What Is Thyroid Cancer

What Is Thyroid Cancer?
Thyroid cancer is a type of cancer that starts in the thyroid gland. Cancer starts when cells begin to grow out of control. (To learn more about how cancers start and spread, see What Is Cancer?)

The thyroid gland makes hormones that help regulate your metabolism, heart rate, blood pressure, and body temperature.

Where thyroid cancer starts
The thyroid gland is in the front part of the neck, below the thyroid cartilage (Adam’s apple). In most people, the thyroid cannot be seen or felt. It is shaped like a butterfly, with 2 lobes — the right lobe and the left lobe — joined by a narrow piece of gland called the isthmus (see picture below).


Illustration showing the parts of the thyroid gland (right lobe, isthmus, left lobe) in relation to the thyroid cartilage and trachea
The thyroid gland has 2 main types of cells:

Follicular cells use iodine from the blood to make thyroid hormones, which help regulate a person’s metabolism. Having too much thyroid hormone (hyperthyroidism) can cause a fast or irregular heartbeat, trouble sleeping, nervousness, hunger, weight loss, and a feeling of being too warm. Having too little hormone (hypothyroidism) causes a person to slow down, feel tired, and gain weight. The amount of thyroid hormone released by the thyroid is regulated by the pituitary gland at the base of the brain, which makes a substance called thyroid-stimulating hormone (TSH).
C cells (also called parafollicular cells) make calcitonin, a hormone that helps control how the body uses calcium.
Other, less common cells in the thyroid gland include immune system cells (lymphocytes) and supportive (stromal) cells.

Different cancers develop from each kind of cell. The differences are important because they affect how serious the cancer is and what type of treatment is needed.

Many types of growths and tumors can develop in the thyroid gland. Most of these are benign (non-cancerous) but others are malignant (cancerous), which means they can spread into nearby tissues and to other parts of the body.

Benign thyroid conditions
Thyroid enlargement
Changes in the thyroid gland’s size and shape can often be felt or even seen by patients or by their doctor.

An abnormally large thyroid gland is sometimes called a goiter. Some goiters are diffuse, meaning that the whole gland is large. Other goiters are nodular, meaning that the gland is large and has one or more nodules (bumps) in it. There are many reasons the thyroid gland might be larger than usual, and most of the time it is not cancer. Both diffuse and nodular goiters are usually caused by an imbalance in certain hormones. For example, not getting enough iodine in the diet can cause changes in hormone levels and lead to a goiter.

Thyroid nodules
Lumps or bumps in the thyroid gland are called thyroid nodules. Most thyroid nodules are benign, but about 2 or 3 in 20 are cancerous. Sometimes these nodules make too much thyroid hormone and cause hyperthyroidism. Nodules that produc