Transient neonatal hypothyroidism due to maternal seaweed soup ingestion: a report of 3 cases.

Seaweed soup (miyeok-guk) is traditionally consumed daily by Korean women during pregnancy and postpartum, often resulting in maternal iodine intake exceeding 2000 µg/day, which can induce neonatal hypothyroidism (NH) in breastfed infants. Neonatal hypothyroidism refers to transient thyroid dysfunction from extrinsic iodine exposure, distinct from congenital hypothyroidism (CH) due to intrinsic thyroid defects. We present 3 infants of Korean descent with NH associated with maternal seaweed soup consumption. Case 1 is a preterm infant with severe hypothyroidism resolving rapidly after maternal discontinuation of seaweed soup, with levothyroxine discontinued at 6 months. Case 2 presented with subclinical hypothyroidism on day of life 11 and an atypical course requiring dose adjustments before proving transient at age 3.5 years. Case 3 had severe hypothyroidism at birth and was treated for 6 years as presumed CH; however, levothyroxine dose remained unchanged despite significant growth, and therapy was discontinued after maternal dietary history was obtained, confirming transient disease. These cases demonstrate that initial thyroid-stimulating hormone severity poorly predicts permanence and that levothyroxine dose trajectory is more reliable. Culturally sensitive history-taking and systematic reevaluation at age 3 can prevent unnecessary lifelong therapy.