Prolactin is a hormone that is only secreted by the lactotroph cells of the pitutiary gland. Although a pituitary tumor is a common cause of high prolactin levels, it is not the only cause. 

Causes of high prolactin levels in the blood:

Primary Hypothyroidismthis is due to increased TRH levels which is known to stimulate prolactin levels   

Renal failureprolactin is mainly excreted from the body from the kidneys. Reduced kidney function can lead to higher levels of prolactin. 

Chest wall stimulation this is most commonly due to suckling from babies when patients are breast feeding.  

Pregnancy: as pregnancy progresses the levels of prolactin increase

Medications:  there are a long list of these that can potentially cause high prolactin levels including: estrogens, neuroleptic drugs, metoclopramide, antidepressant drugs, cimetidine, methyldopa, reserpine, verapamil, phenothiazines, marijuana, opiods, etc 

Pituitary stalk interruption: This is either due to a pituitary tumor or other brain tumor in the area of the pituitary which leads to higher levels of prolactin secretion due to interruption of the natural inhibition of prolactin secretion . Your Houston endocrinologist can explain this in great detail.  Prolactinoma is the most common pituitary tumor that over secretes the  hormone prolactin. 

Idiopathic: This is when no obvious cause is seen and is most commonly due to a pituitary tumor that is too small to see on MRI