What damage can a pituitary tumor cause?

Many: It can do nothing and just sit there for the rest of your life, or it can overproduce or shut down pituitary hormone secretion/production. If it's big enough, it can cause visual problems (loss of peripheral vision).

Variable: Commonly they secrete one or more hormones with resulting side effects. As an example too much growth hormone will result in large face, hands and feet. Local damage may occur in some patients due to compression (blindness) or invasion of bone or brain.