Hypothyroidism in newborns is most likely due to hypothyroidism, but it can also be caused by a low production of thyroid hormones in the newborn’s body. In fact, neonatal hypothyroidism, also known as cretinism, is characterized by delayed resolution of jaundice, abdominal enlargement, delayed defecation, feeding difficulties, low cry, hoarseness, and poor circulation of the extremities. Mental retardation gradually develops with age and is accompanied by short stature. Hypothyroidism is also hereditary. This disease should be treated with oral medication as early as possible after diagnosis and lifelong treatment. Thyroxine preparations are chosen to help maintain the level of thyroid hormone production in newborns. It is important to avoid interruptions in the course of treatment and to adhere to it. The prognosis is good in patients treated promptly with levothyroxine sodium.