Dictionary:

iodine

  (ī'ə-dīn', -dĭn, -dēn')

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.