Thyroid epithelial cells can be classified into 2 types according to embryonic origin, those of foregut endoderm (follicular cells) and those of neuroectodermal deviation (C cells) origin. The production of TG, T4, and T3 is frequently evident with the immunoperoxidase methods is nontoxic thyroid tumors derived from follicular cells. In contrast, medullary thyroid carcinoma originated from thyroid C cells produces and secretes calcitonin. Thyroid transcription factor-1 (TTF-1) regulates the expression of all hormones produced in thyroid. In recent years, evidence has been accumulating for the existence of a tumor group with traits of both follicular and medullary carcinoma, designated as mixed follicular and medullary carcinoma.