Hashimoto's Thyroiditis and Papillary Thyroid Carcinoma: Is There a Correlation?

Context: The link between Hashimoto's thyroiditis (HT) and papillary thyroid cancer (PTC) has long been a topic of controversy. There are conflicting reports; some suggest that these two are positively correlated, whereas other studies report no relationship. Evidence Acquisition: We performed a systematic literature review of original studies that investigated the correlation between HT and PTC. The two main search engines used to identify articles were OVID Medline and PubMed. The included studies were categorized into a fine-needle aspiration biopsy (FNAB) group and an archival thyroidectomy specimen group. Evidence Synthesis: There is no clear evidence to support the correlation between HT and PTC. Population-based FNAB studies report no linkage, whereas many of the studies of thyroidectomy specimens report a positive relationship. The average prevalence rate of PTC in patients with HT was 1.20% in 8 FNA studies of 18 023 specimens and 27.56% in 8 archival thyroidectomy studies of 9 884 specimens. The relative risk ratio of finding evidence of PTC in HT specimens ranged from .39 to 1.00 in the FNA group (average RR + .69) in contrast to 1.15 to 4.16 from the archival thyroidectomy studies (average RR = 1.59). In addition, there are many studies in the literature that propose a genetic link between HT and PTC involving the PI3K/Akt pathway and RET/PTC gene rearrangements. Conclusion: Although limited by the lack of definitive pathology, population-based FNA studies did not find a statistically significant correlation between HT and PTC. Thyroidectomy studies, which reported a statistically significant positive correlation, are subject to selection bias. More prospective studies with longer follow-up are needed to further elucidate this relationship. (J Clin Endocrinol Metab 98: 474-482, 2013)