BACKGROUND
Congenital hypothyroidism refers to children born with hypothyroidism. This condition is uncommon and it affects about 1 child for every 4000 newly born children. However, it was noted that this problem is becoming more common in recent years. There are certain characteristics that are known to be associated with congenital hypothyroidism. For example, congenital hypothyroidism is more common in girls, in children born significantly before their due date, and in twin pregnancies. It can also occur in babies born from mothers with hyperthyroidism, due to either the effects of the medication the mothers received during the pregnancy (antithyroid drugs) or thyroid antibodies in the mother that cross the placenta and affect the baby.

Iodine is important for making thyroid hormones and for normal thyroid function. Iodine in the mother is important for the normal brain development of the baby. However, too much iodine may be harmful to the baby. This study was done to find out if there is a link between the amount of iodine in the mother and hypothyroidism in children from birth to 1 year of age.

THE FULL ARTICLE TITLE
Yokomichi H et 2021 Mother’s iodine exposure and infants’ hypothyroidism: The Japan Environment and Children’s Study. Endocr J. Epub 2021 Aug 26. PMID: 34433732

SUMMARY OF THE STUDY
The authors study over 100,000 children born in different regions of Japan between 2011 and 2014. They carefully collected information regarding the amount of iodine the mothers were exposed to during their pregnancy: from the diet (seaweed) and from two medical sources: a radiological study called hysterosalpinography (HSG) that uses iodine to look at the uterus and tubes before pregnancy and povidone-iodine, a solution that is applied to the mother during delivery in order to kill germs.

To determine congenital hypothyroidism, thyroid blood levels of the newborns were taken about one week after birth. Also, when the children were 1 year of age, the mothers received a questionnaire asking them if a new diagnosis of hypothyroidism was made by their pediatricians.

One in about 700 children were affected by hypothyroidism (about 0.14%). This number is higher than previously reported. Children born prematurely had a higher chance of being hypothyroid at birth (about 1 in 100). Children born to mothers who had received a HSG had less than 1 in 1000 chances of being hypothyroid. Graves’ disease in the mother and treatment with antithyroid drugs had a higher chance of being linked to congenital hypothyroidism, occurring in about 1 in 100 children. Similarly, mothers who were taking levothyroxine, have a higher chance of having children with congenital hypothyroidism at 1.15 in 100. Children born to mothers who took low iodine during the pregnancy had a little over 1 in 1000 chance of congenital hypothyroidism.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study shows that hyper- and hypothyroidism in the mother significantly increases the risk of hypothyroidism at birth, but may resolve. Hypothyroidism can be seen in children born to mothers who were exposed to povidoneiodine or did not have enough iodine in their diet during their pregnancy. However, hypothyroidism in their babies resolved by the age of 1-year-old. Babies born to mothers that received high doses of iodine before pregnancy did not experience higher chances of congenital hypothyroidism. Pregnant women should follow the health organizations’ recommendations of iodine supplementation to their dietary intake by taking a daily prenatal vitamin that contains 150 mcg of iodine.

— Susana Ebner MD

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