Excess maternal iodine linked to thyroid problems in newborns

Dr Kara Connelly of Oregon Health & Science University, USA, and colleagues reveal the details of three infants with congenital hypothyroidism whose mothers had taken 12.5 mg of iodine daily – more than 11 times the safe upper limit – while pregnant and/or breastfeeding.

"The use of iodine-containing supplements in pregnancy and while breastfeeding is recommended,” noted Connelly. “However, these cases demonstrate the potential hazard of exceeding the safe upper limit for daily ingestion."

Connelly and her colleagues called for increased attention a potential increase in the use of nutritional supplements containing iodine in amounts far higher than the recommended daily allowance during pregnancy: “The use of nutritional supplements is increasing in our society due to the belief that they are healthy and safe and can replace dietary deficiencies with minimal side effects.”

“The mothers of these three infants were ingesting a nutritional supplement whose iodine content far exceeded the daily recommended intake and had elevated iodine levels in urine and breast milk samples.”

Iodine excess

Connelly and her team revealed that the three infants had blood iodine levels 10 times higher than healthy control infants.

They added that although the World Health Organization (WHO) recommends 200-300 µg of iodine daily during pregnancy for normal fetal thyroid hormone production and neurocognitive development, 1,100 µg is considered to be the safe upper limit for daily ingestion by the US Institute of Medicine.

Globally congenital hypothyroidism is most commonly caused by a deficiency of iodine. However excess iodine may also be part of the explanation for the reported increasing incidence in recent years.

However, the team noted that because it is not routine practice to ask mothers of infants with congenital hypothyroidism about all nutritional supplements taken during pregnancy, the over-supplementation of iodine – leading to excess iodine – may be a more common practice than currently presumed.