Fast track — ArticlesIodine status of UK schoolgirls: a cross-sectional survey
Introduction
WHO estimates that two billion people worldwide including 285 million children of school age have iodine deficiency despite major national and international efforts to increase iodine intake, mainly through voluntary or mandatory iodisation of salt.1, 2 Iodine deficiency is defined as a population median urine iodine excretion less than 100 μg/L, with deficiency classified as moderate if 20–49 μg/L and severe if less than 20 μg/L.3 Iodine deficiency has substantial effects on growth and development and is the most common cause of preventable mental impairment worldwide. Mild iodine deficiency impairs cognition in children, and moderate to severe iodine deficiency in a population reduces IQ by 10–15 points.4 Iodine supplementation before pregnancy might prevent this adverse effect on the intellectual development of infants and children.5 Mild-to-moderate iodine deficiency occurs in areas that are not immediately recognised as iodine deficient.3 Iodine intake can vary substantially within a region and country because of variations in the natural iodine content of food and water.6
Around 45% of the population of continental Europe continues to have evidence of iodine deficiency, although iodine status in several regions has not been documented.7 No current data are available for the UK population.8 Although the UK population was historically thought to have sufficient iodine intake,9 concern has been expressed about current UK iodine intake.10 We aimed to assess systematically the iodine status of the UK population using the standard approach of targeting the most relevant age group and sex.
Urine iodine excretion is a good marker of dietary intake of iodine over days and is the measure of choice for assessment of iodine status.3 For epidemiological studies, a population distribution of urinary iodine is required and, because the frequency distribution is typically skewed towards high values, the median rather than the mean is judged the best indicator of iodine status. WHO, International Council for Control of Iodine Deficiency Disorders, and UNICEF recommend that for national surveys of iodine nutrition, the median urinary iodine from representative samples of spot urine collections from children aged 6–12 years can be used to define a population's iodine status.3 This study focused on female schoolchildren aged 14–15 years from nine UK centres, since those who might proceed to pregnancy in the short-to-medium term (and their offspring) are the most susceptible to the adverse effects of iodine deficiency.
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Participants
Participants in this cross-sectional survey were recruited from nine centres across the UK (Aberdeen, Belfast, Birmingham, Cardiff, Dundee, Exeter, Glasgow, London, and Newcastle-upon-Tyne), with three different sampling clusters in each centre, broadly in accord with WHO guidance.3 Multicentre ethics approval was obtained (reference 09/H0720/47). Girls aged 14–15 years attending secondary schools were invited to participate. Members of the survey team made a presentation to participants about
Results
810 schoolgirls participated in the survey, of whom 737 provided urine samples. Questionnaire data were available for 664 of these 737 participants. 73 participants provided questionnaire data only. 30 tap water samples were obtained to establish iodine concentration in drinking water in participating centres.
The median urinary iodine excretion in our sample was 80·1 μg/L (IQR 56·9–109·0), for which the 95% CI was 76·7–83·6 μg/L, indicating that the sample size was sufficiently large to
Discussion
Our findings suggest that the UK is now iodine deficient (panel). Endemic goitre associated with iodine deficiency was at one time widespread in the UK.9 In 1924, a national survey of 375 000 12-year old schoolchildren in England and Wales found a prevalence of visible goitre of up to 30%.14 A 1944 Medical Research Council survey reported visible goitre in 50% of adult women and 26–43% of schoolgirls.15 No salt iodisation programme was adopted in the UK, unlike in other European countries.
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