Recommended number of participants in iodine nutrition studies is similar before and after an iodine fortification programme

BACKGROUND: Iodine fortification programs have been applied in many iodine deficient regions. Iodine excess is also unfavourable, and it is recommended to monitor iodine status by measuring urinary iodine concentration (UIC). The number of samples needed in such monitoring depends on the variation in UIC. However, it is not known if variation in UIC differs according to iodine levels. AIM AND METHOD: We aimed to describe the effect of an iodisation program on the individual and group-based variation in UIC in spot urine samples. Group 1 (G1, n = 16) was studied before, and group 2 (G2, n = 21) was studied after an iodine fortification program was implemented. Individual urine samples were collected monthly for one year, 13 samplings. RESULTS: G1s (207 samples) median (interquartile range) UIC was 50 (37-67) μg/L, and G2 (265 samples) was 98 (69-139) μg/L. Median individual coefficient of variation (CV) was 38 % in G1 and 40 % in G2 (p = 0.55), whereas the group-based CV was 50 % in G1 and 53 % in G2. No trend was seen between mean UIC and variation in UIC, neither at the individual (p = 0.36) nor at the group level (p = 0.43). Based on data from both groups, approximately 100 samples were needed to reliably estimate the UIC in a population. CONCLUSION: In two groups studied before and after an iodine fortification program was implemented and with different UIC levels, variation in UIC was comparable both at the individual level and according to UIC level. When mild iodine deficiency is corrected, the number of samples needed to reliably estimate the UIC in a population is unaffected.