Does a prolactinoma affect water retention & weight gain or loss?

Not really: Prolactinoma manifests differently depending on whether you are male, premenopausal female or postmenopausal female. Premenopausal women develop varying degrees of infertility and menstrual abnormalities and galactorrhea. Post menopausal women are usually asymptomatic. Men get lowered libido, infertility, impotence and gynecomastia.

By itself no: A prolactinoma is the most common tumor of the pituitary gland. It secretes the hormone prolactin which by itself does not cause weight gain or water retention. However the tumor may be mixed which means that there are also cells that secrets corticotropin, another pituitary hormone that stimulates the adrenal glands to secrete steroids which can cause water retention and weight gain.

It may: Excessive production of prolactin causes infertility, loss of libido and other sexual dysfunctions. A large tumor will affect vision, cause headaches and deficiency of other pitutiary hormones that may cause the symptoms you listed. See this site for more info: http://www.Ncbi.Nlm.Nih.Gov/pubmedhealth/pmh0001377/.