This results from loss of thyroid function, usually due to failure of the thyroid gland to develop correctly. The incidence is 1 in 3500 to 4000, which may be higher in Asian families, being 1 in 918 in the north-west of England. Neonates are usually normal at birth. Prior to the establishment of neonatal screening programme, hypothyroidism used to be a major cause of mental retardation. (59 If the condition remains untreated, neonates develop lethargy, difficulty with feeding, constipation, enlargement of the tongue, and umbilical hernia. As the development of the brain and normal growth are dependent upon normal levels of thyroid hormone, congenital hypothyroidism leads to growth retardation and mental retardation. Treatment with thyroxine needs to begin as soon as the diagnosis is made; even a delay of 6 months can lead to significant reduction in the IQ. (6,9
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