The biopsy shows well differentiated thyroid tissue, with no cytologic features of papillary thyroid carcinoma. Thyroid differentiation is confirmed by immunohistochemical stains that show expression of thyroglobulin and TTF-1.
This should be considered to be metastatic follicular thyroid carcinoma until proven otherwise. If this patient has no history of previous thyroid surgery, then investigations to identify the site of origin of the tumor are needed. Possible sites include the thyroid gland, sites of potential aberrant localization of thyroid tissue (such as mediastinum), as well as ovarian germ cell tumor.