Follicular Thyroid Carcinoma Arising in a Background of Hashimoto's Thyroiditis.

A prevalent autoimmune condition affecting the thyroid gland known as Hashimoto's thyroiditis (HT) is linked to a higher chance of developing thyroid cancer, especially papillary thyroid carcinoma (PTC) and primary thyroid mucosal-associated lymphoid tissue lymphoma. It is extremely rare for follicular thyroid neoplasms whether adenoma or carcinoma to develop in conjunction with Hashimoto's disease. In this case report, a left thyroid lobe large nodule was diagnosed by US as TIRAD IV. Cytology using fine-needle aspiration was done, and the case was designated as Thy3f (suspicious of follicular neoplasm). The patient underwent total thyroidectomy, gross examination revealed a large capsulated left thyroid lobe nodule and histological examination revealed minimally invasive follicular carcinoma on a background of Hashimoto's disease. The nuclear features of PTC were focal and immunostaining for CK19 was focal and weak. Furthermore, HBME1 IHC was negative and Glypican3 IHC showed focal weak cytoplasmic staining. In conclusion, rather than PTC and lymphoma, HT could coexist with other forms of thyroid neoplasms as follicular carcinoma.