Congenital hypothyroidism (CH) is the most common congenital endocrine disease and avoidable cause of severe mental retardation. L-thyroxine supplementation started by two to three weeks of age can prevent severe neurological damage. Thus, in economically advanced countries, neonatal screening programmes have been instituted to allow early CH detection and initiation of therapy.
Congenital hypothyroidism (CH) is the most common congenital endocrine disease and avoidable cause of severe mental retardation. L-thyroxine supplementation started by two to three weeks of age can prevent severe neurological damage. Thus, in economically advanced countries, neonatal screening programmes have been instituted to allow early CH detection and initiation of therapy.
In the mid-1970s, a newborn screening programme for CH was started in Quebec and rapidly developed in other countries. Two principal screening strategies have been followed: a primary thyroid-stimulating hormone (TSH) method, more common in Europe, Japan and Oceania, and a primary T4 method, more common in North America. The use of these strategies has allowed the early detection of a larger number of CH cases, with a currently reported incidence of 1:3,000–4,000 newborns. A recent European survey on about 6 million newborns, mostly screened using a primary TSH method, reported an overall incidence of 1:2,709.In 1987–2003, the Italian CH Registry reported a national incidence of 1:2,500 out of about 7,520,000 live newborns.
The current understanding of CH indicates that morphogenetic defects (athyreosis, ectopy, hemiagenesis or hypoplasia) account for about 75% of total cases. The remainder have a thyroid gland in situ (GIS) that may be associated with either transient or permanent functional defects. This classification is based on the experience with screening programmes using primary T4 determination or TSH cut-off values of 20–40mU/l in the dry blood-spot. These strategies have been followed mainly in order to avoid excessive recall rates and limit costs, and were justified by the general assumption that milder CH forms are devoid of neurological consequences. However, definitive proof for this hypothesis is lacking.
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