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Thyroid Autoimmunity and Thyroid Cancer: Review Focused on Cytological Studies

The association between Hashimoto thyroiditis (HT) and papillary thyroid carcinoma (PTC) has been originally suggested by retrospective pathological studies and has recently been re-evaluated and proposed on the basis of several fine-needle aspiration cytology (FNAC) studies. In FNAC studies, the association between HT and PTC is based on the comparison of anti-thyroid autoantibodies (ATA) (anti-thyroperoxidase [TPOAb] and anti-thyroglobulin [TgAb]), thyroid function (TSH), and cytology with histology of thyroid nodules and lymphocytic thyroid infiltration (LTI) of operated thyroid glands. Most of the pathological studies found a high prevalence rate of PTC in HT. In most FNAC studies, the risk ratio of PTC in HT patients was evaluated using multivariate statistical analysis: increased TSH levels represented the main and common independent risk factor of malignancy, although it resulted not consistently related to HT. On the other hand, several studies provided a positive relationship between ATA and PTC, particularly with TgAb. Two recent FNAC studies from the same referral center clearly demonstrated an independent risk for thyroid malignancy conferred by both TPOAb and TgAb, confirming the role of increased TSH levels, and found a significant association between PTC and ATA and diffuse LTI at histology. These studies are consistent with the hypothesis that autoimmune thyroid inflammation and increased serum TSH concentration may be involved in thyroid tumor growth. The complex relationship between HT and PTC, which involves immunological/hormonal pathogenic links, needs to be further investigated with prospective studies.