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Iodine and Thyroid Function During Pregnancy

Rebagliato, Marisa; Murcia, Mario; Ballester, Ferrán

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doi: 10.1097/EDE.0b013e3181d74fa0
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The authors respond:

We appreciate the comments of Gonzalez-Romero et al1 on our work.2 We agree that a moderate fall in free thyroxine levels occurs during pregnancy. Accordingly, we compared thyroid hormone levels stratified by trimester of pregnancy and by area; effects of iodine intake on free thyroxine were estimated separately by area, given its heterogeneity, and adjusted for gestational age and for a wide range of covariates. Therefore, we do not find a clear reason why type I error might be high in our study.

WHO recommended that iodized salt coverage of less than 90% of households should be a basis for recommending iodine supplementation during pregnancy.3 It is noteworthy, however, that WHO also recommended that median urinary iodine concentration (UIC) in the general population could be used to decide the iodine nutrition category for a specific area.3 Recent studies in nonpregnant population in several Spanish regions found urinary iodine concentration within the normal range. Furthermore, the concentration recommended by WHO during pregnancy was estimated by applying the same algorithm derived from studies in nonpregnant population.3 More empirical data are needed to evaluate the correspondence between urinary iodine concentration and iodine intake during pregnancy, because pregnancy-related changes in metabolism are excretion may alter this relationship.4

We acknowledge the concern of Gonzalez-Romero et al1 about the complexity of factors affecting thyroid function. Unfortunately, we could not measure thyroid antibodies, which might have helped to explain the observed effect of supplement intake on thyroid dysfunction.

Iodine supplements have appeared to be safe in the few clinical trials carried out to date,5 but their sample sizes were small and no long-term effects were assessed. In the study by Velasco et al,6 (referenced by Gongalez-Romero1) lower levels of free thyroxine were found in supplemented women, which points in the same direction as our results. The better neurodevelopmental outcomes in the treatment group infants reported in that study6 should be taken with caution given its limitations: it was not a randomized controlled trial; there was no adjustment for confounding variables; and infants from the intervention and control groups were evaluated at different ages.

Finally, we agree that further evidence on the safety and effectiveness of iodine supplementation is needed. At the same time, more efforts should be devoted to assessing and improving the basal iodine nutrition status of women in childbearing ages, rather than relying on systematic iodine supplementation during pregnancy.

Marisa Rebagliato

CIBER Epidemiología y Salud Pública (CIBERESP)

Barcelona, Spain

Department of Public Health Miguel Hernández University

San Juan de Alicante, Spain

[email protected]

Mario Murcia

Ferrán Ballester

CIBER Epidemiología y Salud Pública (CIBERESP)

Barcelona, Spain

Centre for Public Health Research (CSISP)

Unit of Environment and Health, Conselleria de Sanidad

Valencia, Spain


1. Gonzalez-Romero S, Soriguer F, Velasco I. Iodine intake and maternal thyroid function during pregnancy [letter]. Epidemiology. 2010;21:429
2. Rebagliato M, Murcia M, Espada M, et al. Iodine intake and maternal thyroid function during pregnancy. Epidemiology. 2010;21:62–69.
3. WHO Secretariat, Andersson M, de Benoist B, Delange F, Zupan J. Prevention and control of iodine deficiency in pregnant and lactating women and in children less than 2-years-old: conclusions and recommendations of the Technical Consultation. Public Health Nutr. 2007;10:1606–1611.
4. Murcia M, Rebagliato M, Espada M, et al. Iodine intake in a population of pregnant women. INMA mother and child cohort study, Spain. J Epidemiol Community Health 2009 [Epub ahead of print].
5. Zimmermann MB. The adverse effects of mildto- moderate iodine deficiency during pregnancy and childhood: A review. Thyroid. 2007; 17:829–835.
6. Velasco I, Carreira M, Santiago P, et al. Effect of iodine prophylaxis during pregnancy on neurocognitive development of children during the first two years of life. J Clin Endocrinol Metab. 2009;94:3234-3241.
© 2010 Lippincott Williams & Wilkins, Inc.