Objective: Seasonality in the incidence of congenital hypothyroidism (CH) has been reported in several countries. The objective of this study was to examine whether seasonality was also observable in Japan and whether this seasonality affected genders differently.
Methods: Confirmed primary CH cases (n = 1586) occurring between 1994 and 2003 were examined through a retrospective review of the nationwide follow-up system at the Aiiku Maternal and Child Health Center, Tokyo, Japan. The number of live births in Japan was obtained from the country's vital statistics. Monthly incidence of CH was analyzed using time series analysis. The relationship between monthly incidence and ambient temperature was also investigated.
Main outcome: The sex ratio (female/male) was 1.25 for CH, against a background ratio of 0.95 for overall live births in Japan. Monthly CH incidence showed an upward time trend and clear seasonality among both males and females, but with different patterns between the two genders: from January to December, males had one peak while females had two peaks, which continued for a longer period than males had. A significant statistical correlation was found between the incidence and temperature (p < 0.001).
Conclusion: In Japan, the monthly incidence of primary CH cases exhibits temperature-associated seasonality, gender-specific patterns, and an upward time trend.