Iodine deficiency

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What is iodine?
Iodine (I2) is a nonmetallic element of the halogen group, naturally occurring in sea water and in certain rocks and sediments. Iodine is a nutritionally essential trace mineral.
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It is essential for the function of thyroid glands and in the synthesis of thyroid hormones. The thyroid hormones, triiodothyronine and its prohormone, thyroxine contain iodine. I2 accounts for more than 50% of molecular weight of the thyroid glands.

Iodine deficiency in the diet can lead to conditions like goiter, cretinism and fibrocystic breast changes. Its deficiency can be controlled or prevented by using iodized foods (salt, milk, flour etc..) and by including sea foods in the diet. People living in mountainous regions and remote inland areas where no sea foods are consumed tend to have low levels of the mineral in their diet. It is estimated that nearly 30% of the world's population, living in about 130 countries, is living in regions of deficiency.

Iodine benefits and functions

Iodine is required for production of thyroid hormones. Each molecule of thyroxine (T4) has four atoms of I2 and each molecule of triiodothyronine (T3) has three I2 atoms. Apart from thyroid glands, iodine is present in body tissues including mucosa, glands, breast tissues, cervix, eyes and choroid plexus. The thyroid hormones are essential for regulating several metabolic functions including energy production and maintenance of body heat. They are essential for embryonic development, general growth, neurological development, skeletal development, gonadal development and their maturation.

Sources of iodine in foods

Sea foods, seaweed, sea vegetables, iodized salt, iodized food products, kelp, dairy products and egg are good sources of iodine. For maintaining normal thyroid function, adults may require about 150 µg (mcg) of daily iodine intake.
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The Recommended Dietary Allowance (RDA) of iodine per day for pregnant women is 220 μg and 290 μg for breastfeeding women. The upper limit of safe daily intake is 1100 µg/day for adults. For children aged 1 to 11 years, adequate intake is 90-120 μg/day and infants below one year the adequate intake is 50-90 µg/day.

Iodine absorption and excretion

Iodine is absorbed in the gut in the form of iodide. It is concentrated into tissues by sodium-iodate symporter. Approximately 90% of ingested mineral is excreted passively in the urine in 24 hours when there is no deficiency. There is also small losses from biliary secretion into the gut.

Causes of iodine deficiency

Some of the causes of I2 deficiency and risk factors that may lead to insufficiency of the mineral in the human body are:
diet poor in iodine,
Selenium deficiency,
use of goitrin,
smoking tobacco,
presence of bromine, perchlorate and fluoride,
presence of lithium,
use of phenobarbitone and
use of rifampin.
A 24-hour urine iodine collection is a useful medical test to diagnose its deficiency, as approximately 90% of ingested mineral is excreted in the urine in 24 hours. Iodine skin test is also helpful in finding whether an individual is having low levels of the element.

Symptoms of iodine deficiency

The symptoms of iodine insufficiency include progressive enlargement of the thyroid glands and onset of goiter. The goiter may develop nodules, enlarge and give rise to symptoms like choking, difficulty in swallowing and breathing. In pregnant women severe deficiency is associated with miscarriages, stillbirth, preterm delivery and congenital abnormalities in the fetus. In such instances the infant develops hypothyroidism and this condition may result in cretinism.

Deficient populations

People not consuming sea foods and living in mountainous regions and remote inland areas and in areas where soil is poor in this element are likely to develop iodine deficiency. In many developed countries, the introduction of iodized salt has reduced this risk. Deficiency of this mineral is still a health issue in several European countries, Australia, New Zealand and most of the third-world nations. The shift towards lower discretionary salt use to lower the risk of cardiovascular disease is resulting in lower levels of iodine intake in the developed countries.

Treatment of iodine deficiency

The insufficiency of I2 is treated by ingesting supplements in the form of iodide salts. Mild cases can be treated by including iodized salt in the diet as well as including iodized flour and multivitamins having iodine. Inclusion of sea foods and seaweed also helps greatly in treating deficient conditions. Levothyroxine (L-T4) treatment and thyroid hormone therapy are carried out under medical guidance.

Iodine supplements

Iodine is supplemented as iodized table salt, injection of iodized oil, iodized water, multivitamins containing iodide salts and natural food rich in the mineral. However I2 supplementation may not reverse cretinism or reduce the size of large nodular goiters.

Iodine deficiency diseases

The insufficiency of iodine can lead to hypothyroidism, which when untreated can cause goiter and fibrocystic breast changes in adults. Untreated maternal hypothyroidism and the resultant congenital hypothyroidism in infants can result in cretinism.

Goiter
Goiter is hyperplasia (enlargement) of the thyroid gland. Worldwide, most of the cases of goiter are caused by iodine deficiency. The lack of dietary iodine causes low levels of thyroxine in the blood and gives rise to high levels of thyroid stimulating hormone (TSH). TSH stimulates the cellular growth and proliferation in the thyroid gland, resulting in the hyperplasia appearing as characteristic swelling in the neck below the Adam's apple. Selenium deficiency, autoimmune disease of thyroid gland, cyanide poisoning, thyroiditis and thyroid cancer are some of the other causes of goiter.

Cretinism
The deficiency of iodine during pregnancy and the resultant maternal hypothyroidism leads to congenital hypothyroidism. Untreated congenital hypothyroidism causes severely impaired physical and mental growth of the infant. Retarded growth, short stature, mental deterioration, neurological impairment, cognitive impairment, swelling of the skin, impaired bone growth, loss of hair and delayed puberty are some of the conditions in cretinism.

Fibrocystic breast changes
Fibrocystic breast disease is an extremely common condition affecting nearly 50% of women at some point in their lives. It is characterized by noncancerous breast lumps and nodules, which are often painful during the menstrual cycle. The unbalanced estrogen production causes breast tissue to be more sensitive to estrogen leading to cyst formation. Breast tissue has a greater concentration of iodine than the rest of the body, offering antioxidant benefits. Deficiency of iodine in diet has been found to be a major cause of these cysts and I2 supplementation had benefited women with fibrocystic breast changes.

Iodine overdose

Though iodine is required in small quantities for human health, elemental I2 is toxic to humans. Overexposure to high concentrations of iodine can occur by frequent skin contact with tinctures of iodine leading to chemical burns. If ingested it may damage the tissues lining the mouth, esophagus, stomach and lungs. Excess use of supplements containing this element, administration of large amounts of iodine through medications, iodinated intravenous dye and also excess use of kelp may cause overdose, leading to thyroid abnormalities, hyperthyroidism, hyperthyroxinemia, risk of autoimmune thyroid diseases and thyrotoxicosis.
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References:
1.World Health Organization/International Council for the Control of the Iodine Deficiency Disorders/United Nations Childrens Fund (WHO/ICCIDD/UNICEF). Assessment of the iodine deficiency disorders and monitoring their elimination. 3rd Edition Geneva: World Health Organization, 2007.
Assey VD, Greiner T, Mzee RK, et al. Iodine deficiency persists in the Zanzibar Islands of Tanzania. Food Nutr Bull 2006; 27: 292–99. 2.Dr Oleg Chestnov. Sustaining the elimination of iodine deficiency disorders (IDD). Noncommunicable Diseases and Mental Health World Health Organization.
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Current topic on nutrition: Iodine deficiency.