Random variability in congenital hypothyroidism from thyroid dysgenesis over 16 years in Quebec

Context: Research on the etiology of congenital hypothyroidism from thyroid dysgenesis (CHTD) (comprising mostly ectopy and agenesis) over the past decade has focused on genetic mechanisms. However, the possibility that environmental factors might be involved has been raised by studies showing a seasonal variability of the incidence of CHTD. Objectives: The objective of this study was to assess the variability in incidence of CHTD in the province of Quebec, Canada. Design, Setting, Patients, and Main Outcome Measure: The Quebec provincial newborn screening database was analyzed from January 1990 to December 2005. Only cases of permanent congenital hypothyroidism with thyroid ectopy or agenesis on scintigraphy were analyzed. Results: During the study period, 1,303,341 children were screened, and 424 cases of permanent congenital hypothyroidism were diagnosed, giving an overall incidence of 1: 3074. Of these, 306 had CHTD (overall incidence 1: 4259) from either ectopy (n = 231) or agenesis (n = 75). Over the 16 yr of the study, this incidence remained stable (P = 0.57), and no significant variability in monthly incidence was found (P = 0.87). Conclusions: The incidence of CHTD did not vary over the observation period, and its monthly variation was random. Therefore, environmental factors do not appear to play a significant role in the etiology of CHTD.