Biology:Prolactin

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Short description: Protein family and hormone


Prolactin (PRL), also known as lactotropin and mammotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans.[1] Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation and nursing. It is secreted heavily in pulses in between these events. Prolactin plays an essential role in metabolism, regulation of the immune system and pancreatic development.[2][3]

Discovered in non-human animals around 1930 by Oscar Riddle[4] and confirmed in humans in 1970 by Henry Friesen,[5] prolactin is a peptide hormone, encoded by the PRL gene.[6]

In mammals, prolactin is associated with milk production; in fish it is thought to be related to the control of water and salt balance. Prolactin also acts in a cytokine-like manner and as an important regulator of the immune system. It has important cell cycle-related functions as a growth-, differentiating- and anti-apoptotic factor. 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. The hormone acts in endocrine, autocrine, and paracrine manners through the prolactin receptor and numerous cytokine receptors.[1]

Pituitary prolactin secretion is regulated by endocrine neurons in the hypothalamus. The most important of these are the neurosecretory tuberoinfundibulum (TIDA) neurons of the arcuate nucleus that secrete dopamine (a.k.a. Prolactin Inhibitory Hormone) to act on the D2 receptors of lactotrophs, causing inhibition of prolactin secretion. Thyrotropin-releasing hormone has a stimulatory effect on prolactin release, although prolactin is the only anterior pituitary hormone whose principal control is inhibitory.

Several variants and forms are known per species. Many fish have variants prolactin A and prolactin B. Most vertebrates, including humans, also have the closely related somatolactin. In humans, 14, 16, and 22 kDa variants exist.[7]

Function

In humans

Prolactin has a wide variety of effects. 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. Prolactin plays an important role in maternal behavior.[8]

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.[9]

In general, dopamine inhibits prolactin[10] but this process has feedback mechanisms.[11]

Elevated levels of prolactin decrease the levels of sex hormones—estrogen in women and testosterone in men.[12] The effects of mildly elevated levels of prolactin are much more variable, in women, substantially increasing or decreasing estrogen levels.

Prolactin is sometimes classified as a gonadotropin[13] although in humans it has only a weak luteotropic effect while the effect of suppressing classical gonadotropic hormones is more important.[14] Prolactin within the normal reference ranges can act as a weak gonadotropin, but at the same time suppresses gonadotropin-releasing hormone secretion. The exact mechanism by which it inhibits gonadotropin-releasing hormone is poorly understood. Although expression of prolactin receptors have been demonstrated in rat hypothalamus, the same has not been observed in gonadotropin-releasing hormone neurons.[15] Physiologic levels of prolactin in males enhance luteinizing hormone-receptors in Leydig cells, resulting in testosterone secretion, which leads to spermatogenesis.[16]

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.[17]

Other actions include contributing to pulmonary surfactant synthesis of the fetal lungs at the end of the pregnancy and immune tolerance of the fetus by the maternal organism during pregnancy. Prolactin promotes neurogenesis in maternal and fetal brains.[18][19]

In music psychology, it is conjectured that prolactin may play a role in the pleasurable perception of sad music, as the levels of the hormone increase when a person feels sad, producing a consoling psychological effect.[20]

In other vertebrates

The primary function of prolactin in fish is osmoregulation,[21] i.e., controlling the movement of water and salts between the tissues of the fish and the surrounding water. 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.[22] In the South American discus, prolactin may also regulate the production of a skin secretion that provides food for larval fry.[23] An increase in brooding behaviour caused by prolactin has been reported in hens.[24]

Prolactin and its receptor are expressed in the skin, specifically in the hair follicles, where they regulate hair growth and moulting in an autocrine fashion.[25][26] Elevated levels of prolactin can inhibit hair growth,[27] and knock-out mutations in the prolactin gene cause increased hair length in cattle[28] and mice.[26] Conversely, mutations in the prolactin receptor can cause reduced hair growth, resulting in the "slick" phenotype in cattle.[28][29] Additionally, prolactin delays hair regrowth in mice.[30]

Analogous to its effects on hair growth and shedding in mammals, prolactin in birds controls the moulting of feathers,[31] as well as the age at onset of feathering in both turkeys and chickens.[32] 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.[33][34]

In rodents, pseudopregnancy can occur when a female is mated with a sterile male. This mating can cause bi-daily surges of prolactin which would normally occur in rodent pregnancy.[35] Prolactin surges initiate the secretion of progesterone which maintains pregnancy and hence can initiate pseudopregnancy. The false maintenance of pregnancy exhibits the outward physical symptoms of pregnancy, in the absence of a foetus.[36]

Prolactin receptor activation is essential for normal mammary gland development during puberty in mice.[37] Adult virgin female prolactin receptor knockout mice have much smaller and less developed mammary glands than their wild-type counterparts.[37] Prolactin and prolactin receptor signaling are also essential for maturation of the mammary glands during pregnancy in mice.[37]

Regulation

In humans, prolactin is produced at least in the anterior pituitary, decidua, myometrium, breast, lymphocytes, leukocytes and prostate.[38][39]

Pituitary

Pituitary prolactin is controlled by the Pit-1 transcription factor, which binds to the gene at several sites including a proximal promoter.[39] This promoter is inhibited by dopamine and stimulated by estrogens, neuropeptides, and growth factors.[40] Estrogens can also suppress dopamine.

Interaction with neuropeptides is still a matter of active research: no specific prolactin-releasing hormone has been identified. It is known that mice react to both VIP and TRH, but humans seem to only react to TRH. There are prolactin-releasing peptides that work in vitro, but whether they deserve their name has been questioned. Oxytocin does not play a large role. 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.[41]

In birds (turkeys), VIP is a powerful prolactin-releasing factor, while peptide histidine isoleucine has almost no effect.[42]

Extrapituitary

Extrapituitary prolactin is controlled by a superdistal promoter, located 5.8 kb upstream of the pituitary start site. The promoter does not react to dopamine, estrogens, or TRH. Instead, it is stimulated by cAMP. Responsiveness to cAMP is mediated by an imperfect cAMP–responsive element and two CAAT/enhancer binding proteins (C/EBP).[39] Progesterone upregulates prolactin synthesis in the endometrium but decreases it in myometrium and breast glandular tissue.[43]

Breast and other tissues may express the Pit-1 promoter in addition to the distal promoter. Oct-1 appears able to substitute for Pit-1 in activating the promoter in breast cancer cells.[41]

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.[39]

Stimuli

Prolactin follows diurnal and ovulatory cycles. Prolactin levels peak during REM sleep and in the early morning. Many mammals experience a seasonal cycle.[34]

During pregnancy, high circulating concentrations of estrogen and progesterone increase prolactin levels by 10- to 20-fold. Estrogen and progesterone inhibit the stimulatory effects of prolactin on milk production. The abrupt drop of estrogen and progesterone levels following delivery allow prolactin—which temporarily remains high—to induce lactation.[44]

Sucking on the nipple offsets the fall in prolactin as the internal stimulus for them is removed. 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 increase 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. The posterior pituitary produces a yet-unidentified hormone that causes prolactin production.[41]

In usual circumstances, in the absence of galactorrhea, lactation ceases within one or two weeks following the end of breastfeeding.

Levels can rise after exercise, high-protein meals, minor surgical procedures,[45] following epileptic seizures[46] or due to physical or emotional stress.[47][48] 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.[48] Stress-induced PRL changes is not linked to the prosterior pituitary in rodents.[41]

Hypersecretion is more common than hyposecretion. Hyperprolactinemia is the most frequent abnormality of the anterior pituitary tumors, termed prolactinomas. Prolactinomas may disrupt the hypothalamic-pituitary-gonadal axis as prolactin tends to suppress the secretion of gonadotropin-releasing hormone from the hypothalamus and in turn decreases the secretion of follicle-stimulating hormone and luteinizing hormone from the anterior pituitary, therefore disrupting the ovulatory cycle.[49] Such hormonal changes may manifest as amenorrhea and infertility in females as well as erectile dysfunction in males.[50][3] Inappropriate lactation (galactorrhoea) is another important clinical sign of prolactinomas.

Structure and isoforms

The structure of prolactin 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 and sulfation, as well as degradation. The non-glycosylated form of prolactin is the dominant form that is secreted by the pituitary gland.[7]

The three different sizes of prolactin are:

  • Little prolactin—the predominant form.[51] It has a molecular weight of approximately 23-kDa.[51] It is a single-chain polypeptide of 199 amino acids and is apparently the result of removal of some amino acids.
  • Big prolactin—approximately 48 kDa.[51] It may be the product of interaction of several prolactin molecules. It appears to have little, if any, biological activity.[52]
  • Big big prolactin—approximately 150 kDa.[51] It appears to have a low biological activity.[53]
  • Other variants with the molecular masses of 14, 16, and 22 kDa.[7]

The levels of larger ones are somewhat higher during the early postpartum period.[54]

Prolactin receptor

Prolactin receptors are present in the mammillary glands, ovaries, pituitary glands, heart, lung, thymus, spleen, liver, pancreas, kidney, adrenal gland, uterus, skeletal muscle, skin and areas of the central nervous system.[55] 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 that initiates the JAK-STAT pathway. Activation also results in the activation of mitogen-activated protein kinases and Src kinase.[55]

Human prolactin receptors are insensitive to mouse prolactin.[56]

Diagnostic use

Prolactin levels may be checked as part of a sex hormone workup, as elevated prolactin secretion can suppress the secretion of follicle stimulating hormone and gonadotropin-releasing hormone, leading to hypogonadism and sometimes causing erectile dysfunction.[57]

Prolactin levels may be of some use in distinguishing epileptic seizures from psychogenic non-epileptic seizures. The serum prolactin level usually rises following an epileptic seizure.[58]

Units and unit conversions

The serum concentration of prolactin can be given in mass concentration (µg/L or ng/mL), molar concentration (nmol/L or pmol/L), or international units (typically mIU/L). The current IU is calibrated against the third International Standard for Prolactin, IS 84/500.[59][60] Reference ampoules of IS 84/500 contain 2.5 µg of lyophilized human prolactin[61] and have been assigned an activity of .053 International Units.[59][60] Measurements that are calibrated against the current international standard can be converted into mass units using this ratio of grams to IUs;[62] prolactin concentrations expressed in mIU/L can be converted to µg/L by dividing by 21.2. Previous standards use other ratios.[63][64][65][66]

The first International Reference Preparation (or IRP) of human Prolactin for Immunoassay was established in 1978 (75/504 1st IRP for human prolactin) at a time when purified human prolactin was in short supply.[62][63] Previous standards relied on prolactin from animal sources.[66] Purified human prolactin was scarce, heterogeneous, unstable, and difficult to characterize. 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.[62][64] It was determined that this preparation behaved anomalously in certain immunoassays and was not suitable as an IS.[62]

Three different human pituitary extracts containing prolactin were subsequently obtained as candidates for an IS. These were distributed into ampoules coded 83/562, 83/573, and 84/500.[59][60][62][65] Collaborative studies involving 20 different laboratories found little difference between these three preparations. 83/562 appeared to be the most stable. This preparation was largely free of dimers and polymers of prolactin. On the basis of these investigations, 83/562 was established as the Second IS for human prolactin.[65] Once stocks of these ampoules were depleted, 84/500 was established as the Third IS for human prolactin.[59][62]

Reference ranges

General guidelines for diagnosing prolactin excess (hyperprolactinemia) define the upper threshold of normal prolactin at 25 µg/L for women and 20 µg/L for men.[55] Similarly, guidelines for diagnosing prolactin deficiency (hypoprolactinemia) are defined as prolactin levels below 3 µg/L in women[67][68] and 5 µg/L in men.[69][70][71] However, different assays and methods for measuring prolactin are employed by different laboratories and as such the serum reference range for prolactin is often determined by the laboratory performing the measurement.[55][72] Furthermore, prolactin levels vary according to factors as age,[73] sex,[73] menstrual cycle stage[73] and pregnancy.[73] The circumstances surrounding a given prolactin measurement (assay, patient condition, etc.) must therefore be considered before the measurement can be accurately interpreted.[55]

The following chart illustrates the variations seen in normal prolactin measurements across different populations. Prolactin values were obtained from specific control groups of varying sizes using the IMMULITE assay.[73]

Typical prolactin values
Proband Prolactin, µg/L (ng/mL)
women, follicular phase (n = 803)
12.1
women, luteal phase (n = 699)
13.9
women, mid-cycle (n = 53)
17
women, whole cycle (n = 1555)
13.0
women, pregnant, 1st trimester (n = 39)
16
women, pregnant, 2nd trimester (n = 52)
49
women, pregnant, 3rd trimester (n = 54)
113
Men, 21–30 (n = 50)
9.2
Men, 31–40 (n = 50)
7.1
Men, 41–50 (n = 50)
7.0
Men, 51–60 (n = 50)
6.2
Men, 61–70 (n = 50)
6.9

Inter-method variability

The following table illustrates variability in reference ranges of serum prolactin between some commonly used assay methods (as of 2008), using a control group of healthy health care professionals (53 males, age 20–64 years, median 28 years; 97 females, age 19–59 years, median 29 years) in Essex, England:[72]

Assay method Mean
Prolactin		 Lower limit
2.5th percentile		 Upper limit
97.5th percentile
µg/L mIU/L µg/L mIU/L µg/L mIU/L
Females
Centaur 7.92 168 3.35 71 16.4 348
Immulite 9.25 196 3.54 75 18.7 396
Access 9.06 192 3.63 77 19.3 408
AIA 9.52[74] 257[74] 3.89[74] 105[74] 20.3[74] 548[74]
Elecsys 10.5 222 4.15 88 23.2 492
Architect 10.6 225 4.62 98 21.1 447
Males
Access 6.89 146 2.74 58 13.1 277
Centaur 7.88 167 2.97 63 12.4 262
Immulite 7.45 158 3.30 70 13.3 281
AIA 7.81[74] 211[74] 3.30[74] 89[74] 13.5[74] 365[74]
Elecsys 8.49 180 3.40 72 15.6 331
Architect 8.87 188 4.01 85 14.6 310

An example of the use of the above table is, if using the Centaur assay to estimate prolactin values in µg/L for females, the mean is 7.92 µg/L and the reference range is 3.35–16.4 µg/L.

Conditions

Elevated levels

Hyperprolactinaemia, or excess serum prolactin, is associated with hypoestrogenism, anovulatory infertility, oligomenorrhoea, amenorrhoea, unexpected lactation and loss of libido in women and erectile dysfunction and loss of libido in men.[75]

Causes of Elevated Prolactin Levels

Decreased levels

Hypoprolactinemia, or serum prolactin deficiency, is associated with ovarian dysfunction in women,[67][68] and arteriogenic erectile dysfunction, premature ejaculation,[69] oligozoospermia, asthenospermia, hypofunction of seminal vesicles and hypoandrogenism[70] in men. In one study, normal sperm characteristics were restored when prolactin levels were raised to normal values in hypoprolactinemic men.[71]

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

In medicine

Prolactin is available commercially for use in other animals, but not in humans.[76] It is used to stimulate lactation in animals.[76] The biological half-life of prolactin in humans is around 15–20 minutes.[77] The D2 receptor is involved in the regulation of prolactin secretion, and agonists of the receptor such as bromocriptine and cabergoline decrease prolactin levels while antagonists of the receptor such as domperidone, metoclopramide, haloperidol, risperidone, and sulpiride increase prolactin levels.[78] 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.[79]

See also

References

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  44. ↑ Calik-Ksepka, Anna; Stradczuk, Monika; Czarnecka, Karolina; Grymowicz, Monika; Smolarczyk, Roman (2022-01-31). "Lactational Amenorrhea: Neuroendocrine Pathways Controlling Fertility and Bone Turnover". International Journal of Molecular Sciences 23 (3): 1633. doi:10.3390/ijms23031633. ISSN 1422-0067. PMID 35163554. 
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  46. ↑ "The approach to patients with "non-epileptic seizures"". Postgraduate Medical Journal 81 (958): 498–504. Aug 2005. doi:10.1136/pgmj.2004.029785. PMID 16085740. 
  47. ↑ "Prolactin". MedLine plus. https://www.nlm.nih.gov/medlineplus/ency/article/003718.htm. 
  48. ↑ 48.0 48.1 "Prolactin, psychological stress and environment in humans: adaptation and maladaptation". Pituitary 6 (1): 35–9. 2003. doi:10.1023/A:1026229810876. PMID 14674722. 
  49. ↑ "Etiology, diagnosis, and treatment of secondary amenorrhea". UpToDate. Waltham, MA. 2020. http://www.uptodate.com/contents/etiology-diagnosis-and-treatment-of-secondary-amenorrhea?detectedLanguage=en&source=search_result&search=etiology+of+secondary+amenorrhea&selectedTitle=1%7E30&provider=noProvider. 
  50. ↑ "Prolactin Biology and Laboratory Measurement: An Update on Physiology and Current Analytical Issues". The Clinical Biochemist. Reviews 39 (1): 3–16. February 2018. PMID 30072818. 
  51. ↑ 51.0 51.1 51.2 51.3 "Prolactin synthesized and secreted by human peripheral blood mononuclear cells: an autocrine growth factor for lymphoproliferation". Proceedings of the National Academy of Sciences of the United States of America 89 (16): 7713–6. Aug 1992. doi:10.1073/pnas.89.16.7713. PMID 1502189. Bibcode: 1992PNAS...89.7713S. , in turn citing: "Size heterogeneity of human prolactin in CSF and serum: experimental conditions that alter gel filtration patterns". The Journal of Clinical Endocrinology and Metabolism 46 (1): 119–24. Jan 1978. doi:10.1210/jcem-46-1-119. PMID 752015. 
  52. ↑ "Heterogeneity of pituitary and plasma prolactin in man: decreased affinity of "Big" prolactin in a radioreceptor assay and evidence for its secretion". The Journal of Clinical Endocrinology and Metabolism 47 (6): 1273–81. Dec 1978. doi:10.1210/jcem-47-6-1273. PMID 263349. 
  53. ↑ "Characterization of big, big prolactin in patients with hyperprolactinaemia". Clinical Endocrinology 37 (4): 365–72. Oct 1992. doi:10.1111/j.1365-2265.1992.tb02340.x. PMID 1483294. 
  54. ↑ "Heterogeneity of human prolactin levels in serum during the early postpartum period". Gynecological Endocrinology 7 (3): 173–7. Sep 1993. doi:10.3109/09513599309152499. PMID 8291454. 
  55. ↑ 55.0 55.1 55.2 55.3 55.4 "Hyperprolactinemia and prolactinomas". Endocrinology and Metabolism Clinics of North America 37 (1): 67–99, viii. Mar 2008. doi:10.1016/j.ecl.2007.10.013. PMID 18226731. 
  56. ↑ "Human prolactin receptors are insensitive to mouse prolactin: implications for xenotransplant modeling of human breast cancer in mice". The Journal of Endocrinology 188 (3): 589–601. Mar 2006. doi:10.1677/joe.1.06560. PMID 16522738. 
  57. ↑ Al-Chalabi, Mustafa; Bass, Autumn N.; Alsalman, Ihsan (2023), "Physiology, Prolactin", StatPearls (Treasure Island (FL): StatPearls Publishing), PMID 29939606, http://www.ncbi.nlm.nih.gov/books/NBK507829/, retrieved 2024-01-10 
  58. ↑ "Serum prolactin in seizure disorders". Indian Pediatrics 41 (8): 827–31. Aug 2004. PMID 15347871. 
  59. ↑ 59.0 59.1 59.2 59.3 "International Standards for human prolactin: calibration by international collaborative study". The Journal of Endocrinology 121 (1): 157–66. Apr 1989. doi:10.1677/joe.0.1210157. PMID 2715755. 
  60. ↑ 60.0 60.1 60.2 "WHO Expert Committee on Biological Standardization". Thirty-ninth Report, WHO Technical Report Series. World Health Organization. 1989. http://whqlibdoc.who.int/trs/WHO_TRS_786.pdf. "86.1520, WHO/BS documents: 86.1520 Add 1, 88.1596" 
  61. ↑ "WHO International Standard, Prolactin, Human. NIBSC code: 84/500, Instructions for use". NIBSC / Health Protection Agency. 1989. http://www.nibsc.ac.uk/documents/ifu/84-500.pdf. 
  62. ↑ 62.0 62.1 62.2 62.3 62.4 62.5 Canadian Society of Clinical Chemists (Dec 1992). "Canadian Society of Clinical Chemists position paper: standardization of selected polypeptide hormone measurements". Clinical Biochemistry 25 (6): 415–24. doi:10.1016/0009-9120(92)90030-V. PMID 1477965. 
  63. ↑ 63.0 63.1 "International Reference Preparation of human prolactin for immunoassay: definition of the International Unit, report of a collaborative study and comparison of estimates of human prolactin made in various laboratories". The Journal of Endocrinology 80 (1): 157–68. Jan 1979. doi:10.1677/joe.0.0800157. PMID 429949. 
  64. ↑ 64.0 64.1 "WHO Expert Committee on Biological Standardization". Thirty-fifth Report, WHO Technical Report Series. World Health Organization. 1985. http://whqlibdoc.who.int/trs/WHO_TRS_725.pdf. 
  65. ↑ 65.0 65.1 65.2 "WHO Expert Committee on Biological Standardization". Thirty-seventh Report, WHO Technical Report Series. World Health Organization. 1987. http://whqlibdoc.who.int/trs/WHO_TRS_760.pdf. [|permanent dead link|dead link}}]
  66. ↑ 66.0 66.1 "The Second International Standard for Prolactin". Bulletin of the World Health Organization 29 (6): 721–8. 1963. PMID 14107744. 
  67. ↑ 67.0 67.1 "Hypoprolactinemia and ovarian function". Fertility and Sterility 49 (3): 437–41. Mar 1988. doi:10.1016/s0015-0282(16)59769-6. PMID 3342895. 
  68. ↑ 68.0 68.1 "Prolactin gene expression and prolactin protein in premenopausal and postmenopausal human ovaries". Fertility and Sterility 68 (4): 696–701. Oct 1997. doi:10.1016/S0015-0282(97)00320-8. PMID 9341613. 
  69. ↑ 69.0 69.1 "Hypoprolactinemia: a new clinical syndrome in patients with sexual dysfunction". The Journal of Sexual Medicine 6 (5): 1457–66. May 2009. doi:10.1111/j.1743-6109.2008.01206.x. PMID 19210705. 
  70. ↑ 70.0 70.1 "Hypoprolactinemia as related to seminal quality and serum testosterone". Archives of Andrology 23 (3): 259–65. 1989. doi:10.3109/01485018908986849. PMID 2619414. 
  71. ↑ 71.0 71.1 "Restoration of normal sperm characteristics in hypoprolactinemic infertile men treated with metoclopramide and exogenous human prolactin". Clinical Pharmacology and Therapeutics 58 (3): 354–9. Sep 1995. doi:10.1016/0009-9236(95)90253-8. PMID 7554710. 
  72. ↑ 72.0 72.1 Table 2 in "Serum total prolactin and monomeric prolactin reference intervals determined by precipitation with polyethylene glycol: evaluation and validation on common immunoassay platforms". Clinical Chemistry 54 (10): 1673–81. Oct 2008. doi:10.1373/clinchem.2008.105312. PMID 18719199. 
  73. ↑ 73.0 73.1 73.2 73.3 73.4 Prolaktin at medical.siemens.com—reference ranges as determined from the IMMULITE assay method
  74. ↑ 74.00 74.01 74.02 74.03 74.04 74.05 74.06 74.07 74.08 74.09 74.10 74.11 The AIA essay values are also from Table 2 in Beltran 2008, like the other values, but it uses a different conversion factor of 27.0 mIU/L per µg/L, taken from the second international standard, IS 83/562).
  75. ↑ Melmed S, Kleinberg D 2008 Anterior pituitary. 1n: Kronenberg HM, Melmed S, Polonsky KS, Larsen PR, eds. Willams textbook of endocrinology. 11th ed. Philadelphia: Saunders Elsevier; 185–261
  76. ↑ 76.0 76.1 Polysaccharides Peptides and Proteins: Pharmaceutical Monographs. Elsevier. 12 May 2014. pp. 153–. ISBN 978-1-4831-9612-1. https://books.google.com/books?id=ojGRBQAAQBAJ&pg=PA153. 
  77. ↑ D.F. Horrobin (6 December 2012). Prolactin: Physiology and Clinical Significance. Springer Science & Business Media. pp. 13–. ISBN 978-94-010-9695-9. https://books.google.com/books?id=h_Q-BAAAQBAJ&pg=PA13. 
  78. ↑ Martin H. Johnson (14 December 2012). Essential Reproduction. John Wiley & Sons. pp. 40–. ISBN 978-1-118-42388-2. https://books.google.com/books?id=qXpBst5hJs0C&pg=PT40. 
  79. ↑ Jan Riordan (January 2005). Breastfeeding and Human Lactation. Jones & Bartlett Learning. pp. 468–. ISBN 978-0-7637-4585-1. https://books.google.com/books?id=aiVesab_2bwC&pg=PA468. 

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