Thyroid cancer is a growth of cells that starts in the thyroid. The thyroid is a butterfly-shaped gland located at the base of the neck, just below the Adam's apple. The thyroid produces hormones that regulate heart rate, blood pressure, body temperature and weight. Thyroid cancer might not cause any symptoms at first. But as it grows, it can cause signs and symptoms, such as swelling in your neck, voice changes and difficulty swallowing.