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 Hypothyroidism: this is due to increased TRH levels which is known to stimulate prolactin levels
Renal failure: prolactin 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