Herein we present the case of a 9-year-old girl who had an enlarged right lobe of the thyroid gland and sub-clinical hypothyroidism (thyroid stimulating hormone at 9.24 mIU/L). The patient had a history of unintentional exposure to radiation while her mother was receiving radionuclide therapy for diffuse toxic goiter. Ultrasonography of the young girl showed right lobe enlargement with diffuse coarse heterogenous echogenicity, compatible with a microcalcification pattern identified in both lobes of the thyroid gland. Histopathology of the tissue from a thyroidectomy revealed papillary thyroid carcinoma in the right lobe and chronic lymphocytic thyroiditis in the remaining tissue. Molecular pathology demonstrated an RET/PTC1 rearrangement in both tumor and non-tumorous tissue harboring thyroiditis. Considering the history of exposure and the characteristics of the thyroid pathology together, the PTC in this patient was likely a secondary-to-genetic alteration induced by external radiation. This case emphasizes the importance of stringent restrictions when giving radioactive iodine therapy to a patient with small children.