Prolactin is secreted solely by the lactotroph cells of the pituitary gland. As a result, hyperprolactinemia results almost exclusively from diseases that cause hypersecretion of prolactin by lactotroph cells. Some of these causes are physiologic and others pathologic.
This topic will review the major causes of hyperprolactinemia. The clinical manifestations, diagnosis, and treatment are discussed separately. (See "Clinical manifestations and evaluation of hyperprolactinemia" and "Treatment of hyperprolactinemia due to lactotroph adenoma and other causes".)
Serum prolactin concentrations normally increase substantially during pregnancy and to a lesser degree in response to nipple stimulation and stress. The upper normal value for serum prolactin in most laboratories is about 20 ng/mL (20 mcg/L SI units).
Pregnancy — Serum prolactin increases throughout pregnancy, reaching a peak at delivery (figure 1) . The magnitude of the increase, however, is quite variable; in one study the mean value at term was 207 ng/mL, but the range was from 35 to 600 ng/mL (35 to 600 mcg/L SI units) . The probable cause of the hyperprolactinemia is the increasing serum estradiol concentrations during pregnancy. By six weeks after delivery, estradiol secretion has decreased and the basal serum prolactin concentration is usually normal even when the mother is breastfeeding.
Nipple stimulation — Nipple stimulation increases serum prolactin concentrations, presumably via a neural pathway. The magnitude of the increase is directly proportional to the degree of preexisting lactotroph hyperplasia due to estrogen. In the first weeks postpartum, as an example, the serum prolactin concentration increases up to a few hundred ng/mL above baseline in response to suckling; in contrast, several months after delivery the increase in prolactin in response to suckling in the breastfeeding woman is usually less than 10 ng/mL (10 mcg/L SI units) above baseline (figure 2) .