The increasing incidence of thyroid cancer may be an artifact of increased diagnostic scrutiny, permitting detection of smaller, subclinical thyroid cancers. Our objective was to examine trends in the incidence of well-differentiated thyroid cancers with large size and adverse pathological features.

Detailed population-based analysis of incidence trends in well-differentiated thyroid carcinoma (1973–2006) in the Surveillance Epidemiology and End Results (SEER) cancer registry, using weighted least squares and Joinpoint regression models.

The incidence of well-differentiated thyroid cancer (WDTC) in the United States has tripled since 1973 (P < .0001). Incidence trends differ significantly between geographic regions and racial groups. Large WDTCs, including those >4 cm or >6 cm, have more than doubled in incidence (P < .0001). Cancers with extrathyroidal extension and with cervical metastases have also more than doubled in incidence (P < .0001).

While the model of improving screening does explain increased diagnoses of small thyroid cancers, significant rises in the incidence of large cancers, and cancers with clinically significant pathological adverse features, are harder to explain. Alternative hypotheses, including a true increase in cancer incidence, would seem to merit exploration.