The patient has a macroprolactinoma, which is best treated with a dopamine agonist such as cabergoline. The most common cause of hyperprolactinemia is a prolactinoma, which is a benign adenoma. Microprolactinomas are less than 10 mm in diameter, and macroprolactinomas are 10 mm or greater in diameter. Prolactinomas are the most common type of secretory pituitary adenoma. The patient's tumor is causing significant mass effect, including compression of the optic chiasm, invasion into the cavernous sinus, secondary hypothyroidism, and likely growth hormone deficiency; however, even with these complications, the macroprolactinoma is best treated with medication instead of surgery. A dopamine agonist can cause a rapid decrease in the serum prolactin level and shrinkage of the prolactinoma. More specifically, in as many as 90% of patients, it can normalize prolactin levels, reverse hypogonadism, and shrink tumors by at least 50%. Because of these rapid decreases in tumor size, dopamine agonists can be used as first-line therapy, even in patients with mild visual field defects, as long as visual acuity is not threatened by rapid progression of the tumor or recent tumor hemorrhage.

Octreotide is used to treat acromegaly but has no role in the first-line treatment of prolactinoma.

Medical therapy is preferred over surgery and radiation for the treatment of prolactinomas. Stereotactic surgery (a specific kind of radiation therapy) would be considered in a patient with a tumor that is refractory to medical therapy and incompletely resectable.

Because this patient has a prolactinoma, the treatment of choice is a dopamine agonist, not surgery. There is an excellent chance that the prolactinoma will respond to cabergoline, and the patient can avoid surgery.