Inadequate thyroid secretion in infants and young children, i.e. hypothyroidism, is often characterized by mental retardation, growth retardation and low basal metabolism, etc. It may be manifested by less fetal movement during maternal pregnancy, and may be characterized by overdue birth, birth weight often greater than 90th percentile at birth, as well as delayed expulsion of fetal stools, prolonged recession of physiological jaundice, abdominal distension and umbilical hernia in infancy and childhood. It is often characterized by delayed reaction, later sitting, crawling, walking and speech development than children of the same age, and the patient has delayed closure of the fontanel, poor appetite, reduced movement, and apathetic expression. For breastfed babies in infancy, the manifestation is mostly atypical because breast milk contains thyroxine, and as the baby grows, the manifestation is mostly obvious at the age of 1-2 years and seeks medical treatment.