It is the purpose of this article to briefly review the initial development and subsequent evolution of newborn screening programs to detect infants with congenital hypothyroidism (CH) and then to provide an update of the advantages and disadvantages of the main test strategies. Pilot programs began screening newborn populations in North America in the mid-1970s using either primary thyroxine (T4)-follow-up thyroid stimulating hormone (TSH) or primary TSH testing. Many programs in the United States and around the world continue to prefer a primary T4-follow-up TSH test strategy. This approach has the advantage of detecting infants with primary CH, as well as cases of hypopituitary hypothyroidism, by follow-up of infants with a T4 below an absolute cutoff or with a persistently low T4 level, necessitating a higher recall rate. With increasing assay sensitivity and specificity, several programs in the United States and worldwide have elected to switch to a primary TSH test strategy. This test strategy has the advantage of detecting primary CH and subclinical hypothyroidism and at a lower recall rate. Programs considering switching to a primary TSH test strategy need to develop age-related TSH cutoffs to maintain an acceptable recall rate. Both test strategies have the potential to detect infants with CH characterized by "delayed TSH rise," but only if they collect a routine or discretionary second specimen, now recommended in low-birth-weight and acutely ill infants. Lastly, a lower TSH cutoff appears to be one of the explanations for the recently described increased incidence of CH.
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