Hashimoto's thyroiditis is negatively associated with lymph node metastasis in PTMC

Purpose The association between Hashimoto's thyroiditis (HT) and lymph node metastasis (LNM) of papillary thyroid microcarcinoma (PTMC) remains poorly understood. We aimed to elucidate the impact of HT on PTMC and its association with LNM.Methods A retrospective cohort study was conducted at a single cancer referral center. Patients diagnosed with PTMC and complete clinicopathological results between January 2013 and June 2018 were included. Propensity score matching (PSM) and logistic regression analysis were performed to evaluate the difference in LNM characteristics between patients with and without HT.Results Among the 9929 PTMC patients, 2389 (24.1%) were pathologically diagnosed with HT. After PSM using variables including age, sex, primary tumor size, central neck dissection, extrathyroidal extension (ETE), gross ETE, multifocality and bilaterality, we identified 2324 pairs of patients for analysis. Patients with HT had a significantly lower incidence of LNM in the central neck (40.9% vs 56.2%, P < 0.001) and lateral neck (11.6% vs 14.2%, P = 0.016), a lower incidence of extranodal extension (ENE) (10.1% vs 17.0%, P < 0.001), fewer positive lymph nodes (median [IQR], 0 [0 to 2] vs 1 [0 to 3], P < 0.001), and a lower lymph node ratio (median [IQR], 0.00 [0.00 to 0.15] vs 0.12 [0.00 to 0.33], P < 0.001) than those without HT. Logistic regression analysis indicated that patients with HT had a significantly reduced risk of CLNM and LLNM compared to those without HT.Conclusions Our study indicated a negative association between HT and LNM in PTMC.