New study associates excess maternal iodine supplementation with congenital hypothyroidism in newborns
07/26/12 Portland, Ore.
Congenital hypothyroidism is thyroid hormone deficiency at birth that, if left untreated, can lead to neurocognitive impairments in infants and children. Although the World Health Organization recommends 200 to 300 µg of iodine daily during pregnancy for normal fetal thyroid hormone production and neurocognitive development, the U.S. Institute of Medicine considers 1,100 µg to be the safe upper limit for daily ingestion. A case series in The Journal of Pediatrics describes three infants who developed congenital hypothyroidism as a result of excess maternal iodine supplementation.
Kara Connelly, M.D., and colleagues from Oregon Health & Science University Doernbecher Children’s Hospital, Boston University School of Medicine, State of Oregon Public Health Laboratory, and Randall Children’s Hospital at Legacy Emanuel describe three infants with congenital hypothyroidism whose mothers had taken 12.5 mg of iodine daily, 11 times more than the safe upper limit, while pregnant and/or breastfeeding. Iodine is transferred from the mother to the infant through the placenta or breast milk. The three infants had blood iodine levels 10 times higher than healthy control infants (measured from newborn screening filter paper).
Excess iodine causes the thyroid to temporarily decrease function to protect against hyperthyroidism (Wolff-Chaikoff effect). Adults and older children are able to “escape” from this effect after several days of excess iodine to avoid hypothyroidism. However, the immature thyroid glands of fetuses and newborns have not developed this protective effect and are more susceptible to iodine-induced hypothyroidism. Although infants recover normal thyroid function after acute iodine exposure (e.g., a few days of topical iodine application), continuous excessive iodine exposure to the fetal and neonatal thyroid gland may cause long-term harmful effects on thyroid function.
Sources of iodine include nutritional supplements, prenatal vitamins, and seaweed (kelp). According to Dr. Connelly, “The use of iodine-containing supplements in pregnancy and while breastfeeding is recommended in the United States. However, these cases demonstrate the potential hazard of exceeding the safe upper limit for daily ingestion.” Excess iodine ingestion from supplementation is often unrecognized because it is not routine practice to ask mothers of infants with congenital hypothyroidism about nutritional supplements taken during pregnancy. Pregnant or breastfeeding women should discuss the safe dosages of nutritional supplements with their doctors prior to including them in their daily regimen.
NOTES FOR EDITORS
“Congenital Hypothyroidism Caused by Excess Prenatal Maternal Iodine Ingestion,” by Kara Connelly, M.D., Bruce Boston, M.D., Elizabeth Pearce, M.D., David Sesser, David Snyder, M.D., Lewis Braverman, M.D., Sam Pino, Stephen LaFranchi, M.D., appears in The Journal of Pediatrics, DOI 10.1016/j.jpeds.2012.05.057, published by Elsevier.
About OHSU Doernbecher Children's Hospital
OHSU Doernbecher Children's Hospital ranks among the top 50 children's hospitals in the United States.* It ranks 36th nationally for NIH-awarded pediatric research funding among children's hospitals affiliated with an academic medical center**, and is one of only 22 NIH-designated Child Health Research Centers in the country.
OHSU Doernbecher cares for tens of thousands of children each year from Oregon, Southwest Washington and around the nation, resulting in more than 175,000 discharges, surgeries, transports and outpatient visits annually.
Nationally recognized OHSU Doernbecher physicians and nurses provide a full range of pediatric care in the most patient- and family-centered environment, and travel throughout Oregon and southwest Washington, providing specialty care to more than 3,000 children at more than 150 outreach clinics in 15 locations. In addition, OHSU Doernbecher delivers neonatal and pediatric critical care consultation to community hospitals statewide through its state-of-the-art telemedicine network.
* U.S. News & World Report 2012-13 Best Children's Hospitals
** Children's Hospital Association (formerly the National Association of Children's Hospitals and Related Institutions)
About The Journal of Pediatrics
The Journal of Pediatrics is a primary reference for the science and practice of pediatrics and its subspecialties. This authoritative resource of original, peer-reviewed articles oriented toward clinical practice helps physicians stay abreast of the latest and ever-changing developments in pediatric medicine. The Journal of Pediatrics is ranked 4th out of 113 pediatric medical journals (2011 Journal Citation Reports, published by Thomson Reuters).