Effects of iodine supplements on maternal thyroid function studied

"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."