Seasonal Iodine Deficiency in Latvian School Children

Dear Editor:

In the year 2000, a nation-wide survey that was conducted to determine the iodine status in Latvia found that the median urinary iodine level was 59 μg/L (1). The Latvian Neonatal Thyrotropin (TSH) Registry data from 2000 showed that TSH levels of >5 mIU/L were found in 16.5% of newborns (2). Both studies support the hypothesis that Latvia is a mild iodine-deficient region. Despite these findings, the introduction of a mandatory salt iodization program in Latvia was rejected, and instead, the voluntary iodine fortification of foods was promoted.

Notably, the nation-wide survey in 2000 was conducted only during spring (1). To determine whether there are seasonal variations with regard to iodine intake in Latvia, we performed a 10-year follow-up cross-sectional school-based cluster survey that included 915 children aged 9–12 from 46 randomly selected schools in all regions of Latvia. Urine samples were collected in October 2010 and April 2011. We also analyzed the Latvian Neonatal TSH Registry data for 31,274 newborns, which were collected between 2009 and 2010.

We found that the median creatinine-standardized urinary iodine (UIC) concentration was 107.3 (interquartile range 69.1–161.7) μg/g. The median UIC was significantly lower in the spring samples (78.3 μg/g Cr) than in the autumn samples (129.7 μg/g Cr). In spring, we found the UIC values to be below 100 μg/g in 63% of schoolchildren and the UIC values to be below 50 μg/g in 28.2% of schoolchildren (see Supplementary Data, available online at www.liebertpub.com/thy). Seasonal fluctuations in the UIC have also been reported in other mild iodine-deficient regions (3).

The data from the Latvian Neonatal TSH Registry also confirmed the seasonal differences in TSH levels (see Supplementary Data). The prevalence of TSH levels >5 mIU/L was 8.7% in 2009–2010; TSH levels >5 mIU/L are indicative of mild iodine deficiency. In accordance with the iodine concentrations in the urine samples, the prevalence of TSH level >5 mIU/L in April was markedly higher than in September (8.3% vs. 4.1%, p<0.001). Despite the observed seasonal pattern, the prevalence of elevated neonatal TSH levels, as well as the lower limit of the 95% confidence interval of the estimate, exceeded 3% even in September.

In countries having no mandatory salt iodination program, foods such as milk and bread are primary sources of iodine due to the use of iodine-containing compounds in agriculture as well as the use of iodized salt in food preparations (4). Therefore, it could be hypothesized that the observed changes in the Latvian iodine status are due to decreased food-related iodine intake during the winter months.

According to the Latvian Food Center data, less than 1% of all edible salt was iodized in 1999 (1). The self-reported prevalence of the consumption of regular iodized salt among Latvian schoolchildren in the present study was 10.2%, but there was no significant association between UIC and the consumption of iodized salt. The increased intake of iodized salt could account for some of the improvement in the iodine status.

In conclusion, in the absence of a mandatory salt iodization program, there is a persisting mild iodine deficiency among Latvian schoolchildren, particularly during spring. The results of our study provide evidence that iodine deficiency studies should be performed during different seasons to account for the seasonality of iodine status.