|Locus||Chr. 6 p22.2-p21.3|
Prolactin (PRL) or Luteotropic hormone (LTH) is a peptide hormone discovered by Dr. Henry Friesen, primarily associated with lactation. In breastfeeding, the act of an infant sucking the nipple stimulates the production of prolactin, which fills the breast with milk via a process called lactogenesis, in preparation for the next feed. Oxytocin, another hormone, is also released, which triggers milk let-down.
Prolactin hormone or luteotropic hormone is synthesised and secreted by sex hormone binding lactotrope cells in the adenohypophysis (anterior pituitary gland). It is also produced in other tissues including the breast, the decidua, parts of the central nervous system, the brain and the immune system.  The gene encoding prolactin in humans is located on chromosome 6.
The system of increased production is a positive feedback cycle. (That is, the effect causes more of itself.) This is one of the few instances in the body where positive feedback is normal.
Pituitary prolactin secretion is regulated by neuroendocrine neurons in the hypothalamus, the most important ones being the neurosecretory tuberoinfundibulum (TIDA) neurons of the arcuate nucleus, which secrete dopamine to act on the dopamine-2 receptors of lactotrophs, causing inhibition of prolactin secretion. Thyrotropin-releasing factor (thyrotropin-releasing hormone) has a stimulatory effect on prolactin release.
It stimulates the mammary glands to produce milk (lactation): Increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and increases the production of milk. However, the high levels of progesterone during pregnancy act directly on the breasts to stop ejection of milk. It is only when the levels of this hormone fall after childbirth that milk ejection is possible. Sometimes, newborn babies (males as well as females) secrete a milky substance from their nipples. This substance is commonly known as Witch's milk. This is caused by the fetus being affected by prolactin circulating in the mother just before birth, and usually stops soon after birth.
Prolactin provides the body with sexual gratification after sexual acts: The hormone counteracts the effect of dopamine, which is responsible for sexual arousal. This is thought to cause the sexual refractory period. The amount of prolactin can be an indicator for the amount of sexual satisfaction and relaxation. Unusually high amounts are suspected to be responsible for impotence and loss of libido (see hyperprolactinemia symptoms). Prolactin also stimulates proliferation of oligodendrocyte precursor cells. These cells differentiate into oligodendrocytes, the cells responsible for the formation of myelin coatings on axons in the central nervous system.
Prolactin also has a number of other effects including contributing to surfactant synthesis of the fetal lungs at the end of the pregnancy and immune tolerance of the fetus by the maternal organism during pregnancy; it also decreases normal levels of sex hormones — estrogen in women and testosterone in men.. It is this inhibition of sex steroids that is responsible for loss of the menstrual cycle in lactating women as well as lactation-associated osteoporosis. Prolactin also enhances luteinizing hormone-receptors in Leydig cells resulting in testosterone secretion which leads to spermatogenesis. Prolactin delays hair regrowth in mice.
During pregnancy, high circulating concentrations of estrogen promote prolactin production. The resulting high levels of prolactin secretion cause further maturation of the mammary glands, preparing them for lactation.
After childbirth, prolactin levels fall as the internal stimulus for them is removed. Sucking by the baby on the nipple then promotes further prolactin release, maintaining the ability to lactate. The sucking activates mechanoreceptors in and around the nipple. 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 cause increased prolactin secretion. The suckling stimulus also triggers the release of oxytocin from the posterior pituitary gland, which triggers milk let-down: Prolactin controls milk production (lactogenesis) but not the milk-ejection reflex; the rise in prolactin fills the breast with milk in preparation for the next feed.
Increasing prolactin levels in women during pregnancy (especially within first 3 months) and then in the breastfeeding period are responsible for cycles of high sexual satiation, which help protect the child and are amidst natural rewards for procreation. However, fluctuations and drops of this hormone occurring in that period can substantially increase sexual tension and in turn often work for higher desire for orgasms.
It has also been found that compared to un-mated males, fathers and expectant fathers have increased prolactin concentrations. 
High prolactin levels also tend to suppress the ovulatory cycle by inhibiting the secretion of both follicle-stimulating hormone (FSH) and gonadotropic-releasing hormone (GnRH). High prolactin levels can also contribute to mental health issues.
Prolactin is a single-chain polypeptide of 199 amino acids with a molecular weight of about 24,000 daltons. Its structure is similar to that of growth hormone and placental lactogen. The molecule is folded due to the activity of three disulfide bonds. Significant heterogeneity of the molecule has been described, thus bioassays and immunoassays can give different results due to differing glycosylation, phosphorylation, sulfation, as well as degradation. The non-glycosylated form of prolactin is the dominant form of prolactin that is secreted by the pituitary gland.
Little prolactin is apparently the result of removal of some amino acids, whereas big prolactin can be the product of interaction of several prolactin molecules.
Pit-1 is a transcription factor that binds to the prolactin gene at several sites to allow for the production of prolactin in the pituitary gland. A key regulator of prolactin production is estrogens that enhance growth of prolactin-producing cells and stimulate prolactin production directly, as well as suppressing dopamine.
Human prolactin receptors are insensitive to mouse prolactin.
Prolactin receptors are present in the mamillary glands, ovaries, pituitary glands, heart, lung, thymus, spleen, liver, pancreas, kidney, adrenal gland, uterus, skeletal muscle, skin and areas of the central nervous system.  When prolactin binds to the receptor it causes it to dimerize with another prolactin receptor. This results in the activation of Janus kinase 2 a tyrosine kinase which initiates the JAK-STAT pathway. The activation of the prolactin receptor also results in the activation of mitogen-activated protein kinases and Src kinase. 
Prolactin levels may be checked as part of a sex hormone workup, as elevated prolactin secretion can suppress the secretion of FSH and GnRH, leading to hypogonadism, and sometimes causing erectile dysfunction in men.
|Women||2.8-29.2||59-619||(World Health Organization [WHO]: Third International Reference Preparation [3rd IRP] of prolactin, human, lyophilized, 0.053 IU / ampoule, for immunoassay [preparation code:84/500])|
|Men||2.1-17.7||45-375||(WHO: 3rd IRP 84/500)|
(Data from The Immunoassay Handbook, Third Edition.)
Hyperprolactinaemia is the term given to having too-high levels of prolactin in the blood.
The World Health Organization states that demand breastfeeding is more than 98% effective as a contraceptive in the first six months postpartum. This effect is said to be responsible for the natural spacing of children seen in countries where contraception is not widely available, and is thought to be an evolutionary means of ensuring adequate care is provided to each newborn. The 98% effectiveness applies only if three criteria are met:
If one or more of these conditions are broken, lactational amenorrhea is no longer a reliable form of birth control. This contraceptive method is highly effective as long as the three conditions stated above are fulfilled. Further, the WHO suggests that a woman who is still amenorrheic has a less-than-5% chance of getting pregnant in the first year of her baby's life, as long as she is still breastfeeding on demand.