Endocrine glands release hormones (chemical messengers) into the bloodstream to be transported to various organs and tissues throughout the body. For instance, the pancreas secretes insulin, which allows the body to regulate levels of sugar in the blood. The thyroid gets instructions from the pituitary to secrete hormones which determine the pace of chemical activity in the body (the more hormone in the bloodstream, the faster the chemical activity; the less hormone, the slower the activity).

The thyroid is a gland located in the neck. It is a part of the endocrine (hormone) system, and plays a major role in regulating the body's metabolism. If a sample of cells is needed from the thyroid gland a fine needle biopsy can be performed. During this procedure, a skinny needle is inserted into the thyroid gland, and a sample of thyroid cells and fluid is drawn into the needle. The needle is then withdrawn and the cells are sent for examination. This test is usually performed to diagnose thyroid disease or thyroid cancer.