Dietary iodine intake in pregnancy: an update.

Iodine deficiency, by depriving the thyroid gland of sufficient raw material to produce thyroid hormones can, in its milder forms, impair cognition in children reducing IQ by up to 10-15 points. 1,2 While the consequences of severe iodine deficiency (population median urinary iodine excretion (UI) < 20 ug/L) can be extreme, including irreversible mental retardation, those arising from more moderate deficiency may take a more subtle form. There is no evidence of a significant level of hypothyroidism or hypothyroxinaemia in Irish or UK mothers or their infants and to date there are no recent studies in Ireland or the UK linking iodine intake to neuropsychological development as has been frequently reported 2 . Interestingly, a 1939 MRC report on iodine deficiency in South Tipperary showed that a higher percentage of schoolchildren with slight or established goitre had what was termed “poor intelligence” 3 . The most recent studies showing borderline iodine deficiency in Ireland 4 have been supported by findings on the iodine status of UK schoolgirls reported in the Lancet by Vanderpump and colleagues 5 . This report was timely, as despite the absence of recent evidence, it had been assumed that the UK population was iodine sufficient. The study carried out on 737 schoolgirls aged 14-15 years found that urinary iodine (UI) concentration, a measure of dietary iodine intake, showed a median of 80.1ug/L with 51% having values < 100μg/L and 17% < 50μg/L. These values are indicative of a mild to moderate iodine deficiency state (WHO) 6 . The lowest UI values came from the Belfast centre where a median value of 64.7μg/L with 31% of values < 50 μg/L were found. The relatively low values from N. Ireland support earlier findings reported in 2006 in first trimester (T1) pregnant women studied in the National Maternity Hospital, Dublin (NMH) 4 in which a median UI of 52 μg/L was observed with 48 % of values indicative of moderate iodine deficiency. Although a more recent study in 20077, found that the median value in T1 pregnant women studied in the NMH was now 79μg/L with the % of values < 50μg/L at 29.3% suggesting the persistence of iodine deficiency. Both the Irish and UK studies showed seasonal variation of UI. In northern Europe milk has been shown to be a major source of dietary iodine intake but agricultural practices arising from climatic conditions result in cattle being brought in from pasture during the winter months and fed on dietary supplements containing iodine. The concentration of iodine in milk and UI are therefore relatively low during the summer months and increased during the winter months. As the fetal thyroid does not develop until 13 15 weeks gestation, neuropsychological development is entirely dependent on maternal thyroid hormone supply. This in turn relies, among other variables, on maternal dietary iodine intake. Although the richest potential dietary sources of iodine come from marine flora and fauna, iodized salt forms a significant source of iodine for people in many countries. However mandatory universal salt iodisation (USI) has not been implemented globally, while in many areas implementation is voluntary. Without salt iodization, the intake of iodine is opportunistic, which in the absence of iodine supplementation results in significant negative maternal iodine balance 2 . The common cause underlying the continuing existence of iodine deficiency in the Irish and UK populations, in contrast to those of other developed countries, is the low availability of iodised salt; < 5% of salt sold is iodised. In contrast, the finding of diminishing population UI values in the absence of adequate iodised salt in Australia and New Zealand has resulted in the introduction of mandatory iodisation of salt in commercially baked bread. 8 In the absence of iodine supplementation, the ability to maintain adequate thyroid hormone production may depend on a woman’s thyroid hormone stores before conception. These in turn reflect long-term dietary iodine intake or previous parity, as it has been shown that multiparous women have larger thyroids than women who have only had one pregnancy. The presence of adequate pre-existing iodine stores may explain why pregnant women in Ireland or the UK display relatively normal thyroid hormone levels despite having daily iodine intakes lower than those recommended by the WHO as indicated by their UI values. Another factor possibly contributing to normal thyroid hormone production in the absence of apparently inadequate iodine intake, may be the ability of the placenta to store iodine. Since it has also been demonstrated that placental iodine storage increases with increased iodine intake, 9 it may be that iodine consumed over the gestation period will be sufficient to meet thyroid hormone requirements for fetal development, even if UI appears to suggest iodine intake below WHO recommendations. There are a number of possible solutions to the problem of inadequate dietary iodine intake particularly in developed countries. The solution of universal salt iodisation recommended by the WHO has the advantage of being a simple intervention which does not require any change in dietary habits, in particular table salt intake, but simply involves the addition of an iodinated substance, usually potassium iodide ( KI) or potassium iodate (KIO3) to table salt at a concentration of 20 to 40 mg/Kg 2 . The population most at risk from iodine deficiency disorders are fetuses, particularly those conceived in the spring or early summer months. However to achieve protection of this cohort, females of child bearing age would require preconception iodine supplementation. The recent report of Moleti et al. 10 supports this view as their findings demonstrated that taking iodine supplements preconception was the most effective in reducing risks of inappropriately low FT4 levels during pregnancy. Such supplementation forms the basis of the USI policy advocated by the WHO. Although USI does not involve increased salt consumption, it could run into presentational problems in light of ongoing campaigns to combat hypertension by reducing salt consumption. Another method would be to use iodised salt in the manufacture of commercially prepared bread as employed in Australia and New Zealand. 9 These methods have the advantage of supplying iodine to the entire population. However if the major at risk group, females of child bearing age subject to non-planned pregnancies, were to be targeted, perhaps the now accepted principle of preconception folate supplementation could be applied to iodine with the potential of providing fetal brain development protection at minimal risk and cost. P Smyth 1 , C O’Herlihy 2 1UCD School of Medicine and Medical Science, Health Sciences Building Lab C3.39,Belfield, Dublin 42Department of Obstetrics and Gynaecology, UCD, National Maternity Hospital, Holles St, Dublin 2 References1. Delange F. Iodine deficiency as a cause of brain damage. Postgraduate Medical Journal 2001; 77:217-220.2. Zimmermann MB. Iodine deficiency. Endocr Rev 2009; 30: 376–408.3. Shee J C (1939) Clinical Hypothyroidism in Endemic Goitre. Effect on Intelligence. Irish J of Med Sci Nov. 1-3.4. Nawoor Z, Burns R, Smith DF, Sheehan S, O’Herlihy C, Smyth PP. Iodine intake in pregnancy in Ireland—a causefor concern? Ir J Med Sci 2006; 175: 21–24.5. Vanderpump MPJ, Lazarus JH, Smyth PP, Boelaert K, Franklyn JA, on behalf of the British Thyroid Association UKIodine Survey Group*et al, on behalf of the British Thyroid Association UK Iodine Survey Group. Iodine status of UKschoolgirls: a cross-sectional survey. Lancet 2011;The Lancet, Volume 377, Issue 9782, Pages 2007 – 20126. WHO, UNICEF (2008) International Council for the Control of Iodine Deficiency Disorders. Assessment of iodinedeficiency disorders and monitoring their elimination, 3rd ed. Geneva, Switzerland:http://www.who.int/nutrition/publications/micronutrients/iodine_deficiency/9789241595827/en/ index.html. World HealthOrganization.7. Smyth P, Burns R & O’Herlihy C. (2010) Changes in dietary iodine intake in the absence of iodised saltavailability. Abstract: 14th International Thyroid Congress, Paris P-0074 http:// www.itc2010.com8. Gallego G, Goodall S, Eastman CJ 2010 Iodine deficiency in Australia: is iodine supplementation for pregnant andlactating women warranted? Med J Aust 192 :461-39. Burns R, Azizi F, Hedayati M, Mirmiran P, O’Herlihy C, Smyth PP 2011. Is placental iodine content related todietary iodine intake? Clin Endocrinol (Oxf). 2011 75:261-410. Moleti M, Trimarchi F, Vermiglio F. Maternal thyroid function in different conditions of iodine nutrition inpregnant women exposed to mild-moderate iodine deficiency: an observational study Clinical Endocrinology 74:762–768. Dietary Iodine Intake in Pregnancy: An Update1 Dietary Iodine Intake in Pregnancy: An Update2