No increase in commonest preventable cause of intellectual disability over 20 years

A new study that was prompted by recent reports of an increase in cases of congenital hypothyroidism in the United States, and aimed at assessing the incidence of this condition among Quebec newborns, suggests that the increase is entirely artifactual. CH is characterized by inadequate thyroid hormone production and is the most common cause of preventable intellectual disability. The results were published in the Journal of Clinical Endocrinology and Metabolism by Dr. Johnny Deladoëy, a pediatric endocrinologist and researcher in metabolic and genetic diseases at CHU Sainte-Justine Research Center and an assistant professor of pediatrics and biochemistry at University of Montreal.

The increase in the number of cases identified may be an artifact due to changes in screening methods and results in the treatment of some children who may, in fact, not benefit from treatment. "Overly sensitive screening methods that identify mild cases of CH represent an obvious shift in neonatal screening from its original purpose, which was to identify severe cases in which the benefits from treatment would be clearly documented", Deladoëy said. "This might explain why, in the United States, more than a third of children labelled as having CH on the basis of neonatal screening no longer receive treatment after four years of age."

The lack of consensus about the thresholds on which to base detection has always presented a challenge for CH screening. Quebec pioneered universal screening for CH in newborns, being the first jurisdiction on the globe to initiate such a program in 1973. Quebec strategies and methodologies have proven to deliver reliable results and may serve as a model in the standardization of CH screening and diagnosis worldwide.

Thanks to a contribution rate of more than 90 % of hospitals and physicians to a provincial database over a period of 20 years, the results from the detection and diagnosis of 620 cases of CH could be analyzed.

Historical data were analyzed for the study jointly with the Québec newborn blood screening laboratory located at the Centre hospitalier universitaire de Québec (CHUQ). More precisely, of the 1,660,857 newborns tested over the 20-year period, 620 cases of CH were detected, diagnosed and treated. During the second half of the period, a slightly more sensitive detection threshold was applied – in addition to those expected, 49 further cases of CH were detected, representing an 18 % increase. Most of these additional cases had mild functional disorders whose impact on mental development is uncertain. Most importantly, the number of detected cases would have remained stable throughout the period studied if the screening threshold had been unchanged.

The study was published ahead of print on June 1, 2011 in the Journal of Clinical Endocrinology and Metabolism by Dr. Johnny Deladoey of the CHU Sainte-Justine Research Center and University of Montreal, jointly with Dr. Guy Van Vliet, also of the CHU Ste-Justine, and Dr. Jean Ruel and Dr. Yves Giguère, of the Québec newborn blood screening laboratory at the CHUQ.

The study was supported by scholarships of the Fonds de la recherche en santé du Québec and by a fellowship from the Canadian Child Health Clinician Scientist Program. Research in pediatric thyroid diseases at CHU Sainte-Justine is supported by the Girafonds/CHU Sainte-Justine Foundation. The University of Montreal is known officially as Université de Montréal.

CH occurs in approximately one in 3,000 newborns, and its major consequence, intellectual disability, can be prevented through a universal screening test done on blood samples in the first few days of life, which enables initiation of thyroid hormone replacement during the second week of life. The test is considered one of the major public health breakthroughs of the last decades and has now long been implemented in all industrialized countries. Further studies assessing the benefits of the treatment of benign and transient cases need to be undertaken in order to evaluate whether or not they should be the object of screening.