Radioactive iodine (RAI) is used effectively in the treatment of hyperthyroidism and thyroid cancer, but it is contraindicated during pregnancy. RAI treatment during pregnancy can lead to fetal hypothyroidism, mental retardation and increased malignancy risk in the infant. Pregnancy tests must be performed before treatment in all women of reproductive age. However, at times, RAI is being used before ruling out pregnancy. We herein present a male newborn infant with congenital hypothyroidism whose mother was given a three-week course of methimazole therapy for her multiple hyperactive nodules and subsequently received 20 mCi RAI during the 12th week of her pregnancy. The patient was referred to our neonatology unit at age two weeks when his thyrotropin (TSH) level was reported to be high in the neonatal screening test. Physical examination was normal. Laboratory investigations revealed hypothyroidism (free triiodothyronine 1.55 pg/mL, free thyroxine 2.9 pg/mL, TSH 452 mU/L, thyroglobulin 20.1 ng/mL). The thyroid gland could not be visualized by ultrasonography. L-thyroxine treatment was initiated.