[5] Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation and nursing.

[6][7] Discovered in non-human animals around 1930 by Oscar Riddle[8] and confirmed in humans in 1970 by Henry Friesen,[9] prolactin is a peptide hormone, encoded by the PRL gene.

[10] In mammals, prolactin is associated with milk production; in fish it is thought to be related to the control of water and salt balance.

As a growth factor, binding to cytokine-like receptors, it influences hematopoiesis and angiogenesis and is involved in the regulation of blood clotting through several pathways.

It stimulates the mammary glands to produce milk (lactation): increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands and prepare for milk production, which normally starts when levels of progesterone fall by the end of pregnancy and a suckling stimulus is present.

[12] It has been shown in rats and sheep that prolactin affects lipid synthesis differentially in mammary and adipose cells.

Prolactin deficiency induced by bromocriptine increased lipogenesis and insulin responsiveness in adipocytes while decreasing them in the mammary gland.

[18] Prolactin within the normal reference ranges can act as a weak gonadotropin, but at the same time suppresses gonadotropin-releasing hormone secretion.

Although expression of prolactin receptors have been demonstrated in rat hypothalamus, the same has not been observed in gonadotropin-releasing hormone neurons.

[19] Physiologic levels of prolactin in males enhance luteinizing hormone-receptors in Leydig cells, resulting in testosterone secretion, which leads to spermatogenesis.

Like mammals, however, prolactin in fish also has reproductive functions, including promoting sexual maturation and inducing breeding cycles, as well as brooding and parental care.

[26] In the South American discus, prolactin may also regulate the production of a skin secretion that provides food for larval fry.

[30] Conversely, mutations in the prolactin receptor can cause reduced hair growth, resulting in the "slick" phenotype in cattle.

[36] Pigeons, flamingos and male emperor penguins feed their young a cheese-like secretion from the upper digestive tract called crop milk, whose production is regulated by prolactin.

[41] Adult virgin female prolactin receptor knockout mice have much smaller and less developed mammary glands than their wild-type counterparts.

[41] In humans, prolactin is produced at least in the anterior pituitary, decidua, myometrium, breast, lymphocytes, leukocytes and prostate.

[42][43] Pituitary prolactin is controlled by the Pit-1 transcription factor, which binds to the gene at several sites including a proximal promoter.

Mice without a posterior pituitary do not raise their prolactin levels even with suckling and oxytocin injection, but scientists have yet to identify which specific hormone produced by this region is responsible.

[46] Extrapituitary prolactin is controlled by a superdistal promoter, located 5.8 kb upstream of the pituitary start site.

[45] Extrapituitary production of prolactin is thought to be special to humans and primates and may serve mostly tissue-specific paracrine and autocrine purposes.

It has been hypothesized that in vertebrates such as mice a similar tissue-specific effect is achieved by a large family of prolactin-like proteins controlled by at least 26 paralogous PRL genes not present in primates.

The abrupt drop of estrogen and progesterone levels following delivery allow prolactin—which temporarily remains high—to induce lactation.

These signals are carried by nerve fibers through the spinal cord to the hypothalamus, where changes in the electrical activity of neurons that regulate the pituitary gland increase prolactin secretion.

Levels can rise after exercise, high-protein meals, minor surgical procedures,[49] following epileptic seizures[50] or due to physical or emotional stress.

[51][52] In a study on female volunteers under hypnosis, prolactin surges resulted from the evocation, with rage, of humiliating experiences, but not from the fantasy of nursing.

[63][64] Reference ampoules of IS 84/500 contain "approximately" 2.5 μg of lyophilized human prolactin[65] and have been assigned an activity of 0.053 International Units by calibrating against the previous standard.

A preparation labeled 81/541 was distributed by the WHO Expert Committee on Biological Standardization without official status and given the assigned value of 50 mIU/ampoule based on an earlier collaborative study.

[76] Hypoprolactinemia can result from hypopituitarism, excessive dopaminergic action in the tuberoinfundibular pathway and ingestion of D2 receptor agonists such as bromocriptine.

[83] D2 receptor antagonists like domperidone, metoclopramide, and sulpiride are used as galactogogues to increase prolactin secretion in the pituitary gland and induce lactation in humans.