The association of Hashimoto thyroiditis and Graves' disease with papillary, follicular, and medullary thyroid cancer has not been comprehensively investigated until now. This comparative clinicopathological study of consecutive patients thyroidectomized at a surgical referral center aimed to explore interdependencies between chronic autoimmune thyroiditis and thyroid cancer. Altogether, there were 852 (58.4%) patients with papillary thyroid cancer, 181 (12.4%) patients with follicular thyroid cancer, and 426 (29.2%) patients with sporadic medullary thyroid cancer, of whom 75 (5.1%) patients also had Hashimoto thyroiditis and 40 (2.7%) patients also had Graves' disease. Patients with papillary (medians of 42 vs. 48 years; P =0.008) and follicular (medians of 33 vs. 63 years; P=0.022) thyroid cancer, unlike patients with medullary thyroid cancer (medians of 57.5 vs. 57 years; P=0.989), were younger at thyroidectomy when they had Hashimoto thyroiditis concomitantly. No such associations were seen with Graves' disease. Primary thyroid cancers tended to be more localized in conjunction with Hashimoto thyroiditis, and less so with Graves' disease, although patterns were not consistent across tumor types. In conclusion, Hashimoto thyroiditis, but not Graves' disease, may be associated with differentiated (papillary and follicular) thyroid cancer but not with medullary thyroid cancer.
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