On the basis of Hashimoto's thyroiditis, to form papillary thyroid carcinoma, metastasized and then to de-differentiate into poorly differentiated squamous cell carcinoma

Thyroid squamous cell carcinoma is very rare. At present, it is limited to case reports. Since the thyroid follicular epithelium is the non-squamous epithelium, how primary squamous cell carcinoma (SCC) of the thyroid occurs is still a controversial issue. Hashimoto’s thyroiditis (HT) is considered to be an independent risk factor for thyroid cancer, under the basis of HT, how tumor cells evolve and develop to papillary thyroid carcinoma (PTC), and particularly to de-differentiate into SCC is elusive. We report a 72-year-old female patient who developed multiple subtypes of PTC on a basis of HT, and finally to de-differentiate into SCC within the local foci of lymph node metastasis. We found that there was a variety of sub-types of PTC in this patient in the background of HT. SCC was found within local lymph node metastasis. Pathomorphology, immunohistochemistry, and molecular pathology have confirmed that the SCC was derived from PTC, and then developed into poorly differentiated SCC and/or anaplastic carcinoma. We also conducted a comprehensive literature review.