[Pathology of benign thyroid tumor]

True benign neoplasm of the thyroid gland is only follicular adenoma, which is a tumor derived from follicular cells. Follicular adenoma is well-circumscribed, with no evidence of capsular or vascular invasion. The architectural pattern of follicular adenoma varies from trabecular to macrofollicular, and in most instances more than one architectural pattern is observed in one tumor. Adenomatous goiter, a hyperplastic lesion of follicles, is the most common tumorous lesion of thyroid gland. The gland is distorted with a nodular surface. The hyperplasia is followed by involution of the follicles leading to large follicles of varying size. Mature or immature teratoma is also observed in the thyroid gland. Mechanical implantation and parasitic nodule should not be misdiagnosed as lymph nodes harboring metastatic carcinoma.