Congenital hypothyroidism is a common preventable cause of mental retardation. The overall incidence is approximately 1:4,000. Females are affected about twice as often as males. Approximately 85% of cases are sporadic, while 15% are hereditary. The most common sporadic aetiology is thyroid dysgenesis, with ectopic glands being more common than aplasia or hypoplasia. In untreated patients, symptoms and signs include the decreased activity, large anterior fontanelle, poor feeding, short stature or failure to thrive, jaundice, decreased stooling or constipation, hypotonia, and hoarse cry. The physical findings of hypothyroidism may or may not be present at birth. Signs include the coarse facial features, macroglossia, large fontanelles, umbilical hernia, mottled, cool, and dry skin, developmental delay, pallor, myxoedema, goitre. In patients properly treated, there are no clinical signs.