Autopsy studies indicate that 25 percent of the American population has small pituitary tumors. Forty percent of these tumors produce prolactin, but most are not considered clinically significant. Clinically significant pituitary tumors affect the health of approximately 14 out of every 100,000 people.
There are several types of pituitary tumors. A pituitary tumor is first grouped based on whether it is non-cancerous (pituitary adenoma) or cancerous (pituitary carcinoma).
Most pituitary tumors are non-cancerous. These tumors are called adenomas. A pituitary adenoma can be 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 or not it produces hormones. 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 being made 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 make up about 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.