is dysgenesis of the thyroid gland After many years of newborn screening, the incidence of CH worldwide had been estimated at 1:3600 births, with no obvious increases in specific ethnic groups. Recent data from certain regions have shown an increase, with New York reporting an incidence of 1:1700. Reasons for the apparent increase in incidence have been studied, but no explanation has been found. Classic CAH, the disorder targeted by newborn screening programs, is caused by a deficiency of the enzyme steroid 21-hydroxylase and comes in two forms – simple virilizing and a salt-wasting form. The incidence of CAH can vary greatly between populations. The highest reported incidence rates are among the Yupic Eskimos of Alaska (1:280) and on the French island of Réunion (1:2100). Hemoglobinopathies Any condition that results in the production of abnormal hemoglobin is included under the broad category of hemoglobinopathies. Worldwide, it is estimated that 7% of the population may carry a hemoglobinopathy with clinical significance. The most well known condition in this group is sickle cell disease. Newborn screening for a large number of hemoglobinopathies is done by detecting abnormal patterns using isoelectric focusing, which can detect many different types of abnormal hemoglobins. In the United States, newborn screening for sickle cell disease was recommended for all infants in 1987, however it was not implemented in all 50 states until 2006. Early identification of individuals with sickle cell disease and other hemoglobinopathies allows treatment to be initiated in a timely fashion. Penicillin has been used in children with sickle cell disease, and blood transfusions are used for patients identified with severe thalassemia. Organic acidemias Most jurisdictions did not start screening for any of the organic acidemias before tandem mass spectrometry significantly expanded the list of disorders detectable by newborn screening. Quebec has run a voluntary second-tier screening program since 1971 using urine samples collected at three weeks of age to screen for an expanded list of organic acidemias using a thin layer chromatography method. Newborn screening using tandem mass spectrometry can detect several organic acidemias, including propionic acidemia, methylmalonic acidemia and isovaleric acidemia. Cystic fibrosis Cystic fibrosis (CF) was first added to newborn screening programs in New Zealand and regions of Australia in 1981, by measuring immunoreactive trypsinogen (IRT) in dried blood spots. After the CFTR gene was identified, Australia introduced a two tier testing program to reduce the number of false positives. Samples with