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Background Increasing incidence rates of thyroid cancer have been noted worldwide, while the underlying reasons remain unclear. Methods Using data from population-based cancer registries, we examined the time trends of thyroid cancer incidence in two largest cities in China, Shanghai and Hong Kong, during the periods 1973-2009 and 1983-2011, respectively. We further performed age-period-cohort analyses to address the possible underlying reasons for the observed temporal trends. Results We observed continuous increases in the incidence rates of thyroid cancer in Shanghai and Hong Kong, since the 1980s, in addition to higher incidence rates in the 1970s in both sexes in Shanghai. The age-standardized incidence rate of thyroid cancer increased by 3.1% [95% confidence interval (CI): 1.0%, 5.1%] and 3.8% (95% CI: 1.9%, 5.7%) per year on average, respectively, in Shanghai men and women during the period 1973-2009, while it increased by 2.2% (95% CI: 1.5%, 2.8%) and 2.7% (1.6%, 3.8%) per year on average, respectively, in Hong Kong men and women during the period 1983-2011. We observed global changes in trends across all age groups in similar ways, in addition to varied trends across different generations (birth cohorts). Conclusions The increased incidence rates of thyroid cancer in these two Chinese populations during recent decades may be contributable to a combination of the introduction of more sensitive diagnostic techniques and the increasing prevalence of environmental exposures in the populations. [ABSTRACT FROM AUTHOR]