Finally, selenium deficiency in rodents was found to have little impact on DIO activities as it appears that selenium is being supplied in priority for adequate synthesis of DIOs at the expense of other selenoenzymes Severe iron -deficiency anemia can impair thyroid metabolism in the following ways: 1 by altering the TSH response of the pituitary gland ; 2 by reducing the activity of thyroid peroxidase that catalyzes the iodination of thyroglobulin for the production of thyroid hormones ; and 3 in the liver by limiting the conversion of T 4 to T 3 , increasing T 3 turnover, and decreasing T 3 binding to nuclear receptors Additional interventions have confirmed that correcting iron-deficiency anemia improved the efficacy of iodine supplementation to mitigate thyroid disorders reviewed in 42 , Vitamin A status , like other nutritional factors, appears to influence the response to iodine prophylaxis in iodine-deficient populations Vitamin A deficiency in animal models was found to interfere with the pituitary - thyroid axis by 1 increasing the synthesis and secretion of thyroid-stimulating hormone TSH by the pituitary gland, 2 increasing the size of the thyroid gland, 3 reducing iodine uptake by the thyroid gland and impairing the synthesis and iodination of thyroglobulin, and 4 increasing circulating concentrations of thyroid hormones reviewed in A cross-sectional study of children with concurrent vitamin A and iodine deficiencies found that the severity of vitamin A deficiency was associated with higher risk of goiter and higher concentrations of circulating TSH and thyroid hormones These children received iodine-enriched salt together with vitamin A , IU at baseline and at 5 months or a placebo in a randomized , double-blind , month trial.
Vitamin A supplementation significantly decreased TSH concentration and thyroid volume compared to placebo In another trial, vitamin A supplementation alone without iodine to iodine-deficient children reduced the volume of the thyroid gland, as well as TSH and thyroglobulin concentrations Some foods contain substances that interfere with iodine utilization or thyroid hormone production; these substances are called goitrogens.
The occurrence of goiter in the Democratic Republic of Congo has been related to the consumption of cassava, which contains linamarin, a compound that is metabolized to thiocyanate and blocks thyroidal uptake of iodine 1.
In iodine-deficient populations, tobacco smoking has been associated with an increased risk for goiter 54, Cyanide in tobacco smoke is converted to thiocyanate in the liver, placing smokers with low iodine intake at risk of developing a goiter.
Some species of millet, sweet potatoes, beans, and cruciferous vegetables e. Further, the soybean isoflavones , genistein and daidzein, have been found to inhibit thyroid hormone synthesis Most of these goitrogens are not of clinical importance unless they are consumed in large amounts or there is coexisting iodine deficiency.
Industrial pollutants, such as perchlorate see Safety , resorcinol, and phthalic acid, may also be goitrogenic 1 , The recommended amounts were calculated using several methods, including the measurement of iodine uptake in the thyroid glands of individuals with normal thyroid function 9.
Radioactive iodine, especially iodine I , may be released into the environment as a result of nuclear reactor accidents, such as the Chernobyl nuclear accident in Ukraine and the Fukushima Daiichi nuclear accident in Japan. Thyroid accumulation of radioactive iodine increases the risk of developing thyroid cancer , especially in children The increased iodine trapping activity of the thyroid gland in iodine deficiency results in increased thyroid accumulation of radioactive iodine I.
Thus, iodine-deficient individuals are at increased risk of developing radiation-induced thyroid cancer because they will accumulate greater amounts of radioactive iodine. Potassium iodide administered in pharmacologic doses up to mg for adults within 48 hours before or eight hours after radiation exposure from a nuclear reactor accident can significantly reduce thyroid uptake of I and decrease the risk of radiation-induced thyroid cancer The prompt and widespread use of potassium iodide prophylaxis in Poland after the Chernobyl nuclear reactor accident may explain the lack of a significant increase in childhood thyroid cancer compared to fallout areas where potassium iodide prophylaxis was not widely used In the US, the Nuclear Regulatory Commission NRC requires that consideration be given to potassium iodide as a protective measure for the general public in the case of a major release of radioactivity from a nuclear power plant Fibrocystic breast changes constitute a benign non-cancerous condition of the breasts, characterized by lumpiness and discomfort in one or both breasts.
The cause of fibrocystic changes is not known, but variations in hormonal stimulation during menstrual cycles may trigger changes in breast tissue A few observational studies also suggested an association between benign breast diseases including but not limited to fibrocystic changes and thyroid disorders.
Recently, a small case-control study cases vs. Conversely, the prevalence of anti-thyroid autoimmunity and hypothyroidism was found to be significantly higher in women with benign breast diseases compared to controls 65, Interestingly, correcting hypothyroidism with supplemental T 4 was found to improve some of the benign breast disease symptoms, including breast pain mastalgia and nipple discharge In estrogen -treated rats, iodine deficiency leads to changes similar to those seen in fibrocystic breasts, while iodine repletion reverses those changes An uncontrolled study of women with fibrocystic changes found that treatment with aqueous molecular iodine I 2 at a dose of 0.
A double-blind , placebo -controlled trial of aqueous molecular iodine 0. A double-blind, placebo-controlled trial in 87 women with documented breast pain reported that molecular iodine 1. In this study, Large-scale, controlled clinical trials are needed to determine the therapeutic value of molecular iodine in fibrocystic breasts. Besides, the doses of iodine used in these studies 1. Seafood is rich in iodine because marine animals can concentrate the iodine from seawater.
Certain types of edible seaweed e. The iodine content of food that is grown or raised on a particular soil depends on the iodine content of this soil.
In the UK and northern Europe, iodine levels in dairy products tend to be lower in summer when cattle are allowed to graze in pastures with low soil iodine content 9. Other good sources of dietary iodine include eggs, fruit, grain products, and poultry Processed foods can contribute to iodine intake if iodized salt or food additives, such as calcium iodate and potassium iodate, are added during production.
Yet, in the US, virtually no iodized salt is used in the manufacturing of processed food and fast food products, and the food industry is not required to list the iodine content on food packaging Because the iodine content of foods can vary considerably, these values should be considered approximate The fortification of salt with iodine is a feasible and inexpensive method to eliminate iodine deficiency, and salt iodization programs have been implemented in almost all countries.
In countries like Denmark 77 , Australia 78, 79 , and New Zealand 80 , the use of iodized salt in the bread-making process is mandated. Additional approaches have been explored, including sugar fortification 81 , egg fortification 82 , use of iodized salt in the preparation of fermented fish and fish sauce 83 , and use of iodine-rich crop fertilizers In addition, fortification of livestock feeds with iodine and the use of iodophors for sanitation during milking contribute to increasing iodine content in dairy products Finally, annual doses of iodized vegetable oil are administered orally or intramuscularly to individuals in iodine-deficient populations who do not have access to iodized salt 4 , Acute iodine poisoning is rare and usually occurs only with doses of many grams.
Symptoms of acute iodine poisoning include burning of the mouth, throat, and stomach, fever, nausea, vomiting, diarrhea, a weak pulse, cyanosis, and coma 1. Iodine supplementation programs in iodine-deficient populations have been associated with an increased incidence of iodine-induced hyperthyroidism IIH , especially in older people with multi-nodular goiter Iodine deficiency increases the risk of developing autonomous thyroid nodules that are unresponsive to TSH control see Function.
These autonomous nodules may then overproduce thyroid hormones in response to sudden iodine supply. IIH symptoms include weight loss, tachycardia high pulse rate , muscle weakness, and skin warmth. IIH can be dangerous in individuals with underlying heart disease. Yet, because the primary cause of nodular goiter and IIH is chronic iodine deficiency, the benefit of iodization programs largely outweighs the risk of IIH in iodine-deficient populations 1.
In iodine-sufficient individuals, excess iodine intake is most commonly associated with elevated blood concentrations of thyroid stimulating hormone TSH that inhibit thyroid hormone production, leading to hypothyroidism and goiter.
A slightly elevated serum TSH concentration without a decrease in serum T 4 or T 3 is the earliest sign of abnormal thyroid function when iodine intake is excessive. Iodine-induced goiter and hypothyroidism are not uncommon in Japan and can be reversed by restricting seaweed intake In newborns, iodine-induced goiter and hypothyroidism can be due to either high maternal intakes or high exposure to iodized antiseptics In order to minimize the risk of adverse health effects, the Food and Nutrition Board of the US Institute of Medicine set a tolerable upper intake level UL for iodine that is likely to be safe in almost all individuals.
The UL values for iodine are listed in Table 4 by age group; the UL does not apply to individuals who are being treated with iodine under medical supervision Individuals with iodine deficiency and those with preexisting thyroid disease, including nodular goiter, autoimmune Hashimoto thyroiditis, Graves disease, and a history of partial thyroidectomy, may be sensitive to iodine intake levels considered safe for the general population and may not be protected by the UL for iodine 9.
Infants, the elderly, and pregnant and lactating women may also be more susceptible to excess iodine see Supplements Over the past decades, the incidence of thyroid cancer has increased worldwide.
The increasing incidence of thyroid cancer worldwide is likely due at least in part to the improved screening and diagnosis activities. However, because it has coincided with the introduction of iodine fortification programs, a possible contribution of increased iodine intakes has been hypothesized.
Yet, in the US, the increasing incidence of thyroid cancers primarily papillary cancer over the last few decades was paralleled with a reduction in average iodine intake Ecologic studies also suggested that iodine prophylaxis in populations that were previously iodine deficient was associated with an increased incidence of the papillary rather than the follicular cancer subtype, and with a reduced incidence of the more aggressive anaplastic thyroid cancer Amiodarone, a medication used to prevent abnormal heart rhythms, contains high levels of iodine and may affect thyroid function 90, Antithyroid drugs used to treat hyperthyroidism , such as propylthiouracil PTU , methimazole, and carbimazole, may increase the risk of hypothyroidism.
Additionally, the long-term use of lithium to treat mood disorders may increase the risk of hypothyroidism Further, the use of pharmacologic doses of potassium iodide may decrease the anticoagulant effect of warfarin coumarin Perchlorate is an oxidizing agent found in rocket propellants, airbags, fireworks, herbicides, and fertilizer.
Mainly as a result of human activity, perchlorate has been found to contaminate drinking water and many foods Among all age groups, children aged two years have the highest estimated perchlorate intakes per day with 0. Average estimated intakes of perchlorate in US adults range between 0. The RDA for iodine is sufficient to ensure normal thyroid function. There is presently no evidence that iodine intakes higher than the RDA are beneficial.
Most people in the US consume sufficient iodine in their diets, making supplementation unnecessary. Because aging has not been associated with significant changes in the requirement for iodine, the LPI recommendation for iodine intake is not different for older adults.
Originally written in by: Jane Higdon, Ph. Updated in April by: Jane Higdon, Ph. Updated in July by: Victoria J. Drake, Ph. Updated in March by: Victoria J. Updated in August by: Barbara Delage, Ph. Reviewed in August by: Elizabeth N. Pearce, M. Zimmermann MB. Iodine and iodine deficiency disorders. Present Knowledge in Nutrition. Laurberg P. Modern Nutrition in Health and Disease. The thyroid gland. Williams Textbook of Endocrinology. Philadelphia: W. Saunders Company; Assessment of iodine deficiency disorders and monitoring of their elimination: a guide for programme managers, 3 rd ed.
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Because your body does not produce iodine, it needs to be supplied in the diet. When iodine intake is poor, the body cannot produce enough thyroid hormones. Iodine deficiency in pregnancy is a worldwide problem and has become a global public health concern since it is identified as the leading cause of preventable brain damage in newborns and infants due to inadequate intake by mothers and infants. Major international efforts are being made to help reduce the problem, mainly through the use of iodized salt and supplements.
Until the early s, iodine deficiency was a common problem in the United States but was significantly improved with the addition of iodine to table salt.
Hypothyroidism, thyroid gland enlargement goiter and weight gain are other conditions that may result from too little iodine in the diet. Many pregnant women in the U. With iodine deficiency listed as the leading cause of intellectual disability around the world, iodine is an important component for healthy brain development. The most damaging consequences are on fetal and infant development of the brain when deficiency can cause irreversible brain damage that lasts a lifetime.
Brain damage, cretinism, intellectual disability and other conditions are additional risks. It is a crucial nutrient throughout life, but especially during pregnancy, infancy and childhood when thyroid hormones regulate growth in the developing brain. Less severe iodine deficiency manifests as below average IQ in children, including impaired brain function in adults too.
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