Well-differentiated thyroid carcinoma with concomitant Hashimoto's thyroiditis present with less aggressive clinical stage and low recurrence.

Papillary thyroid carcinoma (PTC) and follicular thyroid carcinoma (FTC) are the most common differentiated thyroid cancers. Previous studies report that Hashimoto's thyroiditis (HT) concomitant with PTC is unusual and improves prognosis compared to classical PTC. Few previous studies address FTC concomitant with HT. In this study, we retrospectively analyzed data from one institution and compared clinical presentations and results of treatment of PTC and FTC with and without HT. In addition, studies comparing presentation and long term follow-up prognosis in classical PTC and FTC were conducted. A total of 1,788 PTC patients and 209 FTC patients underwent thyroidectomy with or without lymph node dissection and follow-up at Chang Gung Medical Center in Linkou, Taiwan. All thyroid carcinomas were pathologically classified according to World Health Organization criteria. Histological patterns of PTC were categorized as classical PTC, or PTC with HT. Follicular thyroid carcinoma patients were categorized as FTC or FTC with HT. The dataset contained a total of 1,703 PTC cases categorized as classical PTC, 85 cases of PTC with HT, 201 cases of FTC and eight cases of FTC with HT. Analysis of Classification of Malignant Tumors (TNM) stage revealed a higher percentage of classical PTC in stage IV than HT group (12.03% vs. 4.70%). Mean tumor size of classical PTC was larger than HT group. Although 42.3% of FTC cases presented with distant metastases, no cases of FTC with HT presented with distant metastasis. Cancer-specific mortality was higher in classical PTC group than in PTC with HT. There was 53.2% of FTC without HT assigned recurrent status, and six of them died of thyroid cancer. No cancer mortality or recurrence in HT with FTC. PTC and FTC with HT presented with better clinical stage and better prognosis after same therapeutic modality. In conclusions, both PTC and FTC with HT have less aggressive clinical presentation and better prognosis.