Seasons but not ethnicity influence urinary iodine concentrations in Belgian adults

Mild iodine deficiency (MID) is endemic in Belgium. Previous surveys, which assessed iodine nutrition in Belgium, focused on children. The iodine status of adults and the influence of ethnicity or seasonality on urinary iodine concentrations (UIC) have not been investigated. Since the nutritional profile of children differs from that of adults, we may anticipate similar differences in iodine status. Seasonal fluctuations in UIC have also been reported from other MID regions. We aimed at assessing iodine status and its association with ethnicity and seasonality in adults. A stratified random sample of 401 healthy subjects aged between 40 and 60 years, of Belgian, Moroccan, Turkish and Congolese descent residing in Brussels was obtained. Iodine status and thyroid function were determined. Median UIC was 68 mu g/L. The frequency of UIC below 100 mu g/L was 73.3%, of which 41.9% fell between 50 and 99 mu g/L, and 29.8% between 49 and 20 mu g/L. There was no difference in UIC and thyroid function between subjects of different ethnic origins. The frequency of UIC below 50 mu g/L was higher in the fall-winter compared to spring-summer periods (P = 0.004). Serum FT3 concentrations, but not FT4 and TSH, were significantly greater in winter than in summer. Seasonal fluctuations in UIC suggest that the risk of iodine deficiency among adults living in Brussels is higher in fall-winter than in spring-summer. The prevalence of MID in Brussels is high among adults but ethnicity does not appear to influence iodine status.