What causes pituitary tumors?

Unknown. There are malignant and non-malignant pituitary tumors, some that produce hormones, some that don't. Some cause problems because of their size and relationship to the optic chiasm and can cause visual problems and headaches. Causes of pituitary tumors are unknown.
Unknown. No one really knows why pituitary tumors develop. Like most tumors of the body, we believe that a genetic "trigger" causes a cell to divide in an uncontrolled fashion leading to a tumor.