Most pituitary tumors are not cancerous. These non-cancerous pituitary tumors are called adenomas. A pituitary adenoma is classified based on its size:
- Microadenomas are smaller than 10 millimeters
- Macroadenomas are 10 millimeters or larger.
Most pituitary adenomas are microadenomas.
A pituitary adenoma is also classified based on whether it produces hormones or not. A pituitary tumor that makes one or more of the pituitary hormones is called a functioning pituitary tumor. A pituitary adenoma that does not make hormones is called a nonfunctioning pituitary tumor. Each type of functioning pituitary tumor causes different symptoms, depending on the type of hormone that is produced by the tumor. Examples of functioning pituitary tumors include:
- Prolactin-producing tumors (which account for about 43 percent of all pituitary adenomas)
- ACTH-producing tumors (which represent approximately 7 percent of all pituitary adenomas)
- Growth hormone-producing tumors (which account for about 17 percent of all pituitary adenomas)
- Thyroid hormone-producing tumors (which make up about 3 percent of all pituitary adenomas).
Nonfunctioning adenomas make up the remaining 30 percent of all pituitary adenomas.
Cushing's disease occurs when the body's tissues are exposed to excessive levels of cortisol for long periods of time. The cause of Cushing's disease is an ACTH-secreting pituitary adenoma.
ACTH-producing pituitary tumors produce a hormone called adrenocorticotropin (ACTH), which stimulates the adrenal glands to make glucocorticoids. When the body makes too much ACTH, it causes Cushing's disease.