2.2 Iodine Deficiency Disorders Update
The disorders induced by dietary iodine deficiency (IDD) constitute a major global nutrition concern. The effect of iodine deficiency on the thyroid gland has been known for many decades. Knowledge of the impact of iodine deficiency on mental development has played an important role in mobilizing political leaders, public health officials, nutritionists, and private industry worldwide to launch effective national programmes. Progress towards elimination of IDD through universal salt iodization appears to be one of the most significant successes in the field of non-communicable disease.
Iodine is required for the synthesis of thyroid hormones, which are involved in regulating metabolic activities of all cells throughout the life cycle. In addition, it plays a key role in cell replication. This is particularly relevant for the brain since neural cells multiply mainly in utero and during the first two years of life. IDD comprises all the effects of iodine deficiency. In the foetus these effects lead to increased rates of abortion, still-births, congenital anomalies, cretinism, psychomotor defects, and neonatal mortality. In the child and adolescent, the effects manifest as goitre, hypothyroidism, impaired mental function, retarded mental and physical development, and diminished school performance. In adults, goitre and its complications, hypothyroidism, and impaired mental function persist.13
Median urinary iodine and the prevalence of goitre are the most important indicators for assessing IDD and for describing the severity of IDD as a public health problem. School-age children are the most appropriate target group for IDD surveillance.
Urinary iodine is a marker of very recent dietary iodine intake. The normal population median value of urinary iodine is 100-200 µg/L. Values of 50-99 µg/L suggest mild iodine deficiency, while values of 20-49 and below 20 µg/L suggest moderate and severe iodine deficiency, respectively.14 The benchmark for monitoring progress towards elimination of IDD as a public health problem is 50% of the target group with urinary iodine below 100 µg/L and less than 20% with levels below 50 µg/L.
A goitre is an enlarged thyroid. Thyroid size can be determined clinically by inspection and palpation. Goitre is graded according to size: grade 0 is not palpable or visible; grade 1 is a mass in the neck, consistent with an enlarged thyroid, that is not visible when the neck is in the normal position; and grade 2 is a swelling in the neck that is visible when the neck is in a normal position. Palpation becomes less reliable when average goitre size decreases in a population. Ultra-sonography then provides a more precise and objective method. Ultra-sonography is increasingly used to assess population iodine nutrition, and normative values for thyroid volume measured by ultra-sonography in iodine-replete schoolchildren are needed. The Third Report noted that normative values based on pooled samples of school-children living in Europe had been adopted and were applicable worldwide. This has recently been questioned in light of evidence that some populations of school-age children with adequate median urinary iodine have thyroid volumes much lower than the normative values adopted by WHO. The need for regional scales remains a possibility.15
Finally, elevated serum thyroid stimulating hormone (TSH) in the neonate indicates insufficient supply of thyroid hormone to the developing brain. This is the only indicator that allows prediction of possible impairment of mental development at a population-wide level.
Comparing Prevalences and Numbers
In a joint effort WHO, UNICEF, and ICCIDD recently presented data on the status of IDD at the regional and global levels.16 Of the 191 countries assessed, 130 are affected by IDD (Table 2.2). Of the remaining 61 countries, IDD has been eliminated, or is known not to be present in 20. Data are insufficient for 41 countries, more than half of which are small island states, where IDD is unlikely to be severe. Almost every country in Africa has IDD.
TABLE 2.2: Number of countries affected by IDD, 1999
When these figures are transformed into numbers affected by goitre or at risk of IDD (that is, living in a geographical region where the total goitre rate in school-age children is > 5%), it is clear that the scale of the global problem is immense (Table 2.3). Globally about 740 million people are affected by goitre, and more than 2 billion (or over 38% of the population living in 130 countries) are estimated to be at risk of IDD. Many countries - including China and India - have come to regard their entire population as at-risk of IDD.
TABLE 2.3: Current magnitude of IDD, 1999
Prevention and Control
Despite the magnitude of the IDD problem, great progress has been made in recent years towards its elimination. The main thrust has been establishing and sustaining national salt iodization schemes. Effective partnerships have been forged between relevant UN agencies, national and international NGOs, and the salt industry. Globally, 68% of households in countries with IDD now consume iodized salt (Table 2.4). Iodization rates are highest in the Americas at 90%. Africa has achieved a level of 63%.
TABLE 2.4: Current status of household consumption of iodized salt, 1999
Note: These figures reflect household survey data where this is available; otherwise production-level data are used as a proxy. To estimate the overall iodization rate, total population of each country is multiplied by the per cent of households consuming iodized salt. Numbers are then totaled for each region and divided by the total regional population.
Several national programmes in which the salt industry has exercised leadership have shown spectacular success and made enormous strides towards IDD elimination. In China, for example, most provinces now have over 90% coverage with iodized salt. The national median urinary iodine is 314 µg/L, and all provinces except Tibet have a median concentration greater than 100 µg/L. Adjusted for population, the national total goitre rate is estimated as 10.9% by palpation and 9.6% by ultra-sonography, compared with just over 20% in 1995. The keys to China’s success are effectively iodized salt, enforcement of regulations, strong commitment by government at all levels, an intensive educational programme, and monitoring of salt quality and biological impact.17 China’s success clearly demonstrates how rapid increases in rates of effective salt iodization can increase urinary iodine levels and decrease goitre prevalence.
It takes longer, however, to correct the prevalence of goitre than to correct urinary iodine after the implementation of universal salt iodization, according to a recent study of seven African countries.18 Three countries in the sample (DR Congo, Cameroon, and Nigeria) had a particularly long history of severe IDD. The goitrogenic action of iodine deficiency had been aggravated by the long-term consumption of poorly detoxified cyanide-rich cassava. The national implementation of salt iodization ranged from one year (Tanzania and Zimbabwe) to about five years (Kenya). Two to three areas were visited in each country, and goitre prevalence and median urinary iodine levels were determined in school-age children. Median urinary iodine was above 100 µg/L at all sites visited. This is a major public health success, given the remote location of the sites and their long histories of IDD. The prevalence of goitre had decreased in all sites investigated (compared with the period before salt iodization), but goitre rates were still not below the 5% mark, designated as a criterion for the elimination of IDD.
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