Although it is generally acknowledged that exposure to iodine contrast agents can interfere with thyroid function, little is known about the incidence of iodine-induced hypothyroidism in young children (younger than the age of 4 years). This was a retrospective cohort study to estimate the incidence rate of detected hypothyroidism in a US-based general population of pediatric patients exposed to an iodinated contrast agent. The study was conducted in Kaiser Permanente Northern California, an integrated health care delivery system. This study included 2320 pediatric patients younger than 4 years of age who had a diagnostic procedure with an iodinated contrast agent during years 2008 to 2016. Among 2320 young children who met our study criteria, we identified 34 who met the initial criteria to be a case of hypothyroidism. The incidence density ratio for all hypothyroidism in iodine contrast agent-exposed patients was 1.33 per 1000 person months (95% confidence interval, 0.9-1.8). Most cases appeared to have subclinical hypothyroidism. The rate was higher for the probably iodine-induced cases (0.90 per 1000 person months) compared with cases with a possible alternate etiology (0.43 per 1000 person months), for males compared with females, and among children who had a heart catheterization compared with those with a computed tomography scan. It was also highest among the youngest children (younger than 3 months old), and decreased with increasing age. Our finding of hypothyroidism in young children exposed to iodine contrast agents (1.33 per 1000 person months [95% confidence interval, 0.9-1.8]) is broadly consistent with the sparse literature on this outcome.