In more than 95 percent of people with acromegaly, a benign tumor of the pituitary gland, called an adenoma, produces excess growth hormone (GH). Pituitary tumors are labeled either microadenomas or macroadenomas, depending on their size. Most GH-secreting tumors are macroadenomas, meaning they are larger than 1 cm. Depending on their location, these larger tumors may compress surrounding brain structures. For example, a tumor growing upward may affect the optic chiasm-where the optic nerves cross-leading to visual problems and vision loss. If the tumor grows to the side, it may enter an area of the brain called the cavernous sinus where there are many nerves, potentially damaging them.
Compression of the surrounding normal pituitary tissue can alter production of other hormones. These hormonal shifts can lead to changes in menstruation and breast discharge in women and erectile dysfunction in men. If the tumor affects the part of the pituitary that controls the thyroid-another hormone-producing gland-thyroid hormones may decrease. Too little thyroid hormone can cause weight gain, fatigue, and hair and skin changes. If the tumor affects the part of the pituitary that controls the adrenal gland, hormone cortisol may decrease. Too little cortisol can cause weight loss, dizziness, fatigue, low blood pressure (BP), and nausea.
Some GH-secreting tumors may also secrete too much of other pituitary hormones.
Rates of GH production and the aggressiveness of the tumor vary greatly among people with adenomas. Some adenomas grow slowly, and the symptoms of GH excess are often not noticed for many years. Younger patients tend to have more aggressive tumors. Regardless of size, these tumors are always benign.
Most pituitary tumors are the result of a genetic alteration in a single pituitary cell, which leads to increased cell division and tumor formation. This genetic change, or mutation, is not present at birth but happens later in life. The mutation occurs in a gene that regulates the transmission of chemical signals in pituitary cells. It permanently switches on the signal that tells the cell to divide and secrete GH. Events in the cell that cause disordered pituitary cell growth and GH oversecretion are currently the subjects of intensive research.
This information is based on source information from the National Institute of Diabetes and Digestive and Kidney Diseases.