Background: Hashimoto’s thyroiditis (HT) and papillary thyroid microcarcinoma (PTMC) often coexist. Their relationship is paradoxical and unclear

Materials and Methods: This retrospective study reviews 1,509 thyroidectomies from 2018–2025, focusing on PTMC cases compared with benign thyroidectomy controls to assess the link between PTMC and HT.

Results: 37% of PTMC cases have HT, versus 27% of controls. The odds ratio (OR) for PTMC among HT patients is 1.6 (95% CI: 1.046-2.46; p = 0.03), indicating an increased risk. The risk is higher in euthyroid HT, with 63% prevalence in PTMC vs. 12% in controls. The OR for PTMC in euthyroid HT is 13 (95% CI: 4.53-37.04; p < 0.0001). PTMC occurs at a younger age with HT (40 vs. 44; p < 0.05). HT with PTMC reduces lymph node metastasis (LNM) without affecting tumor size. The tumor immune microenvironment (TIME) differs: the immune-inflamed “hot” subtype is more common in HT, while the immune-desert (ID) subtype is more common in non-HT cases. The “hot” type is immunosuppressive and is associated with increased lymph node metastasis, particularly in non-HT-associated PTC.

Conclusion: Euthyroid HT is associated with increased PTMC risk, likely arising early in chronic inflammation before autoimmunity. The dominant “hot” TIME subtype appears immunosuppressive, suggesting a potential role for immunotherapy in PTC.