A carcinoma arising from the thyroid gland. It is usually an adenocarcinoma and includes the following main subtypes: follicular, papillary, medullary, poorly differentiated, and anaplastic. A malignant neoplasm that develops or arises in the thyroid gland. Cancer that forms in the thyroid gland (an organ at the base of the throat that makes hormones that help control heart rate, blood pressure, body temperature, and weight). Four main types of thyroid cancer are papillary, follicular, medullary, and anaplastic thyroid cancer. The four types are based on how the cancer cells look under a microscope.