Oncocytic and Follicular Thyroid Carcinomas Arising in Hashimoto's Thyroiditis: Two Rare Cases Highlighting an Unusual Association.

Hashimoto's thyroiditis (HT) is strongly associated with papillary thyroid carcinoma (PTC). On the other hand, well-differentiated non-PTC thyroid follicular cell-derived malignancies are exceedingly uncommon in a HT milieu. We report two such cases. Two female patients (aged 36 and 23 years) each presented with a thyroid nodule. In both patients, histopathologic assessment confirmed nPTC - one a minimally invasive oncocytic carcinoma (capsular invasion only, pT2) and the other a conventional minimally invasive follicular carcinoma (capsular invasion only, pT3a) - arising on a background of florid chronic lymphocytic thyroiditis and raised anti-thyroid peroxidase antibodies. No features of papillary differentiation or angioinvasion were identified. Both were marginally completely excised, and subsequent completion thyroidectomy specimens showed no residual malignancy. While HT is a known risk factor for PTC, it may also - albeit rarely - be associated with other well-differentiated thyroid follicular cell-derived malignancies. These two cases exemplify rare occurrences of minimally invasive oncocytic and follicular thyroid carcinomas arising in florid HT. Their recognition is essential to avoid misclassification and continues to challenge assumptions regarding malignancy associated with autoimmune thyroiditis, raising questions about the molecular mechanisms at play.