Iodine Exposure in NICU Linked to Increased Risk of Hypothyroidism

July 7, 2020

Results of an NIH-led study suggests infants exposed to iodine in the NICU were at a greater risk of developing congenital hypothyroidism.

New data from a National Institutes of Health (NIH)-led study suggests exposure to iodine in neonatal intensive care units (NICU) could be linked to increased risk of congenital hypothyroidism.

The study, which was performed by investigators at the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development, concluded increased iodine exposure in NICUs was linked to increased risk of congenital hypothyroidism and suggested clinicians should monitor iodine exposure in NICUs.

"Limiting iodine exposure among this group of infants whenever possible may help lower the risk of losing thyroid function," said lead investigator, James L. Mills, M.D., of the Epidemiology Branch at NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development, in a statement.

With previous studies detailing an association between iodine excess and deficiency during pregnancy linked to the development of congenital hypothyroidism, Mills and a team of NIH physicians sought to determine whether iodine status was associated to congenital hypothyroidism in newborns using data from the California Newborn Screening program. With data from the program, the team of investigators designed the current analysis as a nested case control study.

Briefly, the California Newborn Screening program began in 1966 and now collects information related to 80 different genetic and congenital disorders. Using a range from January 2010-March 14, 2015, investigators identified approximately 2.5 million live births. Matching cases with controls by birth month and year, a total of 907 infants with congenital hypothyroidism and 909 controls were identified for inclusion in the study.

Of note, infants with congenital hypothyroidism were significantly more likely to be female, Hispanic, and post-term than controls. These infants were also significantly more likely to have been treated in a NICU.

The median iodine concentration was 20.0 ng/mL in cases and 20.3 ng/mL (IQR, 12.5 – 30.9 ng/mL) in controls (P=.59). Results of the analysis indicated neither very high or very low blood iodine was associated with an increased risk of congenital hypothyroidism when examining the entire study population.

Additional analyses, which examined 114 cases of congenital hypothyroidism and 69 controls, suggested increased levels of iodine among infants treated in a NICU had an increased risk of congenital hypothyroidism, even after adjustment for race/ethnicity, sex, gestational age, nursery type, age at screening, mother’s age, and birth weight.

Investigators pointed out infants with congenital hypothyroidism and a NICU stay were more likely to have a higher blood iodine concentration than those with the condition who did not have a NICU stay.

Investigators highlighted multiple limitations could have impacted the results of their study. These limitations included inability to obtain long-term follow-up data, inability to record long-term history of exposure to drugs or dietary factors, and imposed limits to medical and demographic data to protect confidentiality.

“We expanded on previous reports suggesting that infants treated in a NICU may be at greater risk for congenital hypothyroidism,” investigators wrote. “Showing for the first time that congenital hypothyroidism cases in the NICU had significantly higher blood iodine concentrations than controls. Our results provide some assurance that neither iodine deficiency nor excess during pregnancy is an important contributor to congenital hypothyroidism.”

This study, “Iodine status and congenital hypothyroidism,” was published in the Journal of Nutrition.