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Background: The increasing incidence of thyroid cancer has been described worldwide. Overdiagnosis, improved imaging, and increased environmental risk factors have contributed to the rising incidence. The objective of this study was to analyze the population incidence rate and trends during the period of 2000–2013 in children, adolescents and young adults (AYAs) in Brazil. Methods: Data were extracted from 11 population-based cancer registries (PBCRs) encompassing the five geographic regions of Brazil. Incidence rates per million in children (0–14) and AYAs (15–39) according to world population were analyzed according to sex, age, and type of carcinoma. Incidence trends were evaluated using joinpoint regression. Results: During 2000 to 2013, we identified 11,081 children and AYAs (0–39 years) with thyroid carcinoma in 11 PBCRs, with an age-adjusted incidence rate (AAIR) of 42 cases per million. Females had a higher AAIR of 66 cases per million versus 14 cases per million in males. Age-specific incidence rate (ASR) increased with age. Geographic variation was also observed; the Midwest and Southeast regions had the highest ASR in all age groups. The lowest ASR in all age groups was seen in the North region. Papillary subtype was the most common. Overall, the incidence rates in children and AYAs significantly increased from 0.2 in 2000 to 2.8 in 2013 and from 47.1 to 115.3, respectively, with an annual average percent change of 18.8 [8.1; 30.6] for children and 7.9 [CI 5.6; 10.3] for AYAs. Conclusions: Rates of thyroid cancer, particularly the papillary subtype, are steadily increasing in children and AYAs, especially among females. There are variations among geographic areas. This increased incidence is unlikely to be explained by screening, as children less than 14 years of age do not typically undergo medical surveillance. Environmental risk factors must be investigated. [ABSTRACT FROM AUTHOR]