Iodine and Iodine Deficiency: What is Your Status?

Iodine is a vital mineral nutrient necessary for regulating metabolism, brain development, and other physiological functions in both children and adults. Despite the success of salt-iodization programs in reducing iodine deficiency globally, about 30% of the world's population is still at risk, with the issue re-emerging in developed countries due to changes in dietary patterns and food manufacturing practices. In the U.S., mild-to-moderate iodine deficiency is common and appears to be increasing, while European countries with the highest deficiency rates often lack iodization programs. Monitoring and public health initiatives are needed to address iodine deficiency, especially in at-risk populations such as pregnant women, to prevent associated health issues like intellectual disability in children.

Iodine Intake and Sources

Iodine is an essential nutrient for thyroid hormone synthesis and overall health, with intake historically improved through the fortification of table salt. However, recent trends show a decline in iodine status in industrialized countries like the U.S., UK, and Australia, likely due to changes in dietary patterns, food preparation, and agricultural practices. The decline is particularly concerning among women of reproductive age. Key points include:

• The recommended daily allowance (RDA) for iodine varies by age and life stage, with adults needing 150 µg, pregnant women 220 µg, and lactating women 290 µg.
• Major dietary sources of iodine in the U.S. include bread, dairy products, iodized salt, eggs, fish, and seaweed. However, the actual iodine content can vary significantly, and portion size is crucial in determining intake.
• Fortified foods like bread made with iodate dough conditioners can provide significant iodine, but their use is declining.

Changes in agricultural and industry practices may be reducing the iodine content in the food supply. For example, the reduced use of iodate dough conditioners in bread, iodine-supplemented feed for livestock, and iodophors in milk processing can all contribute to lower iodine levels in food. Additionally, the increasing popularity of organic farming practices, which often use less iodine in animal feed, can further decrease iodine levels in dairy and meat products.

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Consumer practices also play a role in declining iodine intake:

• The use of iodized salt is decreasing due to health messages about reducing sodium intake and the rising popularity of non-iodized salts like sea salt.
• Veganism and certain forms of vegetarianism, which exclude or limit animal products, can lead to lower iodine intake. Plant-based milk alternatives, for example, are often not fortified with iodine and can be less iodine-rich than cow's milk.
• Some foods contain natural goitrogens, substances that can interfere with iodine uptake by the thyroid gland. These include cruciferous vegetables (e.g., broccoli, cabbage, kale) and certain staples in developing countries like cassava. Proper cooking can minimize their goitrogenic effects.

Overall, the decline in iodine intake in developed countries is a multifaceted issue that requires attention to ensure adequate iodine levels in the population, particularly among vulnerable groups like pregnant women.

Assessing iodine status

Assessing iodine status in a population is crucial to ensure adequate intake and prevent deficiency. Four main methods are used for this assessment:

• Urinary Iodine Concentration (UIC):** Reflects recent dietary intake of iodine. It's a sensitive marker that can show changes in iodine intake over a short period.
• Serum Thyroglobulin (Tg):** Represents an intermediate response to iodine intake, showing changes over weeks to months.
• Serum Thyroid-Stimulating Hormone (TSH):** Best indicator of thyroid function, but not sensitive for iodine status in adults. Recommended for neonates as a supplement to UIC screening.
• Thyroid Size:** Changes in thyroid size, such as enlargement or goiter development, reflect long-term iodine nutrition.

However, these methods have limitations and are not equally applicable to all population subgroups. For instance, TSH values may not differ in adults with iodine deficiency as long as daily intake exceeds 50 µg/day, and thyroid hormone concentrations are not recommended as indicators of iodine status since levels often remain normal in iodine-deficient populations.

Adequate iodine status is defined based on median UIC levels:

• Non-pregnant, non-lactating adults and children ≥ 6 years:** 100–199 µg/L.
• Pregnant women:** 150–249 µg/L.
• Lactating women and children < 2 years:** ≥ 100 µg/L.
• Severe iodine deficiency:** UIC concentrations < 20 µg/L.

It's important to monitor iodine status, especially in vulnerable groups like pregnant women and children, to prevent iodine deficiency disorders and ensure optimal health.

Iodine Deficiency

Iodine deficiency is a widespread issue affecting 35–45% of the global population, making it one of the most common nutrient deficiencies. It is the leading cause of goiter, with the incidence of goiter increasing with the severity of iodine deficiency: 5–20% in mild cases, 20–30% in moderate cases, and over 30% in severe cases. In the U.S., around 38% of the population was classified as iodine deficient in 2011–2012. The thyroid gland adapts to iodine deficiency by increasing iodide absorption and metabolism, leading to gland enlargement and goiter. In severe cases, particularly in children, iodine deficiency can lead to endemic cretinism, a condition characterized by intellectual disability and neurological issues. Goiter diagnosis is typically made using ultrasound technology, and multinodular goiter can be classified as euthyroid or toxic based on clinical presentation and pathology.

Excess Iodine

Excessive iodine intake is relatively rare, but it can lead to health issues such as hyperthyroidism, autoimmune thyroid disease, and certain types of thyroid cancer. The upper limit (UL) for iodine intake in the U.S. is set at 1100 µg/day for adults, including pregnant and lactating women. High iodine levels can reduce thyroid hormone production, leading to hypothyroidism and goiter development. In populations with chronic iodine deficiency, a sudden increase in iodine intake can cause iodine-induced hyperthyroidism, especially in older individuals with nodular goiter. While excessive iodine intake is limited, it underscores the importance of maintaining proper iodine levels for overall health.

Iodine Supplementation

Iodine supplementation is crucial to address iodine deficiency disorders (IDD) and improve public health. Universal salt iodization, which involves adding iodine to all salt for human and livestock consumption, is a common strategy, but implementation challenges exist due to poor coordination within the food and agricultural industries. Despite the challenges, iodine supplementation programs have been initiated worldwide, with significant benefits including improved school performance, workforce productivity, and reduced healthcare costs. However, coverage varies by region, with Europe having the lowest extent of salt iodization among WHO countries. Monitoring and surveillance of these programs are essential to prevent complacency, regression, and excessive intake while striving to correct deficiencies.

Optimal Takeaways

• Iodine is essential for metabolism, brain development, and thyroid hormone production, with 70-80% of the body's iodine found in the thyroid gland.
• Despite global efforts, 30% of the world's population is still at risk of iodine deficiency, with a re-emerging concern in developed countries due to changing dietary patterns and food manufacturing practices.
• Mild-to-moderate iodine deficiency is common and increasing in the U.S., while European countries with high deficiency rates often lack iodization programs.
• Pregnant women and women of reproductive age are internationally recognized as high-risk groups for iodine deficiency, which can cause preventable intellectual disability in the developing fetus.
• Urinary iodine concentration (UIC) measurement is the optimal method for assessing iodine status, with salt fortification being the principal measure for population-wide supplementation.
• Industrial, agricultural, and dietary changes in industrialized countries are contributing to declining iodine status, highlighting the need for continuous monitoring and educational campaigns.
• Future research should focus on monitoring iodine status, tracking intake of iodine-containing foods, and evaluating the effectiveness of public health initiatives aimed at increasing iodine intake.
  

Reference

Hatch-McChesney, Adrienne, and Harris R Lieberman. “Iodine and Iodine Deficiency: A Comprehensive Review of a Re-Emerging Issue.” Nutrients vol. 14,17 3474. 24 Aug. 2022, doi:10.3390/nu14173474 This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license https://creativecommons.org/licenses/by/4.0/).

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