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Consequences of the Use of Low Blood-spot Thyroid-stimulating Hormone Cut-offs for the Neonatal Screening of Congenital Hypothyroidism

Published Online: June 6th 2011 European Endocrinology, 2009; 5:64-6; DOI: http://doi.org/10.17925/EE.2009.05.00.64
Authors: Luca Persani, Davide Calebiro
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Abstract
The consequences of using low blood-spot thyroid-stimulating hormone (b-TSH) cut-off values for newborn screening of congenital hypothyroidism (CH) are largely unknown. Therefore, the impact on CH epidemiology and classification generated by the introduction in our Italian region of a low b-TSH cut-off during 1999–2005 was retrospectively examined. This work was recently performed in collaboration with the Laboratory for Neonatal Screening and the Principal Follow-up Centre of the Lombardy region. The incidence of CH in this Italian population was 1:1,446 live births, with a predominance of functional over morphogenetic defects. The use of low b-TSH cut-offs allowed the detection of an unsuspected number of children with neonatal hypothyroidism, evolving to mild permanent thyroid dysfunction later in life. Premature birth was associated with a three- to five-fold increased risk of CH with gland in situ.

Keywords
Newborn screening, congenital hypothyroidism, thyroid-stimulating hormone, thyroid dyshormogenesis, thyroid dysgenesis, premature birth
Disclosure: The authors have no conflicts of interest to declare.
Received: 25 May 2009 Accepted: 21 July 2009
Correspondence: Luca Persani, Department of Medical Sciences, University of Milan, Laboratory of Experimental Endocrinology, IRCCS Istituto Auxologico Italiano, Via Zucchi, 18, 20095 Cusano, Milan, Italy. E: luca.persani@unimi.it

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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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