Pathology and cytologic features of thyroid neoplasms.

Most thyroid neoplasms arise from follicular cells and are well differentiated. Anaplastic and poorly differentiated carcinomas are rare and have a high mortality. Five percent of tumors are of C-cell origin, and 20% to 25% of these are hereditary. Thyroid lymphoma is rare and occurs in the setting of Hashimoto's thyroiditis. Fine needle aspiration biopsy is the best diagnostic tool to classify thyroid neoplasms.