What is thyroid cancer?

Thyroid cancer is a malignant neoplasm that develops from thyroid cells. In the normal state, new thyroid cells are constantly being produced in a regulated way to replace cells that die off. In the course of this process, certain cells may become abnormal, escaping from cycles of growth. When these abnormal cells continue to multiply and expand in an uncontrolled fashion, they become a tumor or neoplasm ("neo" = new, "plasm" = growth). If a neoplasm grows but remains contained within it without spreading outward to invade normal tissue, it is considered to be benign. If a neoplasm grows and spreads outward to invade normal tissue, it is considered to be malignant, in other words, a cancer.