The thyroid is butterfly-shaped organ located in the lower neck, below the larynx. It has two cone-like lobes connected by a tissue called isthmus. In response to the hypophysis (an important gland of the endocrine system located at the base of the brain), through the thyrotropin (thyroid stimulating hormone), the thyroid secretes hormones used to monitor vital functions of the body. The two main thyroid hormones are thyroxine, a hormone containing iodine, which regulates the metabolism so it works normally; and calcitonin, a hormone that keeps calcium levels at normal levels.
All these functions are performed by thousands of cells working synergistically in the thyroid gland. Every day, by a natural process called self-cell destruction or apoptosis, thousands of these cells die to be replaced by new healthy cells. Thyroid cancer occurs when there is abnormal cell production and proliferation in the thyroid gland. The causes of this cellular disorder are not well known by scientists; however, several factors are suspected (see risk factors).