A minority of newly detected thyroid cancer cases were diagnosed because of symptoms. Access to medical care and factors related to cancer aggressiveness were associated with how diagnoses occurred. The likelihood of diagnosing the lowest-risk thyroid cancers was higher in situations related to management of other thyroid conditions. Adherence to thyroid management guidelines could reduce some thyroid cancer over-diagnosis, but ultimately better diagnostic tools are needed to differentiate between indolent cancers and those of clinical significance.