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).
Thyroid
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Iodine's main role in vertebrate 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 iodine 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. T3 acts on small intestine cells and adipocytes to increase carbohydrate absorption and fatty acid release, respectively. A 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 has a nutritional relationship with selenium. A family of selenium-dependent enzymes called deiodinases converts T4 to T3 (the active hormone) by removing an iodine atom from the outer tyrosine ring. These enzymes also convert T4 to reverse T3 (rT3) by removing an inner ring iodine atom; and convert T3 to 3,3'-Diiodothyronine (T2) also by removing an inner ring atom. Both of the latter are inactivated hormones which are ready for disposal and have essentially no biological effects. A family of non-selenium dependent enzymes then further deiodinates the products of these reactions.
Selenium also plays a very important role in the production of Glutathione, the body's most powerful antioxidant. During the production of thyroid hormones, hydrogen peroxide is produced, high Iodine in the absence of selenium destroys the thyroid gland (often felt as a sore throat feeling), the peroxides are neutralized through the production of glutathione from selenium. In turn an excess of selenium increases demand for iodine, and deficiency will result when a diet is high in selenium and low in iodine.
Extrathyroidal iodine
Iodine accounts for 65% of the molecular weight of T4 and 59% of the T3. The human body contains about 15â€"20 mg of iodine, mostly concentrated in thyroid tissue (70-80%). Extra-thyroidal iodine exists in several other organs including the mammary glands, eyes, gastric mucosa, the cervix, ovary and salivary glands. In the cells of these tissues iodide enters directly by sodium-iodide symporter (NIS). Different tissue responses for iodine and iodide occur in the mammary glands and the thyroid gland of rats. The role of iodine in mammary tissue is related to fetal and neonatal development, but its role in the other tissues is unknown. It has been shown to act as an antioxidant and antiproliferant in various tissues that can uptake iodine. Molecular iodine (I2) has a suppressive effect on benign and cancerous neoplasias.
The US Food and Nutrition Board and Institute of Medicine recommended daily allowance of iodine ranges from 150 micrograms/day for adult humans to 290 micrograms/day for lactating mothers. However, the thyroid gland needs no more than 70 micrograms/day to synthesize the requisite daily amounts of T4 and T3. These higher recommended daily allowance levels of iodine seem necessary for optimal function of a number of body systems, including lactating breast, gastric mucosa, salivary glands, oral mucosa, arterial walls, thymus, epidermis, choroid plexus and cerebrospinal fluid, etc. In amphibian metamorphosis iodine and thyroid hormones also exert a well-studied experimental model of apoptosis on the cells of gills, tail, and fins of tadpoles. Moreover, iodine can add to double bonds of docosahexaenoic acid and arachidonic acid of cellular membranes, making them less reactive to free oxygen radicals.
Dietary recommendations
The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for iodine in 2000. For people age 14 and up, the iodine RDA is 150 μg/day. RDA for pregnancy is 220 μg/day. RDA for lactation is 290 μg/day. For children 1â€"8 years, 90 μg/day. For children 8â€"13 years 130 μg/day. As for safety, the IOM sets Tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. The UL for adults is 1,100 μg/day. This UL was assessed by analyzing the effect of supplementation on thyroid-stimulating hormone. Collectively the EARs, RDAs, AIs and ULs are referred to as Dietary Reference Intakes (DRIs).
The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL defined the same as in United States. For women and men ages 18 and older the PRI is set at 150 μg/day. PRI for pregnancy or lactation 200 μg/day. For children ages 1â€"17 years the PRIs increase with age from 90 to 130 μg/day. These PRIs are comparable to the U.S. RDAs with the exception of lactation. The European Food Safety Authority reviewed the same safety question and set its adult UL at 600 μg/day, which is a bit more than half the U.S. value. Japan reduced its adult iodine UL from 3,000 µg to 2,200 µg in 2010 but then increased it back to 3,000 µg in 2015.
For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For iodine labeling purposes 100% of the Daily Value was 150 μg, and remained at 150 μg in the May 27, 2016 revision. A table of the old and new adult Daily Values is provided at Reference Daily Intake. The original deadline to be in compliance was July 28, 2018, but on September 29, 2017 the FDA released a proposed rule that extended the deadline to January 1, 2020 for large companies and January 1, 2021 for small companies.
Food sources
Natural sources of iodine include sea life, such as kelp and certain seafood, as well as plants grown on iodine-rich soil. Iodized salt is fortified with iodine. According to a Food Fortification Initiative 2016 report, 130 countries have mandatory iodine fortification of salt and an additional 10 have voluntary fortification.
Range of observed intakes
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. In Japan, consumption is much higher due to the frequent consumption of seaweed or kombu kelp, The average daily intake ranges from 1,000 to 3,000 μg/day. Previous estimates were of an average intake as high as 13,000 μg/day.
Deficiency
Worldwide, iodine deficiency affects two billion people and is the leading preventable cause of mental retardation. Mental disability is a result which occurs primarily when babies or small children are rendered hypothyroidic by a lack of the element (new hypothyroidism in adults may cause temporary mental slowing, but not permanent damage).
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 also gives rise to hypothyroidism, symptoms of which are extreme fatigue, epidemic goitre (swelling in the thyroid gland), mental slowing, depression, weight gain, and low basal body temperatures.
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. Iodine deficiency is also a problem in certain areas of Europe. In Germany it has been estimated to cause a billion dollars in health care costs per year.
Iodine and cancer risk
- Breast cancer. The mammary gland actively concentrates iodine into milk for the benefit of the developing infant, and may develop a goiter-like hyperplasia, sometimes manifesting as fibrocystic breast disease, when iodine level is low. Studies 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, with elemental iodine (I2) having been found to be more effective in reducing ductal hyperplasias and perilobular fibrosis in iodine-deficient rats than iodide (Iâˆ'). On the observation that Japanese women who consume iodine-rich seaweed have a relatively low rate of breast cancer, iodine is suggested as a protection against breast cancer. Iodine is known to induce apoptosis in breast cancer cells. Laboratory evidence has demonstrated an effect of iodine on breast cancer that is in part independent of thyroid function, with iodine inhibiting cancer through modulation of the estrogen pathway. Gene array profiling of the 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. Whether iodine/iodide will be useful as an adjuvant therapy in the pharmacologic manipulation of the estrogen pathway in women with breast cancer has not been determined clinically.
- Iodine and stomach cancer. Some researchers have found an epidemiologic correlation between iodine deficiency, iodine-deficient goitre, and gastric cancer; a decrease in the death incidence from stomach cancer after iodine-prophylaxis. In the proposed mechanism, the iodide ion functions in gastric mucosa as an antioxidant reducing species that detoxifies poisonous reactive oxygen species, such as hydrogen peroxide.
Iodine, Thyroxine and Apoptosis
Iodine and thyroxine also stimulate the spectacular apoptosis of the cells of the larval gills, tail and fins in amphibians metamorphosis, and stimulate the evolution of their nervous system transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog. In fact, amphibian frog Xenopus laevis serves as an ideal model system for the study of the mechanisms of apoptosis.
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. Although elemental iodine is used in the formulation of Lugols Solution, it becomes tri-iodide upon reacting with potassium iodide used in the solution and is non-toxic. Only a small amount of elemental iodine will dissolve in water, adding potassium iodide allows a much higher amount of elemental iodine to dissolve through the reaction of I2-I3. This allows Lugols to come in strengths varying from 2%-15% iodine.
Elemental iodine (I2) is poisonous if taken orally in larger amounts; 2â€"3 grams of it is a lethal dose for an adult human. Potassium Iodide on the other hand has an LD50 that is high in several other animals; in rabbits it is 10 g/kg, in rats it is 14 g/kg, and in mouse it is 22 g/kg. The tolerable upper intake level for iodine, established by the Food and Nutrition Board, is 1,100 µg/day for adults. The safe upper limit of consumption set by the Ministry of Health, Labor and Welfare in Japan is 3 mg/day (3000 µg/day).
The biological half-life of iodine is different in the various organs of the body, from 100 days in the thyroid, to 14 days in the kidneys and spleen, to 7 days in the reproductive organs. Typically the daily urinary elimination rate ranges from 100 to 200 µg/L in humans. However, the Japanese diet, high in kelp, contains 1,000 to 3,000 µg of iodine per day, and research indicates the body is able to readily eliminate excess iodine that isn't needed for thyroid hormone production. Literature reports as much as 30,000 µg/L (30 mg/L) of iodine being safely excreted in the urine in a single day, with levels returning to the standard range in a couple of days, depending on seaweed intake. One study concluded the range of total body iodine content in males was 12.1 mg to 25.3 mg, with a mean of 14.6 mg. It is presumed that once thyroid-stimulating hormone is suppressed the body simply eliminates excess iodine, and as a result, long term supplementation with high doses of iodine has no additional effect once the body is replete with enough iodine. It is unknown if the thyroid is the rate limiting factor in generating thyroid hormone from iodine and tyrosine, but assuming it isn’t, a short term loading dose of one or two weeks at the tolerable upper intake level could quickly restore thyroid function in iodine deficient patients.
Iodine vapor is very irritating to the eye, to mucous membranes, and in 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. Iodide toxicity is similar to (but not the same as) toxicity to bromides or fluorides. Excess Bromine and Fluorine can be toxic to Iodine uptake (storage and use) in organisms, as both can selectively replace iodine biochemically.
Excess iodine can be more cytotoxic in the presence of selenium deficiency. Iodine supplementation in selenium-deficient populations is theoretically problematic, partly for this reason.
Amino Acid impacts
- Selenocysteine (abbreviated as Sec or U, in older publications also as Se-Cys) is the 21st proteinogenic amino acid, and is the root of Iodide Ion toxicity when there is not enough Selenium biologically available.
- Selenocysteine exists naturally in all kingdoms of life as a building block of selenoproteins. Selenocysteine is a cysteine analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group.
- Selenocysteine is present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5' deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, selenophosphate synthetase 1, methionine-R-sulfoxide reductase B1 (SEPX1), and some hydrogenases).
- Selenomethionine is a naturally occurring amino acid containing selenium.
- The L-enantiomer of selenomethionine, known as L-selenomethionine, is a common natural food source of selenium and is the predominant form of selenium found in Brazil nuts, cereal grains, soybeans, and grassland legumes, while Se-methylselenocysteine, or its γ-glutamyl derivative, is the major form of selenium found in Astragalus, Allium, and Brassica species.
- In vivo, selenomethionine is randomly incorporated instead of methionine.
Hypersensitivity reactions to iodine-containing compounds
Some people develop a sensitivity to compounds of iodine but there are no known cases of people being directly allergic to "elemental" Iodine itself.
- Application of tincture of iodine can cause a rash.
- Some cases of reaction to Povidone-iodine (Betadine) have been documented to be a chemical burn.
- Eating iodine-containing foods can cause hives.
Medical use of iodine compounds (i.e. as a contrast agent, see above) can cause anaphylactic shock in highly sensitive patients, presumably due to sensitivity to the chemical carrier. Cases of sensitivity to iodine compounds should not be formally classified as iodine allergies, as this perpetuates the erroneous belief that it is the iodine to which patients react, rather than to the specific allergen.
Sensitivity to iodine containing compounds is rare but has a considerable effect given the extremely widespread use of iodine-based contrast media.
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