Hypothyroidism or thyroid cancer may develop in children as a result of exposure to radioactive iodine. Therefore, young children and pregnant women in particular should not be in close physical proximity to patients receiving this therapy. We found only three cases of adolescents developing thyroid cancer in the literature. Two were affected by radioactive iodine treatment of themselves and one was affected by treatment of the mother. Here we report on an 11-year-old girl living in the same environment as her mother, who was receiving radioactive iodine therapy. The girl developed transient hypothyroidism in association with her mother's treatment. We discuss the case in light of the literature.