Elsevier

The Lancet

Volume 377, Issue 9782, 11–17 June 2011, Pages 2007-2012
The Lancet

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Iodine status of UK schoolgirls: a cross-sectional survey

https://doi.org/10.1016/S0140-6736(11)60693-4Get rights and content

Summary

Background

Iodine deficiency is the most common cause of preventable mental impairment worldwide. It is defined by WHO as mild if the population median urinary iodine excretion is 50–99 μg/L, moderate if 20–49 μg/L, and severe if less than 20 μg/L. No contemporary data are available for the UK, which has no programme of food or salt iodination. We aimed to assess the current iodine status of the UK population.

Methods

In this cross-sectional survey, we systematically assessed iodine status in schoolgirls aged 14–15 years attending secondary school in nine UK centres. Urinary iodine concentrations and tap water iodine concentrations were measured in June–July, 2009, and November–December, 2009. Ethnic origin, postcode, and a validated diet questionnaire assessing sources of iodine were recorded.

Findings

810 participants provided 737 urine samples. Data for dietary habits and iodine status were available for 664 participants. Median urinary iodine excretion was 80·1 μg/L (IQR 56·9–109·0). Urinary iodine measurements indicative of mild iodine deficiency were present in 51% (n=379) of participants, moderate deficiency in 16% (n=120), and severe deficiency in 1% (n=8). Prevalence of iodine deficiency was highest in Belfast (85%, n=135). Tap water iodine concentrations were low or undetectable and were not positively associated with urinary iodine concentrations. Multivariable general linear model analysis confirmed independent associations between low urinary iodine excretion and sampling in summer (p<0·0001), UK geographical location (p<0·0001), low intake of milk (p=0·03), and high intake of eggs (p=0·02).

Interpretation

Our findings suggest that the UK is iodine deficient. Since developing fetuses are the most susceptible to adverse effects of iodine deficiency and even mild perturbations of maternal and fetal thyroid function have an effect on neurodevelopment, these findings are of potential major public health importance. This study has drawn attention to an urgent need for a comprehensive investigation of UK iodine status and implementation of evidence-based recommendations for iodine supplementation.

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.

Section snippets

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.

References (32)

  • IDD Newsletter. Volume 29, No 3

  • DIW Phillips

    Iodine, milk, and the elimination of endemic goiter in Britain: the story of an accidental public health triumph

    J Epidemiol Community Health

    (1997)
  • T Ohashi et al.

    Simple microplate method for determination of urinary iodine

    Clin Chem

    (2000)
  • Z Nawoor et al.

    Iodine intake in pregnancy in Ireland—a cause for concern?

    Ir J Med Sci

    (2006)
  • C Mian et al.

    Iodine status in pregnancy: role of dietary habits and geographical origin

    Clin Endocrinol (Oxf)

    (2009)
  • P Stocks

    Goitre in the English school child

    Q J Med

    (1928)
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