. Alternatively, iodine may come from iodophors, which contain iodine complexed with a solubilizing agent. Examples of such preparations include:^[14]

• Tincture of iodine (iodine in ethanol, or iodine and sodium iodide in a mixture of ethanol and water)
• Lugol's iodine (iodine and iodide in water)
• Povidone iodine (an iodophor)

Staining

See also: staining

Testing a seed for starch with a solution of iodine

Iodine is a common general stain used in thin-layer chromatography. It is also used in the Gram stain as a mordant, after the sample is treated with crystal violet.

In particular, iodine forms an intense blue complex with starch. Several applications rely on this property:

• Iodometry. The concentration of an oxidant can be determined by adding it to an excess of iodide, to give elemental iodine/triiodide. Starch is used as an indicator. close to the end-point to increase the visual contrast (dark blue/colorless instead of yellow of dilute triiodide/colorless).
• Iodine clock reaction is an extension of the techniques in iodometry.
• Iodine may be used to test a sample substance for the presence of starch.
• Iodine solutions are used in counterfeit banknote detection pens; the premise being that counterfeit banknotes made using commercially available paper contain starch.
• Starch-iodide paper are used to test for the presence of oxidants such as peroxides. The oxidants convert iodide to iodine, which shows up as blue. A solution of starch and iodide can perform the same function.^[15]

Radiocontrast agent

Diatrizoic acid, a radiocontrast

Iodine, as a heavy element, is quite radio-opaque. Organic compounds of a certain type (typically iodine-substituted benzene derivatives) are thus used in medicine as X-ray radiocontrast agents for intravenous injection. This is often in conjunction with advanced X-ray techniques such as angiography and CT scanning

Radioiodine

Some radioactive iodine isotopes can be used to treat thyroid cancer. The body accumulates iodine in the thyroid, thus radioactive iodine can selectively damage growing thyroid cancer cells while the radioactive dose to the rest of the body remains small.

Iodine compounds

See also: Category:Iodine compounds

Iodine forms many compounds. Potassium iodide is the most commercially significant iodine compound. It is a convenient source of the iodide anion; it is easier to handle than sodium iodide because it is not hygroscopic. Sodium iodide is especially useful in the Finkelstein reaction, because it is soluble in acetone, while potassium iodide is poorly so. In this reaction, an alkyl chloride is converted to an alkyl iodide. This relies on the insolubility of sodium chloride in acetone to drive the reaction:

R-Cl (acetone) + NaI (acetone) → R-I (acetone) + NaCl (s)

Iodic acid (HIO₃) and its salts are strong oxidizers. Periodic acid (HIO₄) cleaves vicinal diols along the C-C bond to give aldehyde fragments. 2-Iodoxybenzoic acid and Dess-Martin periodinane are hypervalent iodine oxidants used to specifically oxidize alcohols to ketones or aldehydes. Iodine pentoxide is a strong oxidant as well.

Interhalogen compounds are well known; examples include iodine monochloride and trichloride; iodine pentafluoride and heptafluoride.

HI

He

LiI

BeI2

BI3

CI4

NI3

I2O4, I2O5, I4O9

IF, IF3, IF5, IF7

Ne

NaI

MgI2

AlI3

SiI4

PI3, P2I4

S

ICl, ICl3

Ar

KI

CaI2

Sc

TiI4

VI3

Cr

MnI2

Fe

CoI2

NiI2

CuI

ZnI2

Ga2I6

GeI2, GeI4

AsI3

Se

IBr

Kr

RbI

SrI2

Y

ZrI4

Nb

Mo

Tc

Ru

Rh

Pd

AgI

CdI2

InI3

SnI4, SnI2

SbI3

TeI4

I

Xe

CsI

BaI2

Hf

Ta

W

Re

Os

Ir

Pt

AuI

Hg2I2, HgI2

TlI

PbI2

Bi

Po

At

Rn

Fr

Ra

Rf

Db

Sg

Bh

Hs

Mt

Ds

Rg

Uub

Uut

Uuq

Uup

Uuh

Uus

Uuo

↓

La

Ce

Pr

Nd

Pm

SmI2

Eu

Gd

TbI3

Dy

Ho

Er

Tm

Yb

Lu

Ac

ThI4

Pa

U

Np

Pu

Am

Cm

Bk

Cf

Es

Fm

Md

No

Lr

Organic compounds

See also: Organoiodine compound

Many organoiodine compounds exist, the simplest is iodomethane, approved as a soil fumigant. Iodinated organics are used as synthetic reagents, and also radiocontrast agents.

Biologically active substances like the thyroid hormones are naturally occurring organoiodine compounds.^[16]

Chemistry

Elemental iodine is poorly soluble in water, with one gram dissolving in 3450 ml at 20 °C and 1280 ml at 50 °C. By contrast with chlorine, the formation of the hypohalite ion (IO^–) in neutral aqueous solutions of iodine is negligible.

I₂+ H₂O H⁺ + I⁻ + HIO   (K = 2.0×10⁻¹³)^[17]

Solubility in water is greatly improved if the solution contains dissolved iodides such as hydroiodic acid, potassium iodide, or sodium iodide; this extra solubility results from the high solubility of the I₃⁻ ion. Dissolved bromides also improve water solubility of iodine. Iodine is soluble in a number of organic solvents, including ethanol (20.5 g/100 ml at 15 °C, 21.43 g/100 ml at 25 °C), diethyl ether (20.6 g/100 ml at 17 °C, 25.20 g/100 ml at 25 °C), chloroform, acetic acid, glycerol, benzene (14.09 g/100 ml at 25 °C), carbon tetrachloride (2.603 g/100 ml at 35 °C), and carbon disulfide (16.47 g/100 ml at 25 °C).^[18] Aqueous and ethanol solutions are brown. Solutions in chloroform, carbon tetrachloride, and carbon disulfide are violet.

Elemental iodine can be prepared by oxidizing iodides with chlorine:

2I⁻ + Cl₂ → I₂ + 2Cl⁻

or with manganese dioxide in acid solution:^[17]

2I⁻ + 4H⁺ + MnO₂ → I₂ + 2H₂O + Mn²⁺

Iodine is reduced to hydroiodic acid by hydrogen sulfide:^[19]

I₂ + H₂S → 2HI + S↓

or by hydrazine:

2I₂ + N₂H₄ → 4HI + N₂

Iodine is oxidized to iodate by nitric acid:^[20]

I₂ + 10HNO₃ → 2HIO₃ + 10NO₂ + 4H₂O

or by chlorates:^[20]

I₂ + 2ClO₃⁻ → 2IO₃⁻ + Cl₂

Iodine is converted in a two stage reaction to iodide and iodate in solutions of alkali hydroxides (such as sodium hydroxide):^[17]

I2 + 2OH− → I− + IO− + H2O		(K = 30)
3IO− → 2I− + IO3−		(K = 1020)

Despite having the lowest electronegativity of the common halogens, iodine reacts violently with some metals, such as aluminum:

3I₂ + 2Al → 2AlI₃

This reaction produces 314kj per mole of aluminum, comparable to thermite's 425kj. Yet the reaction initiates spontaneously, and if unconfined, causes a cloud of gaseous iodine due to the high heat.

Organic synthesis

With phosphorus, iodine is able to replace hydroxyl groups on alcohols with iodide. For example, the synthesis of methyl iodide from methanol, red phosphorus, and iodine.^[21] The iodinating reagent is phosphorus triiodide that is formed in situ:

3 CH₃OH + PI₃ → 3 CH₃I + H₃PO₃

Phosphorous acid is formed as a side-product.

The iodoform test uses an alkaline solution of iodine to react with methyl ketones to give the labile triiodomethide leaving group, forming iodoform which precipitates.

Iodine is sometimes used to activate magnesium when preparing Grignard reagents; aryl and alkyl iodides both form Grignard reagents. Alkyl iodides such as iodomethane are good alkylating agents. Some drawbacks to use of iodo-organics in chemical synthesis are:

• iodine compounds tend to be more expensive than the corresponding bromides and chlorides, in that order
• iodides tend to be much stronger alkylating agents, and so are more toxic (e.g. methyl iodide is very toxic (T+)^[22]
• low molecular weight iodides tend to have a much higher equivalent weight, compared with other alkylating agents (e.g. methyl iodide versus dimethyl carbonate), due to the atomic mass of iodine.

Clandestine synthetic chemical use

In the United States, the Drug Enforcement Agency (DEA) regards iodine and compounds containing iodine (ionic iodides, iodoform, ethyl iodide, and so on) as reagents useful for the clandestine manufacture of methamphetamine. Persons who attempt to purchase significant quantities of such chemicals without establishing a legitimate use are likely to find themselves the target of a DEA investigation. Persons selling such compounds without doing due diligence to establish that the materials are not being diverted to clandestine use may be subject to stiff penalties, such as expensive fines or even imprisonment.^[23][24]

Biological role

Iodine is an essential trace element for life, the heaviest element commonly needed by living organisms, and the second-heaviest known to be used by any form of life (only tungsten, a component of a few bacterial enzymes, has a higher atomic number and atomic weight). Iodine's main role in animal biology is as constituents of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). These are made from addition condensation products of the amino acid tyrosine, and are stored prior to release in an iodine-containing protein called thyroglobulin. T4 and T3 contain four and three atoms of iodine per molecule, respectively. The thyroid gland actively absorbs iodide from the blood to make and release these hormones into the blood, actions which are regulated by a second hormone TSH from the pituitary. Thyroid hormones are phylogenetically very old molecules which are synthesized by most multicellular organisms, and which even have some effect on unicellular organisms.

Thyroid hormones play a basic role in biology, acting on gene transcription to regulate the basal metabolic rate. The total deficiency of thyroid hormones can reduce basal metabolic rate up to 50%, while in excessive production of thyroid hormones the basal metabolic rate can be increased by 100%. T4 acts largely as a precursor to T3, which is (with minor exceptions) the biologically active hormone.

Iodine accounts for 65% of the molecular weight of T4 and 59% of the T3. 15–20 mg of iodine is concentrated in thyroid tissue and hormones, but 70% of the body's iodine is distributed in other tissues, including mammary glands, eyes, gastric mucosa, the cervix, and salivary glands. In the cells of these tissues iodide enters direcly by sodium-iodide symporter (NIS). Its role in mammary tissue is related to fetal and neonatal development, but its role in the other tissues is unknown.^[25] It has been shown to act as an antioxidant in these tissues.^[25]

Iodine may have a relationship with selenium, and iodine supplementation in selenium-deficient populations may pose risks for thyroid function.^[25]

Iodine and breast cancer

It is known that a diet lacking in iodine is connected with adverse health effects collectively referred as iodine deficiency diseases or disorders. Studies also indicate that iodine deficiency, either dietary or pharmacologic, can lead to breast atypia and increased incidence of malignancy in animal models, while iodine treatment can reverse dysplasia.^[26][27][28] Laboratory evidences demonstrate that the effect of iodine on breast cancer is in part independent of thyroid function and that iodine inhibit cancer promotion through modulation of the estrogen pathway. Gene array profiling of estrogen responsive breast cancer cell line shows that the combination of iodine and iodide alters gene expression and inhibits the estrogen response through up-regulating proteins involved in estrogen metabolism. This suggests that iodine/iodide may be useful as an important adjuvant therapy in the pharmacologic manipulation of the estrogen pathway in women with breast cancer.^[26]

Iodine and immunity

Iodine has an important action on the immune system. The high iodide-concentration of thymus gives the anatomical rationale for this role of iodine in immune system.^{[29][30][31][32][33]}

Human dietary intake

The United States Recommended Daily Allowance (RDA) is 150 micrograms per day (μg/day) for both men and women, with a Tolerable Upper Intake Level (UL) for adults is 1,100 μg/day (1.1 mg/day).^[34] The tolerable upper limit was assessed by analyzing the effect of supplementation on thyroid-stimulating hormone.^[25]

Natural sources of iodine include sea life, such as kelp and certain seafood, as well as plants grown on iodine-rich soil.^[35][36] Iodized salt is fortified with iodine.^[36]

As of 2000, the median intake of iodine from food in the United States was 240 to 300 μg/day for men and 190 to 210 μg/day for women.^[34] In Japan, consumption is much higher due to the frequent consumption of seaweed or kombu kelp.^[25] Although some Chinese data associate excess iodine with autoimmune thyroiditis and hypothyroidism, these effects have not been observed in Japanese populations, and a protective effect on breast cancer has been hypothesized.^[25]

After iodine fortification programs (e.g. iodized salt) have been implemented, iodine-induced hyperthyroidism has been observed. The condition mainly seems to occur in people over forty, and the risk appears higher when iodine deficiency is severe and the initial rise in iodine intake is high.^[37]

Deficiency

Main article: Iodine deficiency

In areas where there is little iodine in the diet, typically remote inland areas and semi-arid equatorial climates where no marine foods are eaten, iodine deficiency gives rise to hypothyroidism, symptoms of which are extreme fatigue, goitre, mental slowing, depression, weight gain, and low basal body temperatures.^[38]

Iodine deficiency is the leading cause of preventable mental retardation, a result which occurs primarily when babies or small children are rendered hypothyroidic by a lack of the element. The addition of iodine to table salt has largely eliminated this problem in the wealthier nations, but as of March 2006, iodine deficiency remained a serious public health problem in the developing world.^[39] Iodine deficiency is also a problem in certain areas of Europe. In Germany it has been estimated to cause a billion dollars in healthcare costs per year.^[25]

Radioiodine in biology

Radioiodine and the thyroid

Human exposure to radioactive iodine will cause thyroid uptake, as with all iodine, leading to elevated chances of thyroid cancer. Isotopes with shorter half-lives such as I¹³¹ present a greater risk than those with longer half-lives since they generate more radiation per unit of time. Taking large amounts of regular iodine will saturate the thyroid and prevent uptake. Iodine pills are sometimes distributed to persons living close to nuclear establishments, for use in case of accidents that could lead to releases of radioactive iodine.

• Iodine-123 and iodine-125 are used in medicine as tracers for imaging and evaluating the function of the thyroid.
• Noncombined (elemental) iodine is mildly toxic to all living things.

The artificial radioisotope ¹³¹I (a beta emitter), has a half-life of 8.0207 days. Also known as radioiodine, ¹³¹I has been used in treating cancer and other pathologies of the thyroid glands. ¹²³I is the radioisotope most often used in nuclear imaging of the kidney and thyroid as well as thyroid uptake scans (used for the evaluation of Graves' Disease). The most common compounds of iodine are the iodides of sodium (NaI) and potassium (KI) and the iodates (KIO₃).

Potassium iodide (KI tablets, or "SSKI" = "Saturated Solution of KI" liquid drops) can be given to people in a nuclear disaster area when fission has taken place, to block the uptake of iodine-131 by the thyroid. The protective effect of KI lasts approximately 24 hours, so it should be dosed daily until a risk of significant exposure to radioiodines no longer exists.^[40][41] The exposure can be reduced by evacuation, sheltering, and by control of the food supply. Iodine-131 also decays rapidly, with a half-life of 8 days, so that 99.95% of the original radioiodine is gone after three months.

Iodine-129 (¹²⁹I; half-life 15.7 million years) is a product of cosmic ray spallation on various isotopes of xenon in the atmosphere, in cosmic ray muon interaction with tellurium-130, and also uranium and plutonium fission, both in subsurface rocks and nuclear reactors. Nuclear processes, in particular nuclear fuel reprocessing and atmospheric nuclear weapons tests have now swamped the natural signal for this isotope. ¹²⁹I was used in rainwater studies following the Chernobyl accident. It also has been used as a groundwater tracer and as an indicator of nuclear waste dispersion into the natural environment.

Radioiodine and the kidney

In the 1970s imaging techniques were developed in California to utilize radioiodine in diagnostics for renal hypertension.

Precautions and toxicity of elemental iodine

Elemental iodine is an oxidizing irritant and direct contact with skin can cause lesions, so iodine crystals should be handled with care. Solutions with high elemental iodine concentration such as tincture of iodine are capable of causing tissue damage if use for cleaning and antisepsis is prolonged.

Elemental iodine (I₂) is poisonous if taken orally in larger amounts; 2–3 grams of it are a lethal dose for an adult human.

Iodine vapor is very irritating to the eye, to mucous membranes, andin the respiratory tract. Concentration of iodine in the air should not exceed 1 mg/m³ (eight-hour time-weighted average).

When mixed with ammonia and water, elemental iodine forms nitrogen triiodide which is extremely shock sensitive and can explode unexpectedly.

Toxicity of iodide ion

Excess iodine has symptoms similar to those of iodine deficiency. Commonly encountered symptoms are abnormal growth of the thyroid gland and disorders in functioning and growth of the organism as a whole. Iodides are similar in toxicity to bromides.^{[citation needed]}

See also

• Iodide as an antioxidant
• Chemical Oxygen Iodine Laser
• Nutrition facts label
• Starch indicator

References

1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81th edition, CRC press.
2. ^ Guan, L; Suenaga, K; Shi, Z; Gu, Z; Iijima, S (Jun 2007). "Polymorphic structures of iodine and their phase transition in confined nanospace.". Nano letters 7 (6): 1532–5. doi:10.1021/nl070313t. ISSN 1530-6984. PMID 17477579. 
3. ^ ^{a b} Dissanayake, C. B.; Chandrajith, Rohana; Tobschall, H. J. (1999). "The iodine cycle in the tropical environment — implications on iodine deficiency disorders". International Journal of Environmental Studies 56 (3): 357–372. doi:10.1080/00207239908711210. 
4. ^ ^{a b} N. Bell, L. Hsu, D. J. Jacob, M. G. Schultz, D. R. Blake, J. H. Butler, D. B. King, J. M. Lobert, and E. Maier-Reimer (2002). "Methyl iodide: Atmospheric budget and use as a tracer of marine convection in global models". Journal of GeophysicalResearch 107 (D17): 4340. doi:10.1029/2001JD001151. 
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6. ^ Tatsuo Maekawa, Shun-Ichiro Igari and Nobuyuki Kaneko (2006). "Chemical and isotopic compositions of brines from dissolved-in-water type natural gas fields in Chiba, Japan". Geochemical Journal 40: 475–484. doi:10.2343/geochemj.40.475. 
7. ^ Bernard Courtois (1813). "Découverte d'une substance nouvelle dans le Vareck". Annales de chimie 88: 304–10.  In French, seaweed that had been washed onto the shore was called "varec", "varech", or "vareck", whence the English word "wrack". Later, "varec" also referred to the ashes of such seaweed: the ashes were used as a source of iodine and salts of sodium and potassium.
8. ^ Patricia A. Swain (2005). "Bernard Courtois (1777-1838) famed for discovering iodine (1811), and his life in Paris from 1798". Bulletin for the History of Chemistry 30 (2): 103–11. http://www.scs.uiuc.edu/~mainzv/HIST/awards/OPA%20Papers/2007-Swain.pdf. 
9. ^ J. Gay-Lussac (1813). "Sur un nouvel acide formé avec la substance décourverte par M. Courtois". Annales de chimie 88: 311–8. 
10. ^ J. Gay-Lussac (1813). "Sur la combination de l'iode avec d'oxigène". Annales de chimie 88: 319–21. 
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13. ^ Humphry Davy (January 1, 1814). "Some Experiments and Observations on a New Substance Which Becomes a Violet Coloured Gas by Heat". Phil. Trans. R. Soc. Lond. 104: 74–93. doi:10.1098/rstl.1814.0007. http://rstl.royalsocietypublishing.org/content/104/74.full.pdf+html. 
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16. ^ Gribble, G. W. (1996). "Naturally occurring organohalogen compounds - A comprehensive survey". Progress in the Chemistry of Organic Natural Products 68: 1–423. 
17. ^ ^{a b c} F. A. Cotton and G. Wilkinson (1988). Advanced Inorganic Chemistry, 5th ed.. John Wiley & Sons. ISBN 0471849979. 
18. ^ Martha Windholz, editor ; Susan Budavari, associate editor ; Lorraine Y. Stroumtsos, assistant editor ; Margaret Noether Fertig, assistant editor. (1976). Merck Index of Chemicals and Drugs, 9th ed. S.l.: J A Majors Company. ISBN 0911910263. 
19. ^ N.L. Glinka (1981). General Chemistry (volume 2). Mir Publishing. 
20. ^ ^{a b} Linus Pauling (1988). General Chemistry. Dover Publications. ISBN 0486656225. 
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23. ^ "21 USC Sec. 872 01/22/02". http://www.usdoj.gov/dea/pubs/csa/872.htm 21. 
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26. ^ ^{a b} Stoddard II, F. R.; Brooks, A. D.; Eskin, B. A.; Johannes, G. J. (2008). "Iodine Alters Gene Expression in the MCF7 Breast Cancer Cell Line: Evidence for an Anti-Estrogen Effect of Iodine". International Journal of Medical Science 5: 189–196. PMID 18645607. http://www.medsci.org/v05p0189.htm. 
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External links

Wikimedia Commons has media related to: Iodine

Look up iodine in Wiktionary, the free dictionary.

• "Micronutrient Research for Optimum Health", Linus Pauling Institute, OSU Oregon State University
• ATSDR - CSEM: Radiation Exposure from Iodine 131 U.S. Department of Health and Human Services (public domain)
• ChemicalElements.com - Iodine
• who.int, WHO Global Database on Iodine Deficiency
• Network for Sustained Elimination of Iodine Deficiency
• Oxidizing Agents > Iodine
• WebElements.com – Iodine

v • d • e Diatomic chemical elements Hydrogen H2 | Nitrogen N2 | Oxygen O2 | Fluorine F2 | Chlorine Cl2 | Bromine Br2 | Iodine I2 | Astatine At2 |

v • d • e Periodic table
H																															He
Li	Be																									B	C	N	O	F	Ne
Na	Mg																									Al	Si	P	S	Cl	Ar
K	Ca															Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Br	Kr
Rb	Sr															Y	Zr	Nb	Mo	Tc	Ru	Rh	Pd	Ag	Cd	In	Sn	Sb	Te	I	Xe
Cs	Ba	La	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Hf	Ta	W	Re	Os	Ir	Pt	Au	Hg	Tl	Pb	Bi	Po	At	Rn
Fr	Ra	Ac	Th	Pa	U	Np	Pu	Am	Cm	Bk	Cf	Es	Fm	Md	No	Lr	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Uub	Uut	Uuq	Uup	Uuh	Uus	Uuo
Alkali metals Alkaline earth metals Lanthanoids Actinoids Transition metals Other metals Metalloids Other nonmetals Halogens Noble gases

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - jod

Nederlands (Dutch)
jodium

Français (French)
n. - iode, teinture d'iode

Deutsch (German)
n. - Jod

Ελληνική (Greek)
n. - (χημ.) ιώδιο

Italiano (Italian)
iodio, tintura di iodio

Português (Portuguese)
n. - iodo (m) (Quím.)

Русский (Russian)
йод

Español (Spanish)
n. - yodo

Svenska (Swedish)
n. - jod

中文（简体）(Chinese (Simplified))
碘, 碘酒

中文（繁體）(Chinese (Traditional))
n. - 碘, 碘酒

한국어 (Korean)
n. - 요드

日本語 (Japanese)
n. - ヨー素, ヨード, ヨードチンキ

العربيه (Arabic)
‏(الاسم) اليود‏

עברית (Hebrew)
n. - ‮יוד (יסוד כימי)‬

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