The pituitary's hormone production rises and falls depending on hormonal instructions from another gland, the hypothalamus. In the case of most pituitary hormones, including Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH), the presence of hypothalamic hormones signals the pituitary to increase production. For prolactin, however, the signal works in reverse: An increase in the hypothalamic hormone dopamine tells the pituitary to stop releasing prolactin. In some cases, however, the dopamine cannot reach the pituitary gland by passing through the veins of the pituitary stalk. When that happens, there is no signal to suppress the secretion of prolactin, and the pituitary continues to release prolactin.
When prolactin levels rise, the hypothalamus manufactures and releases more dopamine, once called the Prolactin Inhibiting Factor (PIF), in an effort to stop prolactin production. This increase in dopamine has other effects. In particular, it suppresses the hypothalamus' production of the hormones that promote release of FSH and LH. This, in turn, leads to a drop in LH and FSH, causing low estrogen levels and amenorrhea. Prolactin is also reported to be synthesized and secreted by a range of other cells in the body, most prominently various immune cells, the brain and the pregnant uterus.