A Review Article by LC Masur MD*
A June 2011 study from Lancet implied that more than two thirds of “schoolgirls” in the UK are iodine deficient. [1] The participants in that study were 14 to 15 years of age whereas, in the context of discussing iodine status world-wide, the World Health Organization (WHO) defines “school-aged children” (SAC) as being 6 to 12 years of age. [2] Iodine deficiency is not unique to the UK as similar deficiencies exist in many European countries.2
Iodine, a trace element, is present in soil and is found naturally as a micronutrient in some foods, may be present in other foods as an additive, and may be ingested as a nutritional supplement. It is essential for the production of thyroid hormones which have roles in protein synthesis, metabolic and enzymatic activity, as well as in skeletal development and in neurological development and function.
Less than 30% of total body iodine may be present in the thyroid gland while the rest is non-hormonal and is concentrated in extra-thyroidal tissues where its role is largely unknown.[3] Iodine may play a role in the immune response and supplementation may have beneficial effects in mammary dysplasia and fibrocystic breast disease. [4]
Iodine deficiency is one of the three most common micronutrient deficiencies in the world today (the other two being deficiencies of iron and of vitamin A). [5] Iodine deficiency is the world’s most prevalent and preventable cause of mental impairment.5,[6] Deficiency during pregnancy is well known as a cause of stillbirth, spontaneous abortion, and congenital abnormalities such as cretinism.[7] Iodine deficiency disorders (IDD) throughout life can result in growth retardation, decreased fertility, development of goiter, hypothyroidism (iodine intake below 10-20 mcg/day causes hypothyroidism, often accompanied by goiter)[8] and may be reflected as a pervasive reduction in intellectual capacity.
Soil world-wide generally is deficient in iodine; 29% of the world’s population (more than 2 billion people) resides in areas in at least 130 countries where the soil is deficient in iodine. [9] Cultivation of iodine depleted soil results in the production of iodine deficient crops. Iodine deficiency is an on-going public health problem in poor, land-locked countries: Tibet provides a prime example of this situation where goiter rates are as high as 50%, cretinism in some villages runs at 13% and the average IQ of children is only 85. [10]
In some developed areas of the world the possibility of iodine deficiency may not be a high public health priority. After all, it is easily prevented with the addition of iodine to common foods such as salt, milk and edible oils and/or the ingestion of nutritional supplements containing iodine. The cross-sectional study of urine iodine levels in teen-aged girls in the UK, as initially referenced above, should decrease such complacency and move the possibility of iodine deficiency back into perspective as an on-going potential public health concern. In that UK study, the mean urinary iodine level reported for 700 school-aged females was only 80.1 mcg/L (N>100 mcg/L of urine) where 51% of the participants had a mild deficiency of iodine (i.e. 50-99 mcg/L), 16% had a moderate deficiency (20-49 mcg/L) and 1% were severely deficient (< 20 mcg/L); thus, fully 2/3 of the participants were iodine deficient according to WHO standards. [11]
The data from the above study are made even more relevant in that iodine is not added to salt in the UK whereas salt is commonly iodized in more than 70 other countries. For example, in the USA [and Canada] table salt has been iodized since the early 20th century. [12] Recent National Health and Nutrition Examination Survey (NHANES) data show on-going stability of population-based 24-hour urine iodine excretion at approximately160 mcg/L. [13] This is an improvement over the 1988-91 NHANES median of 145 mcg/L but is still only half as good as it was in the early 1970s when the median urine iodine level was 320 mcg/L). [14] Although no recent Canadian population health surveys have examined iodine status, Canada is considered by the International Council for the Control of Iodine Deficiency Disorders (ICCIDD) likely to be iodine sufficient to the same degree as the USA. [15]
Sources of Iodine.
One of the best dietary sources of iodine is seaweed. Other good sources include fish and shellfish, dairy products (when iodized feed supplements are used) and eggs. Breast milk (depending on maternal iodine stores) and infant formula also are excellent sources of iodine for infants. Grain and vegetable crops will contain variable amounts of iodine depending on the iodine content of the soils upon which they are grown as well as upon fertilisation and irrigation practices. Supplemental sources of iodine include foods such as salt, milk and edible oils fortified with iodine and over-the-counter mineral preparations.
Goitrogens.
Goitrogens suppress the function of the thyroid by interfering with the normal uptake of iodine and can result in enlargement of the gland (goiter). Goitrogen-containing foods include soy and cassava, cabbage, broccoli, cauliflower, and other cruciferous vegetables; excessive intake of these foods can make iodine deficiency worse. Deficiencies of iron and of vitamin A also can be goitrogenic.
Testing for iodine deficiency.
Evaluation of the 24-hour urinary excretion of iodine has long been recognized as the best test for identifying iodine deficiency on a population basis; urine iodine concentrations of 50-99 mcg/L indicate mild; 20-49 mcg/L moderate; and <20mcg/L severe iodine deficiency.1,2 Other tests used in population-based studies of iodine status include the random urine iodine-to-creatinine ratio and spot urine iodine testing.
The assessment of iodine concentration in a 24 hour urine sample also is appropriate for the evaluation of iodine status in individuals. [16] The same guidelines of 50-99 mcg/L for mild; 20-49 mcg/L for moderate; and < 20 mcg/L for severe iodine deficiency, as used in population-based studies, apply.
Traditional recommendations regarding daily dietary iodine intake.
Traditional recommendations concerning how much dietary iodine should be ingested are available from the National Institutes of Health, Office of Dietary Supplements [17] and are outlined below.
Intake recommendations for iodine and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies (formerly National Academy of Sciences). DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. These values, which vary by age and gender, include:
- Recommended Dietary Allowance (RDA): average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals.
- Adequate Intake (AI): established when evidence is insufficient to develop an RDA and is set at a level assumed to ensure nutritional adequacy.
- Estimated Average Requirement (EAR): average daily level of intake estimated to meet the requirements of 50% of healthy individuals. It is usually used to assess the adequacy of nutrient intakes in populations but not individuals.
- Tolerable Upper Intake Level (UL): maximum daily intake unlikely to cause adverse health effects.
Table 1 lists the current RDAs for iodine. For infants from birth to 12 months, the FNB published an AI for iodine that is equivalent to the mean intake of iodine in healthy, breastfed infants in the United States.
| Age | Male | Female | Pregnancy | Lactation |
| Birth to 6 mo | 110 mcg* | 110 mcg* | ||
| 7–12 months | 130 mcg* | 130 mcg* | ||
| 1–3 years | 90 mcg | 90 mcg | ||
| 4–8 years | 90 mcg | 90 mcg | ||
| 9–13 years | 120 mcg | 120 mcg | ||
| 14–18 years | 150 mcg | 150 mcg | 220 mcg | 290 mcg |
| 19+ years | 150 mcg | 150 mcg | 220 mcg | 290 mcg |
* Adequate Intake (AI)
Table 1: Recommended Dietary Allowances (RDAs) for Iodine
The World Health Organization (WHO), United Nations Children’s Fund (UNICEF), and the International Council for the Control of Iodine Deficiency Disorders (ICCIDD) recommend a slightly higher iodine intake for pregnant women of 250 mcg per day. [18],[19]
Complementary and alternative health recommendations regarding daily iodine intake.
The Japanese daily diet, high in seaweed, may contain up to 13.8 mg of iodine. [20] Seemingly based on this contention, many complementary and alternative medicine (CAM) authors/clinicians [21] recommend daily iodine intakes that are much higher than the daily recommendation of either the Institute of Medicine or the WHO and generally suggest iodine doses from 6.25 to 50 mg per day. Most patients do not require these doses on a long-term basis and it is recommended that reassessment of iodine status and thyroid function should be done on a regular basis, perhaps every three to four months while on iodine therapy. [22]
Laboratory testing for the assessment of iodine intake.
The kidneys normally excrete most of any ingested iodine. In the investigation of population-based and individual iodine status 24-hour urine iodine concentration, random urine iodine-to-creatinine ratio and spot urine iodine testing all may be useful tools for the assessment of dietary iodine intake.
Assessment of iodine status – the iodine loading test.
The iodine loading test is based on the concept that the body has specific and saturable mechanisms to take up iodine/iodide. The daily allowance for iodine recommended by the Institute of Medicine (see Table 1 above) is 150 mcg. To perform the iodine loading test a much higher dose of iodine/iodide (e.g. 50 mg in total comprising 20 mg iodine and 30 mg iodide) is given and urine is collected for twenty-four hours. On the supposition that 90% of the ingested load of iodine should be excreted in the urine, levels of excretion less than 90% are interpreted by some proponents of this test22,[23] as being indicative of sub-optimal iodine status.
With treatment of sub-optimal iodine states, and as maximal retention is attained, the percentage of the iodine/iodide load that is retained on subsequent iodine loading tests should decrease and the percentage of the iodine load excreted in the urine should increase toward 90%.
Criticisms of this test [24] mainly revolve around questions concerning the degree of intestinal absorption of the iodine/iodide loading dose and also seek clarification of the claim that orally ingested iodine is “quantitatively absorbed” [25] – the implication from proponents of the test being that only negligible amounts of an oral loading dose are not absorbed (i.e. very little of the ingested iodine dose is excreted in feces).
A large selection of iodine testing is available [26] including:
- The Urine Iodine Pre- and Post-loading Combination which gives, among other useful results, the percentage of an iodine/iodide loading dose that is excreted over a 24 hour period;
- The Urine Halides Pre- and Post-loading Combination which reports the percentage of iodine excreted after a loading dose and, additionally, reports both bromine and fluoride levels in “pre-“ and “post-“ (24 hour) samples;
- The 24-Hour Urine Iodine test (mg/24 hours);
- The 24-Hour Urine Fluoride analysis (mcg/ml and mg/24 hours); and
- The 24-Hour urine Halides assessment, which reports iodine and bromine (mcg/mg creatinine and mg/24 hours) and fluoride (mcg/ml and mg/24 hours) plus the percentage urinary iodine excreted over a 24 hour collection period (% of loading dose excreted).
* L. Charles Masur MD
Scientific Support Physician
Doctor’s Data, Inc
3755 Illinois Avenue
St Charles, IL
60174-2420 USA
Other useful web site references:
http://ods.od.nih.gov/factsheets/Iodine/#h4
http://ceecis.org/iodine/10_monitoring/02_monitoring_iodine_status/mon_uri_art_controvs_%202002.pdf
http://www.iodine4health.com/index.htm
Iodine Status World-Wide http://whqlibdoc.who.int/publications/2004/9241592001.pdf
References
[1] Vanderpump MPJ, Lazarus JH, Smyth PP, Laurberg P, Holder RL, Boelaert K, Franklyn JA, on behalf of the British Thyroid Association UK Iodine Survey Group. Iodine status of UK schoolgirls: a cross-sectional survey. Lancet 2011; 377: 2007-2012. View Abstract
[2] Iodine Status World-Wide, World Health Organization, Geneva, 2004 [Accessed 15 Dec 2011 at http://whqlibdoc.who.int/publications/2004/9241592001.pdf]
[3] Aceves C. The Extrathyroidal Functions of Iodine. Editorial; Current Chemical Biology 2011; 5(3):154-154. View Abstract
[4] Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academy Press, 2001.
[5] World Health Organization. http://www.who.int/nutrition/topics/micronutrients/en/index.html. Accessed 30 Nov 2011.
[6] Editorial. Iodine deficiency—way to go yet. The Lancet; 372 (9633): 88; 12 July 2008. View Abstract
[7] Zimmermann MB. Iodine deficiency. Endocr Rev. 2009 Jun;30(4):376-408. View Abstract
[8] National Research Council, Committee to Assess the Health Implications of Perchlorate Ingestion. Health Implications of Perchlorate Ingestion. Washington, DC: The National Academies Press, 2005.
[9] de Benoist B, McLean E, Andersson M, Rogers L. Iodine deficiency in 2007: global progress since 2003. Food Nutr Bull. Sep 2008;29(3):195-202 View Abstract
[10] Li M and Eastman CJ. Working with funding agencies in the delivery of health care in the Asia Pacific region. Med J Aust 178:13-16 (2003). View Abstract
[11] WHO, UNICEF, ICCIDD. Assessment of iodine deficiency disorders and monitoring their elimination. Geneva, World Health Organization, 2001 (WHO/NHD/01.1). View PDF
[12] McClure RD (October 1935). Goiter prophylaxsis with iodized salt. Science (New York, N.Y.) 82 (2129): 370–371. View Abstract
[13] Caldwell KL, Jones R, Hollowell JG. Urinary iodine concentration: United States National Health And Nutrition Examination Survey 2001-2002. Thyroid. Jul 2005;15(7):692-699. View Abstract
[14] Hollowell JG, Staehling NW, Hannon WH, Flanders DW, Gunter EW, Maberly GF, Braverman LE, Pino S, Miller DT, Garbe PL, DeLozier DM, Jackson RJ. Iodine nutrition in the United States. Trends and public health implications: iodine excretion data from National Health and Nutrition Examination Surveys I and III (1971-1974 and 1988-1994). J Clin Endocrinol Metab. Oct 1998;83(10):3401-8. View Abstract
[15] International Council for the Control of Iodine Deficiency Disorders, http://www.iccidd.org (accessed July 9, 2007).
[16] Patrick L. Iodine: deficiency and therapeutic considerations. Altern Med Rev. 2008 Jun;13(2):116-127. View Abstract
[17] National Institutes of Health, Office of dietary Supplements. Dietary Supplement Fact Sheet: Iodine, 24 June 2011 [Accessed 13 Dec 2011 at http://ods.od.nih.gov/factsheets/Iodine/#h4 ].
[18] World Health Organization. United Nations Children’s Fund & International Council for the Control of Iodine Deficiency Disorders. Assessment of iodine deficiency disorders and monitoring their elimination. 3rd ed. Geneva, Switzerland: WHO, 2007. View PDF
[19] WHO Secretariat, Andersson M, de Benoist B, Delange F, Zupan J. Prevention and control of iodine deficiency in pregnant and lactating women and in children less than 2-years-old: conclusions and recommendations of the Technical Consultation. Public Health Nutr. 2007 Dec;10(12A):1606-1611. View Abstract
[20] Nagataki, S., Shizume, K., Nakao, K., Thyroid Function in Chronic Excess Iodide Ingestion: Comparison of Thyroidal Absolute Iodine Uptake and Degradation of Thyroxine in Euthyroid Japanese Subjects, J. Clin Endo: 27:638-647, 1967. View Abstract
[21] Iodine4Health.com. Clinicians using iodine. [Accessed 19 Dec 2011 at http://www.iodine4health.com/overviews/clinicians/clinicians.htm ].
[22] Brownstein D. Iodine: why you need it, why you can’t live without it. Medical Alternatives Press, West Bloomfield, MI, 2004.
[23] Abraham GE, Flechas J, Brownstein D. Orthoiodosupplementation [Accessed 14 Dec 2011 at http://www.iodine4health.com/ortho/ortho.htm ].
[24] Gaby A. Iodine: A Lot to Swallow. Townsend Letter, Aug-Sep 2005 [Accessed 14 Dec 2011 at http://www.townsendletter.com/AugSept2005/gabyiodine0805.htm ].
[25] Abraham GE. The concept of orthoiodosupplementation and its clinical implications. The Original Internist, 11(2):29-38, 2004.
[26] Doctor’s Data Inc, 3755 Illinois Avenue, St Charles, IL, 60174-2420, USA. www.doctorsdata.com
