Review
Iodine deficiency and thyroid disorders

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Summary

Iodine deficiency early in life impairs cognition and growth, but iodine status is also a key determinant of thyroid disorders in adults. Severe iodine deficiency causes goitre and hypothyroidism because, despite an increase in thyroid activity to maximise iodine uptake and recycling in this setting, iodine concentrations are still too low to enable production of thyroid hormone. In mild-to-moderate iodine deficiency, increased thyroid activity can compensate for low iodine intake and maintain euthyroidism in most individuals, but at a price: chronic thyroid stimulation results in an increase in the prevalence of toxic nodular goitre and hyperthyroidism in populations. This high prevalence of nodular autonomy usually results in a further increase in the prevalence of hyperthyroidism if iodine intake is subsequently increased by salt iodisation. However, this increase is transient because iodine sufficiency normalises thyroid activity which, in the long term, reduces nodular autonomy. Increased iodine intake in an iodine-deficient population is associated with a small increase in the prevalence of subclinical hypothyroidism and thyroid autoimmunity; whether these increases are also transient is unclear. Variations in population iodine intake do not affect risk for Graves' disease or thyroid cancer, but correction of iodine deficiency might shift thyroid cancer subtypes toward less malignant forms. Thus, optimisation of population iodine intake is an important component of preventive health care to reduce the prevalence of thyroid disorders.

Section snippets

Introduction: iodine deficiency disorders

Iodine deficiency impairs thyroid hormone production and has many adverse effects throughout the human life cycle, collectively termed the iodine deficiency disorders (panel 1).1 Although goitre is the most visible effect of iodine deficiency, the most serious is cognitive impairment—normal concentrations of thyroid hormones are needed for neuronal migration, glial differentiation, and myelination of the central nervous system.2 Because iodine deficiency continues to affect large populations,

Assessment, epidemiology, and iodine deficiency in industrialised countries

WHO has established recommended nutrient intakes for iodine (Panel 2).4 The iodine status of populations can be assessed by using a biomarker of exposure, urinary iodine concentration (UIC), biomarkers of function, goitre, and thyroid function tests (table 1).8 When assessing populations, UIC is the biomarker of choice;4 it measures the latest iodine intake, because the kidney excretes more than 90% of dietary iodine in the subsequent 24–48 h.8 To classify national iodine status, WHO, UNICEF,

Adaptation of the thyroid to iodine deficiency

The relation between iodine intake and thyroid disorders in populations is U-shaped because both deficient and excessive iodine intakes can impair thyroid function. Moreover, even small increases in iodine intake in previously iodine-deficient populations change the pattern of thyroid diseases.17 Thus, researchers undertaking epidemiological studies need to consider not only present intakes, but also the history of iodine intake of the population. Researchers also need to consider varying

Prevention and treatment

In nearly all countries the best strategy to provide additional dietary iodine is the addition of iodine to salt: it is simple, effective, safe, and inexpensive.4 Iodine can be added to salt in the form of potassium iodide (KI) or potassium iodate (KIO3). Because KIO3 has increased stability in the presence of salt impurities, humidity, and porous packaging, it is the recommended form.4 Iodine is usually added at a concentration of 20–40 mg iodine per kg salt, depending on local salt intake.4

Conclusion and future research

As a population moves from severe iodine deficiency to mild iodine deficiency and then to iodine sufficiency, there is a shift from excess hypothyroidism to excess hyperthyroidism, which is transient, and then a small shift back towards excess mild subclinical hypothyroidism. Severe iodine deficiency causes hypothyroidism because, despite an increase in thyroid activity to maximise iodine uptake and recycling, iodine concentrations are simply not high enough to maintain thyroid hormone

Search strategy and selection criteria

We searched PubMed, Web of Science, and the Cochrane Libraries for articles published in English, French, and German between 1960 and June 28, 2014 with the search terms “iodine deficiency”, “iodine excess”, “iodine status”, “iodised salt”, “urinary iodine”, “goitre”, “nodules” “hyperthyroidism”, “hypothyroidism”, “thyroid antibodies”, “thyroid autoimmunity”, and “thyroid cancer”. Articles were selected for this review if they were original research papers that reported novel findings.

References (93)

  • WD Alexander et al.

    Influence of iodine intake after treatment with antithyroid drugs

    Lancet

    (1965)
  • IM Grais et al.

    Thyroid and the heart

    Am J Med

    (2014)
  • AG Vagenakis et al.

    Adverse effects of iodides on thyroid function

    Med Clin North Am

    (1975)
  • S Andersen et al.

    Iodine deficiency influences thyroid autoimmunity in old age--a comparative population-based study

    Maturitas

    (2012)
  • KE Charlton et al.

    Improvement in iodine status of pregnant Australian women 3 years after introduction of a mandatory iodine fortification programme

    Prev Med

    (2013)
  • MB Zimmermann et al.

    Increasing the iodine concentration in the Swiss iodized salt program markedly improved iodine status in pregnant women and children: a 5-y prospective national study

    Am J Clin Nutr

    (2005)
  • RC Gordon et al.

    Iodine supplementation improves cognition in mildly iodine-deficient children

    Am J Clin Nutr

    (2009)
  • MB Zimmermann et al.

    Iodine supplementation improves cognition in iodine-deficient schoolchildren in Albania: a randomized, controlled, double-blind study

    Am J Clin Nutr

    (2006)
  • SC Bath et al.

    Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC)

    Lancet

    (2013)
  • E Ausó et al.

    A moderate and transient deficiency of maternal thyroid function at the beginning of fetal neocorticogenesis alters neuronal migration

    Endocrinology

    (2004)
  • Assessment of iodine deficiency disorders and monitoring their elimination. A guide for programme managers

    (2007)
  • Aburto N, Abudou M, Candeias V, Wu T. Effect and safety of salt iodization to prevent iodine deficiency disorders: a...
  • K Bougma et al.

    Iodine and mental development of children 5 years old and under: a systematic review and meta-analysis

    Nutrients

    (2013)
  • MB Zimmermann et al.

    Assessment of iodine nutrition in populations: past, present, and future

    Nutr Rev

    (2012)
  • MB Zimmermann et al.

    Thyroglobulin is a sensitive measure of both deficient and excess iodine intakes in children and indicates no adverse effects on thyroid function in the UIC range of 100–299 μg/L: a UNICEF/ICCIDD study group report

    J Clin Endocrinol Metab

    (2013)
  • EM Wong et al.

    Comparison of median urinary iodine concentration as an indicator of iodine status among pregnant women, school-age children, and nonpregnant women

    Food Nutr Bull

    (2011)
  • ICCIDD global network

  • The WHO Vitamin and Mineral Nutrition Information System (VMNIS) on iodine deficiency disorders

  • MB Zimmermann

    Symposium on ‘Geographical and geological influences on nutrition’: Iodine deficiency in industrialised countries

    Proc Nutr Soc

    (2010)
  • KL Caldwell et al.

    Iodine status in pregnant women in the National Children's Study and in U.S. women (15–44 years), National Health and Nutrition Examination Survey 2005–2010

    Thyroid

    (2013)
  • SC Bath et al.

    Iodine deficiency in pregnant women living in the South East of the UK: the influence of diet and nutritional supplements on iodine status

    Br J Nutr

    (2014)
  • P Laurberg et al.

    The Danish investigation on iodine intake and thyroid disease, DanThyr: status and perspectives

    Eur J Endocrinol

    (2006)
  • I Okayasu et al.

    Racial and age-related differences in incidence and severity of focal autoimmune thyroiditis

    Am J Clin Pathol

    (1994)
  • JG Hollowell et al.

    Serum TSH, T(4), and thyroid antibodies in the United States population (1988 to 1994): National Health and Nutrition Examination Survey (NHANES III)

    J Clin Endocrinol Metab

    (2002)
  • MB Zimmermann

    Iodine deficiency

    Endocr Rev

    (2009)
  • CD Thomson et al.

    Urinary iodine and thyroid status of New Zealand residents

    Eur J Clin Nutr

    (2001)
  • P Vejbjerg et al.

    Thyroglobulin as a marker of iodine nutrition status in the general population

    Eur J Endocrinol

    (2009)
  • H Studer et al.

    Simple goiter and its variants: euthyroid and hyperthyroid multinodular goiters

    Endocr Rev

    (1982)
  • R Gutekunst et al.

    Goitre epidemiology: thyroid volume, iodine excretion, thyroglobulin and thyrotropin in Germany and Sweden

    Acta Endocrinol (Copenh)

    (1986)
  • P Laurberg et al.

    High incidence of multinodular toxic goitre in the elderly population in a low iodine intake area vs high incidence of Graves' disease in the young in a high iodine intake area: comparative surveys of hyrotoxicosis epidemiology in East-Jutland Denmark and Iceland

    J Intern Med

    (1991)
  • N Knudsen et al.

    Comparative study of thyroid function and types of thyroid dysfunction in two areas in Denmark with slightly different iodine status

    Eur J Endocrinol

    (2000)
  • F Meng et al.

    Assessment of iodine status in children, adults, pregnant women and lactating women in iodine-replete areas of China

    PLoS One

    (2013)
  • IJ Chopra et al.

    Serum thyroid hormone and thyrotropin levels in subjects from endemic goiter regions of New Guinea

    J Clin Endocrinol Metab

    (1975)
  • F Delange et al.

    Relationship between the serum thyrotropin level, the prevalence of goiter and the pattern of iodine metabolism in Idjwi Island

    J Clin Endocrinol Metab

    (1971)
  • F Delange et al.

    Circulating thyroid hormones in endemic goiter

    J Clin Endocrinol Metab

    (1972)
  • C Stevenson et al.

    Thyroxine (T4) and triiodothyronine (T3): effects of iodine on the serum concentrations and disposal rates in subjects from an endemic goiter area

    J Clin Endocrinol Metab

    (1974)
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