Iodine plays a crucial role as a mineral nutrient, vital for regulating various essential physiological functions, such as metabolism and brain development in children and adults. Consequently, monitoring iodine intake and status in populations has become an area of significant concern and research interest. While salt-iodization programs have historically helped decrease iodine deficiency worldwide, approximately 30% of the global population remains at risk.
Even industrialized nations, including the U.S., are grappling with iodine nutrition issues due to declining iodine intake caused, in part, by shifts in dietary patterns and food manufacturing methods. The absence of universal salt iodization policies in many countries and variations in agriculture and industry practices and regulations have led to disparities in iodine supplementation approaches.1
Iodine, classified as a trace element, holds essential significance as it forms a fundamental part of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones are crucial for the proper functioning of various organs such as the liver, kidney, muscles, brain, and central nervous system. Additionally, they play a vital role in regulating the metabolic activities of most cells and are particularly significant during the early growth and development of organs, especially the brain. In humans, the brain's early development occurs during the fetal and early postnatal stages.
An adult body typically contains 15-20 mg of iodine, with 70-80% concentrated in the thyroid gland. During pregnancy, maintaining sufficient iodine levels is of utmost importance, as deficiency during this period is a major preventable cause of intellectual disability in the developing fetus. Therefore, the iodine status of pregnant women and women of reproductive age is a recognized international concern.2,3
Causes of iodine deficiency
Iodine, in the form of iodide, is present in the earth's environment, but its distribution is uneven. Many regions have experienced leaching due to glaciations, flooding, and erosion, leading to a depletion of iodide in surface soils. The majority of iodide is concentrated in the oceans. Since iodide is found in seawater, it can volatilise into the atmosphere and eventually return to the soil. However, in non-coastal regions, this cycle remains incomplete, resulting in the depletion of iodide in plant foods and drinking water. Consequently, historical instances of iodine deficiency have been observed in populations residing in inland regions (such as central Asia and Africa, central and eastern Europe, and the central U.S.), mountainous areas (like the Alps, Andes, Atlas, and Himalayas), and regions prone to frequent flooding (e.g., Southeast Asia).2
Iodine deficiency in regions with insufficient iodine levels in the soil poses a significant challenge as the food grown there cannot adequately supply iodine to the population and livestock. Unlike nutrients like iron, calcium, or vitamins, iodine is not naturally present in specific foods but is rather absorbed from the soil by the crops grown in that area. The main cause of iodine deficiency lies in the lack of iodine in the earth's crust, making it a geological issue rather than a social or economic one. It cannot be addressed solely by changes in dietary habits or by consuming specific types of locally grown foods.4
In addition to nutritional iodine deficiency, other environmental, socio-cultural, and economic factors contribute to exacerbating the problem and causing thyroid dysfunctions. These factors include protein-energy malnutrition linked to poverty, consumption of goitrogens through unusual diets (particularly among the impoverished), bacteriologically contaminated drinking water, and diets rich in bulky, high-residue foods that hinder the absorption of iodine in the intestines. Consequently, addressing iodine deficiency requires a comprehensive approach that considers various factors influencing iodine availability and consumption.4
Signs and symptoms of iodine deficiency
Various signs and symptoms indicate iodine deficiency, emphasizing its essential role in the body. Common manifestations include an enlarged thyroid gland (goitre), fatigue, weakness, impaired cognition, and hair loss. Dry and rough skin, as well as challenges in weight management, are prevalent. Cold intolerance, irregular menstrual cycles, muscle weakness, and facial puffiness may also arise. Additionally, a slowed heart rate and weakened immune system increase susceptibility to infections. Addressing iodine deficiency is vital for overall health maintenance. Seeking medical evaluation for accurate diagnosis and treatment can prevent complications and improve the quality of life for those impacted.
Health consequences of iodine deficiency
Iodine is a crucial dietary element necessary for the synthesis of thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3). These hormones, formed from iodinated molecules of the essential amino acid tyrosine, play a vital role in regulating cellular oxidation, which in turn influences calorigenesis, thermoregulation, and intermediary metabolism. The presence of T4 and T3 is essential for various physiological processes, including protein synthesis, nitrogen retention, glycogenolysis (breakdown of glycogen into glucose), intestinal absorption of glucose and galactose, lipolysis (breakdown of fats), and glucose uptake by adipocytes (fat cells).4
iodine deficiency has notable implications on cognition, intelligence quotient (IQ), and attention-deficit hyperactivity disorder (ADHD) in children. Higher levels of urinary iodine are linked to elevated IQ scores, while a lack of iodine is associated with an increased risk of ADHD. Additionally, iodine plays a vital role in the normal growth and development of breast tissue, and its deficiency is correlated with heightened risks of breast, endometrial, and ovarian cancer. The administration of iodine supplements has demonstrated promising outcomes in curtailing breast growth and alleviating fibrocystic breast disease symptoms.
Furthermore, gastrointestinal health is also impacted by iodine deficiency, with connections to atrophic gastritis and an augmented susceptibility to gastric cancer. Iodine inhibitors, like nitrates, thiocyanate, and sodium chloride (salt), have increased the likelihood of gastric cancer. In addition, iodine deficiency may have implications for prostate health, as higher levels of urinary iodine excretion are linked to a decreased risk of prostate cancer.
Groups at risk
Currently, the World Health Organization (WHO) approximates that almost 2 billion people suffer from inadequate iodine intake, including approximately one-third of all school-age children. This deficiency persists as a public health concern in 47 countries. Groups at risk of iodine deficiency include children in South Asia, where only 49% of households have access to iodized salt, leaving about 17 million newborns vulnerable to brain damage annually. Economically disadvantaged groups in countries like India also face poor coverage of iodized salt. In Sub-Saharan Africa, iodized salt usage varies widely between countries, posing challenges in implementing effective programs, especially in regions affected by conflict and instability. Convincing small-scale salt producers to iodize their salt is another obstacle in iodine deficiency control.
Children worldwide are at risk, as the International Child Development Steering Group recognizes iodine deficiency as a significant global risk factor for impaired child development.2 Ensuring intervention and addressing iodine deficiency in children is urgent. Moreover, maintaining sustainable iodized salt programs in countries is crucial to prevent the recurrence of iodine deficiency in previously eliminated areas.5
Diagnosis of iodine deficiency
Diagnosing iodine deficiency involves a comprehensive medical evaluation and various laboratory tests. A thorough physical examination can reveal noticeable signs like an enlarged thyroid gland (goitre) and accompanying symptoms such as hair loss, dry skin, and fatigue.
Crucial blood tests are conducted to measure thyroid hormone levels, including thyroxine (T4) and triiodothyronine (T3), as well as thyroid-stimulating hormone (TSH). Reduced T4 and T3 levels combined with elevated TSH indicate hypothyroidism, a potential consequence of iodine deficiency.
Moreover, urine tests are employed to assess urinary iodine concentration (UIC) as an indicator of iodine intake. The integration of clinical assessment and laboratory findings enables healthcare professionals to accurately diagnose and effectively manage iodine deficiency.
Prevention and treatment
Iodine supplementation proves effective in combating population iodine deficiency, yet caution must be exercised to avoid excessive intake. The goal is to increase iodine intake to prevent iodine deficiency disorders (IDD) without surpassing safe levels. The bioavailability of iodine in food varies and is challenging to evaluate, and interactions between different foods within the food matrix remain poorly understood. The most feasible and economical approach to supplement iodine-deficient populations involves using iodized salt, endorsed by international organizations like WHO, the United Nations Children's Fund, and the International Council for Control of Iodine Deficiency Disorders. Other methods include iodized water, iodized oil, and iodine tablets. Iodized salt typically contains 20-40 mg iodine/kg salt, while iodized oil is orally or intramuscularly administered at doses of 200-400 mg iodine per year, commonly targeted at women of childbearing age, pregnant women, and children.5
Presently, iodine deficiency is considered the leading preventable cause of brain damage and mental retardation worldwide. The negative impact of insufficient iodine on the cognitive and physical development of children, as well as adult productivity, is well acknowledged. While some progress has been achieved in improving iodine status in various regions, there remain areas, specific populations, and remote regions that necessitate iodine supplementation, especially women of reproductive age, on a global scale. However, augmenting iodine intake in deficient populations carries risks. Mild iodine deficiency may be linked to reduced risks of overt and subclinical hypothyroidism, as well as autoimmune thyroiditis. Nevertheless, if iodine prophylaxis programs are carefully monitored for both deficiency and excess, the relatively minor risks of iodine excess are significantly outweighed by the substantial risks of iodine deficiency, including pregnancy loss, goitre and mental retardation, which still affect around one-third of the global population.
- Hatch-McChesney A, Lieberman HR. Iodine and iodine deficiency: a comprehensive review of a re-emerging issue. Nutrients [Internet]. 2022 Aug 24 [cited 2023 Aug 7];14(17):3474. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9459956/
- Zimmermann MB. Iodine deficiency. Endocrine Reviews [Internet]. 2009 Jun 1 [cited 2023 Aug 7];30(4):376–408. Available from: https://academic.oup.com/edrv/article/30/4/376/2355070
- Louise B. Iodine intake for pregnant and breastfeeding women and their infants remains a global concern. The Journal of Nutrition [Internet]. 2021 Dec [cited 2023 Aug 7];151(12):3604–5. Available from: https://linkinghub.elsevier.com/retrieve/pii/S0022316622004631
- Kapil U. Health consequences of iodine deficiency. Sultan Qaboos Univ Med J [Internet]. 2007 Dec [cited 2023 Aug 7];7(3):267–72. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3074887/
- De Benoist B, McLean E, Andersson M, Rogers L. Iodine Deficiency in 2007: Global Progress since 2003. Food Nutr Bull [Internet]. 2008 Sep [cited 2023 Aug 7];29(3):195–202. Available from: http://journals.sagepub.com/doi/10.1177/156482650802900305