"Good iodine nutritional status at the start of and during pregnancy is essential for maintaining proper thyroid function in the mother and encouraging healthy brain development in the foetus and psycho-motor development in the child," says Marisa Rebagliato, lead author of the study and a researcher at the INMA Project, a research network of Spanish groups studying the effects of environmental contaminants during pregnancy and the start of life.

"When women begin pregnancy with sufficient levels of iodine through having previously taken iodine in their diet and iodised salt, the iodine reserves in their thyroid glands are sufficient to ensure proper synthesis of thyroid hormones, and pharmacological supplements are not recommended," says the scientist.

The research, published recently in the journal Epidemiology, evaluated the consumption of iodine in foodstuffs, iodised salt and vitamin supplements, as well as ioduria (iodine content in urine) of 1,844 pregnant women in the Spanish provinces of Guipúzcoa, Valencia, and the city of Sabadell, between 2004 and 2008. The objective was to study the relationship between their iodine nutritional status and thyroid function during the first half of pregnancy.

Contrary to what the authors had expected, pregnant women who consumed 200 μg or more in iodine supplements each day "were at greater risk of having high levels of thyroid stimulating hormones (TSH), indicators of possible thyroid dysfunction."

Out of all the pregnant women, 44% ate iodised salt and 49% took multivitamins containing iodine or specific iodine supplements containing at least 100 μg of iodine. The overall results show that the 'median ioduria, an indicator used to assess iodine nutritional status, was 137 μg/litre, and was within the correct limits for the population at large, although "slightly" below the levels recommended by the World Health Organisation (WHO) for pregnancy (150-249μg/l).

Various studies on the general population have shown a link between high iodine consumption and hypothyroidism. "However, data for pregnant woman are scarce and inconclusive. There is consensus, though, that the risks stemming from iodine deficiency, for the health of both mother and child, are greater than those from risks due to excessive iodine consumption," points out the researcher.

The team stresses a basic message. "Epidemiological monitoring of nutritional iodine status should be carried out on this population before making any automatic recommendations about taking iodine supplements during pregnancy. And above all, people should be encouraged to take iodised salt to ensure they have sufficient iodine levels long before pregnancy."

Adapted from materials provided by FECYT - Spanish Foundation for Science and Technology, via EurekAlert!, a service of AAAS.

Journal Reference:

1.Rebagliato M., Murcia M., Espada M., Álvarez-Pedrerol M., Bolúmar F., Vioque J., Basterrechea M., Blarduni E., Ramón R., Guxens M., Foradada C. M., Ballester F., Ibarluzea J., y Sunyer J. Iodine Intake and Maternal Thyroid Function During Pregnancy :. Epidemiology, 2010; 21 (1): 62 DOI: 10.1097/EDE.0b013e3181c1592b

Abstract

Background: An adequate iodine intake during pregnancy is essential for the synthesis of maternal thyroid hormones and normal brain development in the fetus. Scant evidence is available on the effects and safety of iodine supplementation during pregnancy in areas with adequate or mildly deficient iodine intake. We examined the association of maternal iodine intake and supplementation with thyroid function before 24 weeks of gestation in population-based samples from 3 different areas in Spain.

Methods: A cross-sectional study of 1844 pregnant women (gestational age range 8–23 weeks) was carried out in 3 areas in Spain (Guipúzcoa, Sabadell, Valencia), during the period 2004–2008. We measured levels of free thyroxine and thyroid-stimulating hormone (TSH) in serum, iodine in a spot urine sample, and questionnaire estimates of iodine intake from diet, iodized salt and supplements. Adjusted associations were assessed by multiple linear regression and logistic regression analyses.

Results: There was an increased risk of TSH above 3 μU/mL in women who consumed 200 μg or more of iodine supplements daily compared with those who consumed less than 100 μg/day (adjusted odds ratio = 2.5 [95% confidence interval = 1.2 to 5.4]). We observed no association between urinary iodine and TSH levels. Pregnant women from the area with the highest median urinary iodine (168 μg/L) and highest supplement coverage (93%) showed the lowest values of serum free thyroxine. (geometric mean = 10.09 pmol/L [9.98 to 10.19]).

Conclusions: Iodine supplement intake in the first half of pregnancy may lead to maternal thyroid dysfunction in iodine-sufficient or mildly iodine-deficient populations.