Overdiagnosis of Papillary Thyroid Cancer.

Importance: The magnitude of thyroid cancer overdiagnosis in the US, accounting for the true increase in underlying disease, has not been quantified. The implications of reducing thyroid ultrasonography use for these rates has not been calculated. Objective: To estimate US population-level rates of papillary thyroid cancer (PTC) overdiagnosis, and to identify the implications of reducing thyroid ultrasonography use for thyroid cancer incidence and overall mortality. Design, Setting, and Participants: This study used a validated simulation model (Papillary Thyroid Carcinoma Microsimulation Model) of contemporary PTC incidence in the adult US population between 1991 and 2019. Data analysis was conducted from June 2024 to August 2025. Exposures: Thyroid cancer incidence and referrals for ultrasonography of nonpalpable thyroid nodules. Main Outcomes and Measures: Model-estimated relative and absolute rates of PTC overdiagnosis, stratified by sex. The model estimated changes in the rates of thyroid cancer incidence and overall mortality if ultrasonography rates were reduced. Results: The simulation included all adults, aged 18 years or older, in the US. Between 1991 and 2019, the model estimated that 72% to 94% of PTC cases were overdiagnosed. While the proportion of overdiagnosis was slightly higher for women compared with men (75%-95% vs 63%-90%), the absolute rate of overdiagnosis was higher among women compared with men (13-17 per 100 000 individuals vs 3-5 per 100 000 individuals). This rate translated into 443 212 to 573 705 women and 107 804 to 154 504 men who were overdiagnosed with PTC over 28 years. Reducing use of thyroid ultrasonography for nonpalpable nodules by 33% and 67% would have decreased PTC incidence by 17% (18 to 15 per 100 000 individuals) and 41% (18 to 11 per 100 000 individuals) in 2019, respectively, a less than 0.1% change in overall mortality. Conclusions and Relevance: This study found that thyroid cancer overdiagnosis in the US remains substantial, even after accounting for possible increases in true incidence. This finding suggests an opportunity to reduce unnecessary thyroid ultrasonography referrals, particularly for nonpalpable nodules, and to reduce unnecessary diagnoses and treatment-related harms without increasing mortality.