Normal level: up to 25 ng/mL

Cause:

1. Interference with hypothalamic-pituitary axis:
   1. hypothalamic tumor
   2. parasellar tumor
   3. pituitary adenoma
   4. sarcoidosis
   5. histiocytosis
   6. traumatic infundibular transection
2. Pharmacologic agents
   alpha-methyldopa, reserpine, phenothiazine, butyrophenone, tricyclic antidepressants, oral contraceptives
3. Hypothyroidism (TRH also stimulates prolactin)
4. Renal failure
5. Cirrhosis
6. Stress / recent surgery
7. Breast examination
8. Pregnancy
9. Lactation