Cancer begins in cells, the building blocks that make up tissues. Tissues make up the organs of the body.

Normal, healthy cells grow and divide to form new cells as the body needs them. When normal cells grow old or get damaged, they die, and new cells take their place.

Sometimes, this orderly process goes wrong. New cells form when the body does not need them, and old or damaged cells do not die as they should. The build-up of extra cells often forms a mass of tissue called a growth or tumor.

Growths on the thyroid are often called nodules. Most thyroid nodules (more than 90 percent) are benign (not cancer). Benign nodules are not as harmful as malignant nodules (cancer):

• Benign nodules
  • are rarely a threat to life
  • don't invade the tissues around them
  • don't spread to other parts of the body
  • usually don't need to be removed
  
  
• Malignant nodules
  • may sometimes be a threat to life
  • can invade nearby tissues and organs
  • can spread to other parts of the body
  • often can be removed or destroyed, but sometimes the cancer returns

Cancer cells can spread by breaking away from the original tumor. They enter blood vessels or lymph vessels, which branch into all the tissues of the body. The cancer cells attach to other organs and grow to form new tumors that may damage those organs. The spread of cancer is called metastasis.

See the "Staging" section for information about thyroid cancer that has spread.

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