Congenital hypothyroidism (CH) is a condition in which a deficiency of thyroid hormones is present at birth. It is the most common endocrine disorder in children that causes mental retardation. The prevalence of CH in China is about 1 in 2000, with more females than males and a female/male ratio of 2:1. Since CH in newborns can be asymptomatic or mild, population screening of newborns is an important tool for early diagnosis of congenital hypothyroidism. Since the beginning of CH newborn screening in China, more than 50% of newborns nationwide are screened for hypothyroidism each year. Clinical manifestations Most congenital hypothyroidism is born with mild or no clinical symptoms, which is related to the protective effect of maternal thyroid hormones through the placenta. The clinical signs and symptoms of neonatal hypothyroidism are atypical, and most symptoms are mild. Clinical signs include overdue delivery, gigantism, hoarse cry, lethargy, feeding difficulties, constipation, delayed jaundice, low muscle tone, hypermobility, unclosed fontanelle, low nasal bridge, perinasal and perilabial cyanosis, giant tongue, hairiness, umbilical hernia, dry, florid skin, poor peripheral circulation, and in a few cases, goiter. Treatment The timing of treatment initiation, dosage and maintenance up to 2-3 years of age are closely related to the final level of intelligence of the child. Treatment with levothyroxine (L-T4) is usually used to bring blood FT4 and TSH back into normal range as soon as possible. Treatment should be started as early as possible. The initial L-T4 treatment dose of 10-15 μg/kg.d (about 37.5 to 50 μg/d) should bring FT4 and TSH to normal range after 2 weeks of treatment. Based on newborn screening results newborns with more than 2 clinical manifestations, or with delayed ossification centers in the distal femur and significantly elevated TSH values on dried blood filter smear (initial TSH screening values above 30 mU/L) should be started on L-T4 therapy immediately without waiting for the results of venous blood tests. Treatment should also be started immediately if thyroid ultrasound reveals an absent or dysplastic thyroid gland. Screen-positive newborns who do not meet these criteria should wait for the results of the venous blood test before deciding whether to give treatment. For patients with normal FT4 and TSH above 10 mU/L after 2 weeks of life, most pediatric endocrinologists believe they should be treated. For those who do not receive treatment, FT4 and TSH should be rechecked every 2-4 weeks to observe the trend of changes, and if FT4 or and TSH are still abnormal then treatment should be started as soon as possible. The management plan for infants with TSH consistently maintained at 6-10 mu/l within the first month of life is still controversial, and close follow-up of thyroid function is required for infants with this condition. Regular daily dosing is required. For small infants, L-T4 tablets should be crushed and taken in a spoon with a little water or milk, not in a bottle, and avoid concomitant administration with foods or medications that may reduce LT4 absorption, such as soy milk, iron, calcium, cholestyramine, fiber, and aluminum thioglycollate. Subsequent follow-up L-T4 maintenance doses are individualized and adjusted according to blood FT4 and TSH concentrations. Blood FT4 should be maintained within the mean to upper normal range, and TSH should be maintained within the normal range. Lifelong treatment is required for those with thyroid dysplasia and ectopic. Other children can stop the drug for 1 month in 2-3 years of regular treatment, and if TSH is increased or accompanied by reduced FT4, L-T4 should be given for life; if the thyroid function is normal for temporary hypothyroidism, stop the drug and follow up regularly, some patients will have TSH re-elevation. If TSH re-elevation occurs, treatment needs to be started again.