Author reply Re: Urinary iodine excretion: A valuable tool for monitoring salt iodisation programme

We thank the reader for the observations. Our study¹ was mainly aimed at the use of urinary iodine excretion (UIE) for monitoring the salt iodisation programme. It is correct that Student’s t-test was not appropriate to assess statistical differences between groups because the UIE data does not follow a normal distribution. We have redone the analysis using the Mann-Whitney U test to assess statistical significance.

At enrolment in 2017 versus 2019, the data was not significantly different. After 1 year of use in both study A and study B, there was no difference in UIE in groups receiving iodized salt (IS) or double fortified salt (DFS). However, the UIE in study A IS group was significantly higher than in study B IS (p<0.001) and in study A DFS group compared with study B DFS group (p<0.001). At enrolment, intrafamily differences were significantly different by the Mann–Whitney test for boys versus women (p=0.003), boys versus men (p=0.002) and for boys versus girls (p=0.04).

The inference remains unaltered. There has been a reduction in the median UIE during 2017–2021; there are intrafamily differences in the median UIE, and there were no significant differences in UIE between IS and DFS users.

We have hypothesized that reduction in the median UIE levels over time between 2017 and 2021 might be due to the efforts of FSSAI and other stakeholders to sensitise salt manufacturers that iodine content should not exceed 30 ppm at the manufacturing level, increasing awareness of adverse consequences of excess iodine intake, and industry complying with the modification of FSSAI standards that iodine content of salt at manufacturers level should be 20–30 ppm. This is a possible explanation for the observed finding.

The statement in the methodology that ‘the quality of the salt was closely monitored’ was made because the quality of salt was tested by spot iodine and iron testing kits in each batch of the salt received from the manufacturer and periodically in a sub-sample of salt collected from the user households.

The objective of the study was to assess the usefulness of UIE for monitoring iodine content under program conditions. Under program conditions, quantitative iodine estimation in salt samples will not be possible; interruptions in supply (as occurred in this study due to the Covid-19 pandemic) are likely to recur, and urine samples can be collected only from available family members.