Seasonal Fluctuation in the Incidence of Congenital Hypothyroidism Across Different Temperate Zones of China.

Background: Seasonality in the incidence of congenital hypothyroidism (CH) has been identified in several countries and different conclusions have been drawn. The objective of this study was to examine whether this seasonality is also observable in China and how it manifests across different temperate zones.

Methods: Data on CH cases and screened neonates between January 1, 2014, and September 30, 2022, by year and season, were sourced from the Chinese Newborn Screening Information System. The overall CH incidence and incidence across different temperate zones was analyzed by using the seasonal unit root test, seasonal decomposition, and deterministic seasonal means regression model.

Results: A total of 29,259 CH cases were reported nationwide from season one of 2014 to season three of 2022. Quarterly CH incidence showed an upward time trend and significant seasonality among all zones, but with different patterns. Overall, season one was the peak period with an incidence rate of 7.09 per 10,000 neonates, whilst season two was the trough period with an incidence rate of 5.89. Subtropical, warm, and medium temperate zones had one peak period in season one, whilst the tropical zone had two peak periods in seasons one and three. In comparison, the plateau zone had a trough season in season one.

Conclusion: Our study found the quarterly CH incidence exhibited clear seasonality, temperate zone-specific patterns, and an upward time trend in China. This finding is particularly concerning given China's decline in the number of births, underscoring the urgency of allocating resources appropriately in screening programs.