Nodules in Autoimmune Thyroiditis Are Associated With Increased Risk of Thyroid Cancer in Surgical Series But Not in Cytological Series: Evidence for Selection Bias

Background: The association of thyroid cancer and autoimmune thyroiditis (AIT) has been widely addressed, with conflicting results in surgical and cytological series, likely affected by selection bias. Objective: The objective of the study was to evaluate the association between the cytological features suggestive or indicative of malignancy and AIT in 2504 consecutive patients (2029 females and 475 males, mean age 58.3 +/- 14.1 y) undergoing fine-needle aspiration cytology for thyroid nodules. Patients: Based on the clinical diagnosis, patients were divided into four groups: AIT with nodules (N-AIT, 14.9%); nodular Graves disease (N-GD, 2.8%); nodular goiter and negative thyroid antibodies (NGAb-, 68.4%); and nodular goiter with positive thyroid antibodies (NGAb+, 13.9%). Results: The prevalence of patients with cytological features suggestive (Thy4) or indicative of malignancy (Thy5) was 4.5 % in the N-AIT group, not different compared with the other groups (N-GD, 5.6%; NGAb-, 5.0%; NGAb-, 4.3%). No difference was also found in the other categories (Thy2 and Thy3). When the same analysis was performed in the subgroup of patients (14.3%) with a histological confirmation, we found that the prevalence of differentiated thyroid cancer was significantly higher (P = .01) in the N-AIT group (67.8%) compared with the other groups (N-GD, 40.0%; NGAb-, 37.2%; NGAb+, 36.9%). Conclusions: The results of our cytological series do not support a link between N-AIT and thyroid cancer. The association between cancer and N-AIT found in the histology-based series is likely due to a selection bias represented by the fact that the prevalent indication for surgery in the N-AIT group was suspicious cytology (60.7% of patients) more frequently than in the other groups.