A tumor is an abnormal group of cells.
A pituitary tumor, therefore, is an abnormal group of cells that grows in your pituitary gland. Nearly always benign — meaning not cancerous, and unlikely to spread to other organs — tumors in this gland can present problems in two ways. It can influence your body’s production of certain hormones; this is known as a functional pituitary tumor. Or, it may be a non-functional pituitary tumor, not making enough of any particular hormone to cause a hormonal problem, but growing too large and putting pressure on the parts of the body around it.
Sometimes these tumors are so small that their presence is undiagnosed. In fact, a pituitary tumor is often only found by chance, during tests for other conditions. According to the American Cancer Society, about 10,000 are diagnosed in the United States every year.