In this report describing the iodine status of the cohort of young people exposed to Chornobyl fallout in Belarus, the most striking observations related to temporal trends. Over the two time periods examined—a combined total of 8 calendar years—median urine iodine concentration for the cohort as a whole increased significantly from 65.3 to 111.5
μg/L, in a range considered by WHO to represent adequate iodine intake. The prevalence of goiter, reflecting iodine status over a longer period than urinary iodine concentrations, decreased almost threefold, to a rate of 6.1% for the cohort overall, close to the 5% prevalence WHO defines as indicating iodine sufficiency. Thus, the temporal changes in these two population measures of iodine status are consistent with one another and coincide with public health programs to improve iodine intake that were undertaken during this time period.
In March 2000, a governmental decree by the Chief Sanitary Doctor of the Ministry of Health (N11) mandated that all table salt should be iodized using potassium iodate, rather than the less stable potassium iodide used previously, at a concentration of 40.0
mg/kg of salt. In addition, a nation-wide program was mounted to educate the population concerning the benefits of iodized salt. However, according to republican law, markets in Belarus had to stock both noniodized and iodized salt and it was left to consumers to make the choice. As a result, reported use of iodized salt in cycle 2, while substantially increased, did not reach the level of 100%. In April 2001, the Belarus Council of Ministers issued a decree (N484) mandating the use of iodized salt in the production of processed foods.
The increases in dietary intake of iodized salt and other iodine-containing products were observed in all parts of Belarus, suggesting that these official iodinization initiatives were effective throughout the country. Although the governmental approaches were presumably easier to implement for urbanites, the impact of the program reached rural populations as well, due in part to construction of new shops supplying iodized processed foods to rural areas. In fact, the upward trend in urinary iodine levels from the first to the second time period was more marked in rural than in urban residents.
In both cycles 1 and 2, the lowest levels of urinary iodine were found in Mogilev oblast. The low levels at cycle 1 may reflect the iodine content in local soil and water. A map of stable iodine in soil based on measurements made in the 1960s (18
) indicates that, historically, the Mogilev area had levels at the lowest end of the range (0.56–0.94
mg/kg). Gomel oblast had a wide range of soil iodine levels up to and including 5.0–18.2
mg/kg, and measurements in Minsk city/oblast showed soil iodine content in an intermediate range. The lower levels at cycle 2 probably result principally from the low baseline levels since implementation of the government's iodine program was intended to be uniform across oblasts. As mentioned above, in spite of its low median urine iodine levels in cycle 1, Mogilev showed a large relative increase in median levels compared to other oblast categories at the time of cycle 2.
The descriptive study reported here has several strengths, including a large sample size, data on many factors of interest, and a standardized approach to urine collection and handling as well as repeated measurements over a time span of eight years. A limitation is the reliance on nonfasting spot urine samples. The gold standard for estimating individual iodine excretion has been a 24-hour urine collection. For large studies, however, such an approach is untenable. So long as the study sample is sufficiently large, the median figure for the spot urinary iodine concentrations is a reliable measure for application to populations since, with large numbers, the variation in daily iodine intake and urinary volume is leveled out (19
). Andersen et al.
) have estimated that a precision range of
5% requires 200–500 spot urine samples per subgroup for confidence intervals corresponding to 95% and 80%, respectively, whereas a precision range of
10% requires from 50 to 100 spot samples per subgroup. According to these guidelines, our urinary iodine data based on spot samples are sufficiently reliable for the descriptive analyses we have carried out.
Urinary iodine concentration reflects only current iodine status. To provide a better indication of iodine nutrition over an extended period, we analyzed data on prevalence of diffuse goiter by palpation. In the absence of extensive training, estimation of diffuse goiter by palpation can be subject to both inter-examiner variation as well as overdiagnosis of small thyroid enlargements, particularly in populations with mild iodine deficiency (21
). However, given the extensive, standardized program of training and certification our examiners received, we consider the goiter rates reported here to be sufficiently reliable and valid.
Thyroid enlargement can also be determined based on ultrasonography examination (23
). Although sonographic measurements are considered more precise than thyroid palpation, they are also somewhat subjective and can be affected by measurement error and variation in the shape of the thyroid lobes (24
). Moreover, interpretation of goiter prevalence based on such estimates requires age-appropriate normative data from local controls (25
), ideally children with adequate iodine intake (26
). Since there are no widely accepted referent data for our young population, we had reservations about utilizing ultrasonography-based measures of thyroid enlargement. Nonetheless, we did conduct an analysis to examine whether, within each oblast, the distribution of absolute thyroid volumes categorized by quintiles from low to high was associated with the adjusted median urine iodine concentrations. None of the oblasts studied showed a trend toward lower levels of median urine iodine with increasing thyroid size, either in cycle 1 or cycle 2 (data not shown). While urinary iodine concentrations represent current iodine status, increases in thyroid volume from insufficient iodine intake may resolve more slowly when iodine intake improves (2
). In addition, because the vast majority of our goiters were Grade 1, it is possible the lack of association could partly reflect the limited range in thyroid size. When we split the thyroid volume data into two categories [low vs. high using the criteria from Rasmussen et al.
)], we observed a significant association between enlarged thyroid volume and reduced urine iodine levels, particularly in Minsk and Mogilev oblasts (not shown). The association was no longer significant in cycle 2, after the introduction of government iodination programs.
The urinary iodine findings reported here for Belarus can be compared with those we published previously based on measurements of 11,926 subjects in a parallel screening study in Ukraine (12
). Although the iodination programs and/or implementation may have been different in Ukraine and Belarus, in general terms the results were similar. Iodine concentration levels increased significantly in the time period following government initiatives; median concentrations varied by place of residence, were higher in urban than rural areas, and increased more rapidly in rural regions; reported dietary intake of iodine-containing products rose significantly in the second screening cycle. However, the study area in Ukraine was more iodine deficient at the start, with a cycle 1 median urine iodine level of 41.7
μg/L compared to 65.3
μg/L in Belarus, and improvements in iodine intake over time were more modest (an increase of ~14% in median urine iodine levels vs. an increase of >70% in Belarus). As a result, by the time of the second screening in Ukraine, none of the study regions (Zhytomyr, Chernihiv, and Kyiv oblasts as well as Kyiv City) and only a small percentage of the cohort (18%) had urinary iodine levels indicative of iodine sufficiency, although the period of follow-up was shorter than in Belarus.
Earlier surveys of iodine excretion and goiter prevalence in the Chornobyl region beginning 5 years after the accident also found the affected areas of Ukraine to be more iodine deficient than Belarus (9–11). In a 1991 study of diffuse goiter diagnosed using ultrasonography-based thyroid volumes (10
), rates in the Kyiv (54%) and Zhytomyr (38%) regions of Ukraine were more than double those in the Gomel (18%) and Mogilev (22%) regions of Belarus. Under these circumstances, more time and sustained effort may be required to bring widespread iodine sufficiency to northern Ukraine.
Differences in iodine status should be borne in mind when interpreting results from studies of 131
I exposure from the accident at Chornobyl and estimated risk of thyroid cancer among exposed children and adolescents. Historical evidence based on iodine content in soil (18
) and reported trends in populations [reviewed in (2
) and (27
)] as well as data gathered 5–10 years after the accident (9
) and those presented here appear to indicate that heavily contaminated Gomel oblast seems to have a lower level of iodine deficiency than some other parts of Belarus. At the same time, the strongest evidence to date for a modifying effect of stable iodine on radiation risk of thyroid cancer comes from a study (5
) in which 68% of total cases were drawn from Gomel, and iodine status for individual subjects was based on the content in soil in the settlement of residence at the time of the accident. The important issue of joint effects on thyroid cancer risk from iodine deficiency and radioiodine exposure deserves to be pursued.