Iodine is an essential component of the thyroid hormones, and adequate iodine intake is necessary for normal thyroid function 1)Gregory A. Brent. Thyroid Function Testing. 2010; page 45. Thyroid hormones regulate many important biochemical reactions, including protein synthesis and enzymatic activity, and are critical determinants of metabolic activity. They are also required for proper skeletal and central nervous system development in fetuses and infants 2)http://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/. The optimal range of iodine intake to prevent thyroid disease is relatively narrow. Both low and high iodine intakes may interfere with thyroid function 3)Gregory A. Brent. Thyroid Function Testing. 2010; page 45.
Over the past century, many food vehicles have been fortified with iodine to prevent iodine deficiencies: bread, milk, water and salt. Salt is the most commonly used vehicle for several reasons 4)http://www.dionex.com/en-us/webdocs/81306-AN236-LC-Iodide-Seawater-15Sept2009-LPN2312.pdf. Salt is one of the few commodities consumed by everyone. Salt consumption is fairly stable throughout the year. Salt production is usually in the hands of few producers. Salt iodization technology is easy to implement and available at a reasonable cost (0.4 to 0.5 US cents/kg, or 2 to 9 US cents per person/year). The addition of iodine to salt does not affect its color, taste or odor. The quality of iodized salt can be monitored at the production, retail and household levels and salt iodization programs are easy to implement 5)http://whqlibdoc.who.int/publications/2004/9241592001.pdf?q=iodine-status-worldwide. Potassium iodide and potassium iodate are safe sources of dietary iodine used to iodize salt and prevent iodine deficiencies. In the United States and Canada, potassium iodide is frequently used to iodize salt whereas in many other countries, potassium iodate is preferred due to its greater stability. Iodide can be oxidized to iodine under many conditions, including exposure to humidity, reaction with existing moisture present in the salt, exposure to sunlight,and exposure to heat. This conversion is also catalyzed by metal ions, particularly ferrous ions. Iodine readily sublimates, and is therefore easily lost from iodized salt. Therefore, the packaging of iodized salt is very important. To avoid losses as high as 75% over a period of nine months, waterproof packaging is required, as reemphasized in the recent amended Codex Alimentarius 6)http://www.who.int/nutrition/publications/VMNIS_Iodine_deficiency_in_Europe.pdf. However, iodate is comparably more stable and is not lost by such pathways 7)http://www.dionex.com/en-us/webdocs/81306-AN236-LC-Iodide-Seawater-15Sept2009-LPN2312.pdf. Iodide is readily and completely absorbed from the gastrointestinal tract. Other forms of iodine in foods are reduced to iodide before absorption. Absorbed iodide is distributed throughout the body via the circulatory system. A portion (approximately 30%) is removed by the thyroid for hormonal synthesis. Iodine intake in excess of requirement is excreted primarily through the urine. To ensure an adequate supply of thyroid hormones, the thyroid must trap about 0.060 mg of iodine per day. The daily iodine requirement for prevention of goiter in adults is 0.050-0.075 mg. To provide a margin of safety, an allowance of 0.150mg (150μg) is recommended for adolescents and adults 8)http://www.inchem.org/documents/jecfa/jecmono/v024je11.htm.
What is the Recommended Iodine Content of Iodized Salt?
In order to meet the iodine requirements of a population it is recommended to add 20 to 40 parts per million (ppm) of iodine to salt 9)http://whqlibdoc.who.int/publications/2004/9241592001.pdf?q=iodine-status-worldwide. Iodization levels vary greatly and can range from 5–100 μg/g (ppm) of iodine in salt depending on the country of manufacture and the storage conditions 10)http://www.dionex.com/en-us/webdocs/81306-AN236-LC-Iodide-Seawater-15Sept2009-LPN2312.pdf. US iodized salt contains 46–77 ppm (parts per million), whereas in the UK the iodine content of iodized salt is recommended to be 10–22 ppm 11)http://en.wikipedia.org/wiki/Salt. The concentration of the fortification levels is 5 ppm in Norway and 70 ppm in Sweden. Albania’s level is even higher is 75 ppm 12)http://www.who.int/nutrition/publications/VMNIS_Iodine_deficiency_in_Europe.pdf. Table salt in Italy is iodized at 30 ppm, which is typical of many European programs 13)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2453047/.
The recently recommended daily salt intake is 5 g/day (about a teaspoon of table salt) in adults and 3g/day in children. A daily consumption of 5 g of salt iodized at 30 mg/kg (30μg/g) provides an optimal daily iodine intake in adults. In children the recommended iodine intake is achieved with a daily consumption of 3 g of iodized salt 14)http://www.nature.com/ejcn/journal/v69/n2/full/ejcn2014206a.html.
It is interesting that however, table salt in Italy is iodized enough, Italy is considered a country with mild iodine deficiency because there is the low levels of purchase of iodized salt (and thus, low percentage of households using iodized salt. This has led the Italian Government to pursue an agreement with the salt industry and scientists that might be described as a “semi-compulsory” compromise. Non-iodized salt must be specified on the label so that consumers know whether salt they buy is iodized or not. If a purchase is made without this specification, the retailer will automatically provide iodized salt. Underlying the agreement is a stipulation that all retailers in Italy must have iodized salt on the shelves. It follows that when appropriate levels of iodine concentrations are found in national salt systems, but the penetration rates of iodized salt into households is low, or if the salt used in food industries is not iodized, then the positive impact on public health will be muted. It is also worth mentioning that an increasing amount of the proportion of total salt consumed comes from sources other than table salt. In countries of western and central Europe about 80% of salt is consumed in processed foods such as in bread, sausages, canned and other ready-to-eat foods, as so-called “hidden salt”. Consequently, if this hidden salt is not iodized, it is extremely difficult for a population to achieve adequate iodine intakes. The trend in some segments of the population to consume “cottage salts” and “plain” sea salt can also contribute to reducing the consumption of iodized salt. A few European countries (Denmark, Germany, the Netherlands and Switzerland) require iodized salt in food processing but most do not 15)http://www.who.int/nutrition/publications/VMNIS_Iodine_deficiency_in_Europe.pdf.
Could a Different Iodine Fortification of Salt in Various Countries Cause Thyroid Problems?
As mentioned above, because iodide fortification is higher in North America, there is an increased iodine intake. In Japan, where foods rich in iodine are consumed regularly, the intake may be as high as over 1000 micrograms/day. Although iodine consumption is generally lower in Europe, the people in these countries do not usually develop thyroid disease 16)http://www.medicinenet.com/script/main/art.asp?articlekey=18119&page=2. In areas of iodine sufficiency (North America, Canada, Switzerland, France, United Kingdom, Netherland, Slovakia, Japan) most healthy adults are remarkably tolerant to iodine intakes up to 1 mg/day (1000μg/day), as the thyroid is able to adjust to a wide range of intakes to regulate the synthesis and release of thyroid hormones. Large amounts of iodine given for days to months in small groups of healthy subjects have shown few adverse effects 17)Gregory A. Brent. Thyroid Function Testing. 2010; page 57. However, when people with past or present thyroid abnormalities are exposed to unaccustomed, large amounts of iodine (such as moving to North America and increasing their iodine intake), they can develop thyroid disease 18)http://www.medicinenet.com/script/main/art.asp?articlekey=18119&page=2.
Even minor differences in iodine intake between populations are associated with differences in the occurrence of thyroid disorders. Both iodine intake levels below and above the recommended interval are associated with an increase in the risk of disease in the population 19)http://www.ncbi.nlm.nih.gov/pubmed/20172467. In iodine-replete areas, people do not suffer from thyroid disease caused by severe iodine deficiency but most persons with thyroid disorders have autoimmune disease 20)http://bmb.oxfordjournals.org/content/99/1/39.full. Reporting in the June of 2006 issue, the researchers concluded that more than adequate or excessive iodine intake may lead to iodine-induced hypothyroidism, autoimmune thyroiditis 21)http://thyroid.about.com/od/newscontroversies/a/toomuchiodine.htm and may increase the severity of existing autoimmune thyroiditis 22)http://www.ncbi.nlm.nih.gov/pubmed/1345585 especially for susceptible populations with recurring thyroid disease, the elderly, fetuses, and neonates 23)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4192807/. Many thyroid processes are inhibited when iodine intake becomes high, and the frequency of apoptosis of follicular cells becomes higher 24)http://www.ncbi.nlm.nih.gov/pubmed/20172467. In a damaged thyroid gland, the normal down-regulation of iodine transport into the gland may not occur 25)Gregory A. Brent. Thyroid Function Testing. 2010; page 57. Vulnerable patients with autoimmune thyroid disease; a previous history of surgery, 131I or antithyroid drug therapy for Graves disease; subacute thyroiditis; postpartum thyroiditis; type 2 amiodarone-induced thyrotoxicosis (AIT); hemithyroidectomy; IFNα therapy; and concomitant use of potential goitrogens, such as lithium might have an increased risk of failing to escape from the acute Wolff–Chaikoff effect, which may lead to iodine-induced hypothyroidism 26)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3976240/#!po=26.4706.
The mechanisms that initiate the development of the abnormal immune response and the relationship of autoimmune thyroid disease with excess iodide are poorly understood. There is evidence that an increase in the iodination of thyroglobulin (Tg) enhances its immunogenicity 27)http://www.medicinenet.com/script/main/art.asp?articlekey=18119&page=2. Other mechanisms include as mentioned above direct toxic effect of iodine on thyroid cells via free oxygen radical generation, and immune stimulation by iodine 28)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2935336/. To complicate matters, a rapid increase in iodine intake can lead to iodine-induced hyperthyroidism in individuals who have previously had chronic iodine deficiency 29)http://whqlibdoc.who.int/publications/2004/9241592001.pdf?q=iodine-status-worldwide. 10 european countries are still characterized by mild to moderate iodine deficiency (Estonia, Latvia, Lithuania, Albania, Turkey, Hungary, Ireland, Italy, Spain, France, Belgium, and Denmark) 30)http://www.who.int/nutrition/publications/VMNIS_Iodine_deficiency_in_Europe.pdf. In mild and moderate iodine deficiency, the normal thyroid gland is able to adapt and keep thyroid hormone production within the normal range. However, the prolonged thyroid hyperactivity associated with such adaptation leads to thyroid growth, and during follicular cell proliferation there is a tendency to mutations leading to multifocal autonomous growth and function. In populations with mild and moderate iodine deficiency, such multifocal autonomous thyroid function is a common cause of iodine-induced hyperthyroidism especially in elderly people when it is presented with increased iodine intake. The average serum TSH tends to decrease with age in such populations caused by the high frequency of autonomous thyroid hormone production 31)http://www.ncbi.nlm.nih.gov/pubmed/20172467. It is worth mentioning that iodine-induced hyperthyroidism (IIH) is the most common complication of iodine prophylaxis and it has been reported in almost all iodine supplementation programmes in their early phases. IIH occurs in the early phase of the iodine intervention and primarily affects the elderly, as already mentioned, who have longstanding thyroid nodules. However, it is transient and its incidence reverts to normal after one to ten years 32)http://whqlibdoc.who.int/publications/2004/9241592001.pdf?q=iodine-status-worldwide. Thus, even in so-called iodine-sufficient populations, because many individuals will have past or present thyroid disorders, it is to be expected that changes in population iodine intake will be an important determinant of the pattern of thyroid diseases 33)Gregory A. Brent. Thyroid Function Testing. 2010; page 57.
If a person, who has moved to a country with higher iodine fortification of salt, starts to feel either symptoms of hypothyroidism (fatigue, increased sensitivity to cold, dry skin, constipation, unexplained weight gain, depression) or symptoms of hyperthyroidism (sudden weight loss, rapid heartbeat, increased appetite, sweating, increased sensitivity to heat, difficulty sleeping), he/she should see a doctor and let their thyroid gland tested. Iodine supplements should be avoided because they may worsen thyroid problems.
Monitoring Iodine Levels of Salt
Iodine fortification and supplementation are safe if the amount of iodine administered is within the recommended range. For more than 50 years iodine has been added to salt and bread without noticable toxic effects 34)http://whqlibdoc.who.int/publications/2004/9241592001.pdf?q=iodine-status-worldwide. When salt is adequately iodized, it is likely that a population’s iodine status will improve and the thyroid function of that population will normalize. Monitoring the population’s iodine status is nevertheless necessary since dietary habits may change in some segments of the population or the iodine level of salt may not be sufficient to meet the requirements of some groups, in particular pregnant women 35)http://whqlibdoc.who.int/publications/2004/9241592001.pdf?q=iodine-status-worldwide.