Iodine supplements usually are not necessary if patients live in iodine-replete areas, such as the United States or in most developed countries. Some alternative medicine practitioners recommend iodine tablets or kelp supplements which are high in iodine for people with hypothyroidism 1)http://www.mayoclinic.org/diseases-conditions/hypothyroidism/expert-answers/hypothyroidism-iodine/faq-20057929.
It is true that severe iodine deficiency can cause hypothyroidism 2)http://www.mayoclinic.org/diseases-conditions/hypothyroidism/expert-answers/hypothyroidism-iodine/faq-20057929. Iodine deficiency is the most common cause of thyroid disorders worldwide, leading to goitre formation and hypothyroidism. Populations at particular risk tend to be remote and live in mountainous areas in South-East Asia, Latin America and Central Africa 3)bmb.oxfordjournals.org/content/99/1/39.full. The thyroid uses iodine to produce thyroid hormones, making iodine an essential, necessary nutrient. Iodine is particularly critical for pregnant women and fetuses, as well as young children. According to the World Health Organization (WHO), an estimated 2 billion people, including 285 million school-age children, are iodine deficient. And among them, iodine deficiency disorders (IDD) affect some 740 million with almost 50 million of them suffer from some form of brain damage resulting from the iodine deficiency 4)http://thyroid.about.com/od/newscontroversies/a/toomuchiodine.htm.
But iodine deficiency is rare in the United States and other developed countries since the addition of iodine to salt (iodized salt) and other foods 5)http://www.mayoclinic.org/diseases-conditions/hypothyroidism/expert-answers/hypothyroidism-iodine/faq-20057929. The diet is therefore the main way of achieving adequate iodine nutrition. Dairy products (due to the use of iodophor cleaners for milk cans and teats), some breads (due to the use of iodate bread conditioners), seaweed and other seafood (which are naturally abundant in iodine) and iodized salt are the most common iodine-containing foods. From all these sources of iodine, salt iodization is viewed as one of the safest (salt is consumed in relatively similar quantities by individuals worldwide 6)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2453047/) and most effective methods of achieving iodine sufficiency across a population. In children from the USA aged 6–12 years, dairy intake is a particularly good source of adequate iodine, probably due to the abundance of dairy content in the diet of children in this age range 7)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3976240/#!po=26.4706.
If iodine deficiency isn’t the cause of hypothyroidism, then iodine supplements provide no benefit 8)http://www.mayoclinic.org/diseases-conditions/hypothyroidism/expert-answers/hypothyroidism-iodine/faq-20057929.
In iodine-replete areas, most persons with thyroid disorders have autoimmune disease 9)bmb.oxfordjournals.org/content/99/1/39.full. Hashimoto’s thyroiditis, also known as chronic autoimmune hypothyroidism, is the most common cause of hypothyroidism in the United States 10)http://www.thyroid.org/patient-thyroid-information/ct-for-patients/vol-7-issue-9/vol-7-issue-9-p-10-11/ and is not caused by iodine deficiency. This disease is characterized by the presence of thyroid peroxidase (TPO) and thyroglobulin (Tg) antibodies (Ab). They are implicated in perpetuating the intrathyroidal inflammatory reaction, ultimately resulting in tissue destruction 11)http://thyroid.about.com/od/isthatso/f/iodine.htm. It has been estimated that a genetic predisposition plays a key role in the development of autoimmune thyroid disease but it may be precipitated by exposure to certain environmental factors, such as increased iodine intake 12)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2935336/, 13)http://www.ncbi.nlm.nih.gov/pubmed/1345585. Ingestion of iodine in excess of the recommended daily intake level is common because of iodine in dietary sources such as dairy, egs, meat, bread, and seaweed, overlapping iodine supplementation, or that in pharmacologic sources such as the cardiac antiarrhythmic drug amiodarone 14)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4139880/, 15)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1638306/pdf/envhper00309-0035.pdf.
For adults who are not lactating or pregnant, the US Institute of Medicine, and jointly by the WHO, United Nations Children’s Fund (UNICEF) and the International Council for the Control of Iodine Deficiency Disorders (ICCIDD), recommend a daily iodine intake of 150 μg and state a tolerable upper level (the approximate threshold below which notable adverse effects are unlikely to occur in the healthy population) of 1,100 μg per day in adults. However, iodine is present in concentrations up to several thousand-fold higher than these amounts in medications, supplements and in the iodinated contrast agents used for radiologic studies (Box 1). In some susceptible individuals, the use of these iodine-containing substances can result in thyroid dysfunction as a result of the high iodine load. In certain circumstances, iodine excess can result in adverse thyroidal effects after only a single exposure to an iodine-rich substance 16)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3976240/.
More than adequate or excessive iodine intake may lead to iodine-induced hypothyroidism, autoimmune thyroiditis 17)http://thyroid.about.com/od/newscontroversies/a/toomuchiodine.htm and may increase the severity of existing autoimmune thyroiditis 18)http://www.ncbi.nlm.nih.gov/pubmed/1345585 especially for susceptible populations with recurring thyroid disease, the elderly, fetuses, and neonates 19)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4192807/. According to the intervention study, deliberate exposure to 500 μg of iodine provoked thyroid autoimmunity in 20% of previously healthy individuals 20)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3271310/#!po=40.9091. In 1998, Reinhardt et al. reported that when patients with Hashimoto’s thyroiditis took 250 µg iodine supplement daily, the occurrence of thyroid dysfunction in the thyroiditis group was significantly higher than in the control group 21)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4192807/.
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 22)http://www.ncbi.nlm.nih.gov/pubmed/1345585. Other mechanisms include a direct toxic effect of iodine on thyroid cells via free oxygen radical generation, and immune stimulation by iodine 23)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2935336/. According to animal studies, high iodine intake can initiate and worsen infiltration of the thyroid by lymphocytes. Lymphocytes are white blood cells that accumulate due to chronic injury or irritation 24)http://thyroid.about.com/od/newscontroversies/a/toomuchiodine.htm.
As already mentioned medications or foods containing excess iodine may cause hypothyroidism in susceptible individuals with underlying thyroid diseases 25)http://www.mythyroid.com/drugs.html. In areas of iodine sufficiency, most healthy adults are remarkably tolerant to iodine intakes up to 1 mg/day 26)Gregory A. Brent. Thyroid Function Testing. 2010; page 57.
In healthy individuals, the thyroid gland has several intrinsic autoregulatory mechanisms to handle the iodine efficiently, even when iodine intake exceeds the tolerable upper intake levels for iodine (1,100 μg per day). The sodium-iodide symporter system contributes most to this stability. Faced with an iodine excess, the sodium/iodide symporter (NIS) throttles the transport of iodide into the thyroid cells, the rate-limiting step of hormone synthesis. Even before the iodine symporter reacts, a sudden iodine overload paradoxically blocks the second step of hormone synthesis, the organification of iodide 27)http://www.ncbi.nlm.nih.gov/pubmed/20172475. This so-called Wolff-Chaikoff effect is an effective means of rejecting the large quantities of iodide and therefore preventing the thyroid from synthesizing large quantities of thyroid hormones 28)http://www.ncbi.nlm.nih.gov/pubmed/11396709. The block does not last long (only few days, 5-7 days 29)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2935336/), because after a while the sodium-iodide symporter shuts down; this allows intracellular iodide to drop under The ULs, which allows the organification of intrathyroidal iodide to resume and the normal synthesis of thyroxine (T4) and triiodothyronine (T3) to return back to normal 30)http://www.ncbi.nlm.nih.gov/pubmed/20172475, 31)http://www.ncbi.nlm.nih.gov/pubmed/11396709. This is so-called “escape” phenomenon 32)http://www.ncbi.nlm.nih.gov/pubmed/11396709. For this reason, most people can tolerate high doses of iodine without developing thyroid abnormalities 33)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1638306/pdf/envhper00309-0035.pdf. However, in some susceptible individuals (e.g. people with autoimmune thyroiditis) as a result of a damaged thyroid gland 34)Gregory A. Brent. Thyroid Function Testing. 2010; page 57, the sodium-iodide symporter fails to shut down, the intracellular concentration of iodide remains high and chronic hypothyroidism ensues 35)http://www.ncbi.nlm.nih.gov/pubmed/23529350 because the accumulated iodine inhibits the synthesis of hormones 36)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1638306/pdf/envhper00309-0035.pdf.
Moreover, people with autoimmune thyroid disease may experience adverse effects with iodine intakes considered safe for the general population 37)http://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/. Doses of iodine in the microgram range (lower than ULs) may cause hyperthyroidism or hypothyroidism in those with past or present thyroid abnormalities 38)Gregory A. Brent. Thyroid Function Testing. 2010; page 57. Therefore, if there is anybody who shouldn’t take iodine, it is thyroid patients 39)http://health.clevelandclinic.org/2014/09/thyroid-issues-what-you-need-to-know-about-diet-and-supplements/.
Moreover, the American Thyroid Association recommends against ingestion of an iodine or kelp daily supplement containing >500 μg iodine for all individuals, except for certain medical indications 40)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3976240/.
BOX 1 41)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3976240/
Sources of iodine exposure and potential excess
Kelp (per g): 16–8,165 μg36
Bread (per slice): 2.2–587.4 μg80
Milk (per 8 oz): 88–168 μg80
Fish fillet (per g, dry weight): 0.73 μg81
Iodized salt: Variable
Vitamins (prenatal, labelled content per daily serving): 75–200 μg46
Amiodarone (per 200 mg): 75,000 μg
Iodinated contrast (free iodine content, per CT scan): 13,500 μg
Topical iodine (povidone iodine): variable, usually 1–5%
Expectorants, mouthwashes, vaginal douches: variable
Saturated solution of potassium iodide (per drop): 50,000