Thyroid cancer is a disease in which malignant (cancer) cells form in the tissues of the thyroid gland. Thyroid nodules are common but usually are not cancer. There are different types of thyroid cancer. Age, gender, and being exposed to radiation can affect the risk of thyroid cancer. Medullary thyroid cancer is sometimes caused by a change in a gene that is passed from parent to child. Signs of thyroid cancer include a swelling or lump in the neck. Tests that examine the thyroid, neck, and blood are used to detect (find) and diagnose thyroid cancer. Certain factors affect prognosis (chance of recovery) and treatment options. Thyroid cancer is a disease in which malignant (cancer) cells form in the tissues of the thyroid gland. The thyroid is a gland at the base of the throat near the trachea (windpipe). It is shaped like a butterfly, with a right lobe and a left lobe. The isthmus, a thin piece of tissue, connects the two lobes. A healthy thyroid is a little larger than a quarter. It usually cannot be felt through the skin.