Breastfeeding After Maternal Iodine Contrast: Neonatal Hypothyroidism Revealing an Underlying Congenital Disorder

Abstract Background: Iodine plays a critical role in producing thyroid hormones essential for brain development. Both iodine deficiency and excess can disrupt thyroid function, particularly in neonates. Iodine-induced hypothyroidism is rare but has been reported, particularly in premature infants exposed to excess iodine. Currently, Iohexol, a nonionic radiocontrast agent, has limited data but is considered compatible with breastfeeding. This is a case of a small for gestational age infant that was found to have hypothyroidism after ingesting breastmilk from a mom who was given IV Iohexol. Case Presentation: A small for gestation neonate was delivered at 36 weeks and 3 days of gestation and admitted to the neonatal intensive care unit for respiratory distress and prematurity. Newborn Screens done at admission and repeated after 48 hours of life were negative for thyroid abnormalities. At 5 days of life, the mother underwent a contrast-enhanced imaging study using iohexol, after which the infant received breast milk from days 5-11. On day 11, the neonate had an elevated thyroid-stimulating hormone (TSH) and low free thyroxine (T4) levels, consistent with hypothyroidism. Urine iodine level in the infant was checked and found to be elevated, which prompted the medical team’s concern for the exposure to iodine in breastmilk. However, the thyroid levels in the infant remained abnormal, which was unexpected from a transient process such as exposure to iodine. Genetic testing revealed missense variants in the Iodide Deiodinase Deficiency (IYD) gene, which are associated with thyroid dyshormonogenesis, as well as a likely pathogenic intragenic deletion in the gene RPS6KA3, which is consistent with Coffin–Lowry syndrome. Conclusions: This case highlights the complex interplay between genetic factors and environmental influences in the development of neonatal hypothyroidism. While iodine contrast exposure through breast milk is generally considered safe, this case underscores the potential risks in preterm infants. Initially, iodine exposure through breastmilk was considered a contributor to thyroid dysfunction, but with further time and evaluation, congenital thyroid dysgenesis with underlying genetic findings complicated the diagnosis. Caution should be taken for iodine-exposed breastmilk, but because iodine is only one potential cause of neonatal hypothyroidism, persistent thyroid abnormalities warrant a complete congenital hypothyroidism work-up to assess for alternative or coexisting etiologies.