Prevalence of Hashimoto Thyroiditis in Adults With Papillary Thyroid Cancer and Its Association With Cancer Recurrence and Outcomes.

Importance: Hashimoto thyroiditis (HT) has been suggested to be associated with papillary thyroid cancer (PTC) development. However, its association with PTC progression remains unclear. Objective: To examine the association between HT and PTC presentation and outcomes. Design, Setting, and Participants: This retrospective cohort study included a review of patients aged 18 to 75 years who had pathologically confirmed PTC treated at a single center in China from January 1, 2001, to December 31, 2014. Data analysis was performed from November 1 to December 31, 2020. Exposures: Coexistent HT was defined according to evaluation of postoperative paraffin sections. Main Outcomes and Measures: The primary outcome was the association of HT with PTC-related mortality, assessed using Cox proportional hazards regression models. The secondary outcome was the association of HT with aggressive characteristics and structural recurrence of PTC, assessed using logistic regression and Cox proportional hazards regression with and without adjustment for related factors. Results: Of 9210 patients with PTC (mean [SD] age, 43.6 [12.0] years; 6872 [75%] women) included in the analysis, 1751 (19%) had HT. In the logistic regression model, HT was negatively associated with frequencies of primary tumor size of 4 cm or greater (adjusted odds ratio [aOR], 0.20; 95% CI, 0.12-0.33; P < .001), gross extrathyroidal extension (aOR, 0.44; 95% CI, 0.36-0.54; P < .001), extranodal extension (aOR, 0.66; 95% CI, 0.55-0.80; P < .001), and distant metastasis (aOR, 0.17; 95% CI, 0.04-0.71; P = .02). After a median follow-up of 85 months (range, 12-144 months), 131 PTC-related deaths were identified in the cohort; 2 patients who died had HT. Patients with HT had significantly superior outcomes compared with patients without HT in terms of unadjusted 10-year disease-specific survival (99.9% vs 96.6%; log-rank P < .001) and recurrence-free survival (92.0% vs 87.6%; log-rank P = .001). After adjusting for sex, age, primary tumor size, extrathyroidal extension, lymph node metastasis, distant metastasis, extent of surgery, and radioactive iodine ablation, HT was associated with decreased PTC-related mortality (hazard ratio [HR], 0.19; 95% CI, 0.05-0.76; P = .02). Stratified analysis showed that HT was associated with less frequent structural recurrence in patients with extrathyroidal extension (HR, 0.52; 95% CI, 0.38-0.71; P < .001; P = .002 for interaction) or after total thyroidectomy (HR, 0.50; 95% CI, 0.35-0.69; P < .001; P = .009 for interaction). Conclusions and Relevance: In this cohort study, patients with coexistent HT had less aggressive characteristics at presentation and better outcomes of PTC than did patients without HT. The findings suggest that autoimmune thyroiditis has a protective role in association with thyroid cancer.