Hair loss

{{Short description|Loss of hair from the head or body}} {{Pp-vandalism|small=yes}} {{Redirect-several|dab=off|Balding (surname)|Bald (disambiguation)|Alopecia (disambiguation)}}{{Redirect-distinguish|Baldness|Boldness}}{{Infobox medical condition | name = | pronounce = Alopecia: {{IPAc-en|ˌ|æ|l|ə|'|p|iː|ʃ|(|i|)|ə|,_|-|s|i|.|ə}}<ref>{{cite Dictionary.com|alopecia}}</ref> | synonyms = Alopecia, baldness | image = alopecia.jpg | caption = A bald spot on a man | field = [[Dermatology]] | symptoms = Loss of [[hair]] from part of the head or body.<ref name=NHS2013/> | complications = [[Psychological distress]]<ref name=Nal2016/> | onset = | duration = | types = [[androgenetic alopecia|Male-pattern hair loss]], [[female-pattern hair loss]], [[alopecia areata]], [[telogen effluvium]]<ref name=Var2015/> | causes = | risks = | diagnosis = | differential = | prevention = | treatment = Accepting the condition, medications, shaving of balding areas, surgery<ref name=Var2015/> | medication = [[Pattern hair loss]]: [[minoxidil]], [[finasteride]]<ref name=Mc2012/><br />[[Alopecia areata]]: [[steroid]] injections<ref name=Var2015/> | prognosis = | frequency = 50% of males, 25% of females (pattern hair loss by age 50)<ref name=Var2015/><ref name=Le2008/> | deaths = }} [[File:Wie kommt es zu Haarausfall.webm|thumb|265px|Explanatory video with English subtitles: What causes hair loss? (in 96 seconds)]] <!-- Definition and symptoms --> '''Hair loss''', also known as '''alopecia''' or '''baldness''', refers to a loss of [[hair]] from part of the head or body.<ref name=NHS2013>{{cite web|title=Hair loss|url=http://www.nhs.uk/conditions/Hair-loss/Pages/Introduction.aspx|work=NHS Choices|access-date=22 September 2013|url-status=live|archive-url=https://web.archive.org/web/20130927020342/http://www.nhs.uk/conditions/Hair-loss/Pages/Introduction.aspx|archive-date=27 September 2013}}</ref> Typically at least the head is involved.<ref name=Var2015/> The severity of hair loss can vary from a small area to the entire body.<ref name=DermNet2016>{{cite web |url=https://dermnetnz.org/hair-nails-sweat/hair-loss.html |title=Hair loss |access-date=2016-08-03 |work=DermNet| archive-url= https://web.archive.org/web/20160625012752/http://www.dermnetnz.org/hair-nails-sweat/hair-loss.html| archive-date= 2016-06-25| url-status= live}}</ref> [[Inflammation]] or [[scarring]] is not usually present.<ref name=Var2015/> Hair loss in some people causes [[psychological distress]].<ref name=Nal2016>{{cite journal|last1=Nalluri|first1=R|last2=Harries|first2=M|title=Alopecia in general medicine.|journal=Clinical Medicine|date=February 2016|volume=16|issue=1|pages=74–78|pmid=26833522|pmc=4954340|doi=10.7861/clinmedicine.16-1-74}}</ref>

<!-- Common cause --> Common types include [[androgenetic alopecia|male- or female-pattern hair loss]], [[alopecia areata]], and a thinning of hair known as [[telogen effluvium]].<ref name=Var2015/> The cause of male-pattern hair loss is a combination of [[genetics]] and [[androgen|male hormones]]; the cause of female pattern hair loss is unclear; the cause of alopecia areata is [[autoimmune]]; and the cause of telogen effluvium is typically a physically or psychologically stressful event.<ref name=Var2015/> Telogen effluvium is very common following [[pregnancy]].<ref name=Var2015>{{cite journal|last1=Vary JC|first1=Jr|title=Selected Disorders of Skin Appendages – Acne, Alopecia, Hyperhidrosis.|journal=The Medical Clinics of North America|date=November 2015|volume=99|issue=6|pages=1195–1211|pmid=26476248|doi=10.1016/j.mcna.2015.07.003}}</ref>

<!-- Less common causes --> Less common causes of hair loss without inflammation or scarring include the [[Traction alopecia|pulling out of hair]], certain medications including [[chemotherapy]], [[HIV/AIDS]], [[hypothyroidism]], and [[malnutrition]] including [[Vitamin B12 deficiency|vitamin B₁₂]] and [[Iron deficiency|iron]] deficiencies.<ref name="Nal2016" /><ref name="Var2015" /> Causes of [[scarring hair loss|hair loss that occurs with scarring]] or inflammation include [[tinea capitis|fungal infection]], [[lupus erythematosus]], [[radiation therapy]], and [[sarcoidosis]].<ref name=Nal2016/><ref name=Var2015/> Diagnosis of hair loss is partly based on the areas affected.<ref name=Var2015/>

<!-- Treatment and epidemiology --> Treatment of pattern hair loss may simply involve accepting the condition, which can also include [[Head shaving|shaving one's head]].<ref name=Var2015/> Interventions that can be tried include the medications [[minoxidil]] (or [[finasteride]]) and [[hair transplant surgery]].<ref name=Mc2012>{{cite journal |last1=McElwee |first1=Kevin J. |last2=Shapiro |first2=Jerry |title=Promising therapies for treating and/or preventing androgenic alopecia. |journal=Skin Therapy Letter |date=June 2012 |volume=17 |issue=6 |pages=1–4 |pmid=22735503 |url=https://www.skintherapyletter.com/alopecia/promising-therapies/ }}</ref><ref name=Le2008>{{cite journal | last1 = Leavitt | first1 = M. | title = Understanding and Management of Female Pattern Alopecia | journal = Facial Plastic Surgery | volume = 24 | issue = 4 | pages = 414–427 | year = 2008 | pmid = 19034818 | doi = 10.1055/s-0028-1102905 | s2cid = 260136951 }}</ref> Alopecia areata may be treated by [[corticosteroid|steroid]] injections in the affected area, but these need to be frequently repeated to be effective.<ref name=Var2015/> Hair loss is a common experience.<ref name=Var2015/> Pattern hair loss by age 50 affects about half of men and a quarter of women.<ref name=Var2015/> About 2% of people develop alopecia areata at some point in time.<ref name=Var2015/> {{TOC limit|3}}

== Terminology == Baldness is the partial or complete lack of hair growth, and part of the wider topic of "hair thinning". The degree and pattern of baldness varies, but its most common cause is [[androgenic hair loss]], ''alopecia androgenetica'', or ''alopecia seborrheica'', with the last term primarily used in Europe.{{citation needed|date=September 2013}}

===Hypotrichosis=== Hypotrichosis is a condition of abnormal hair patterns, predominantly loss or reduction. It occurs, most frequently, by the growth of [[vellus hair]] in areas of the body that normally produce [[terminal hair]]. Typically, the individual's [[Human hair growth|hair growth]] is normal after birth, but shortly thereafter the hair is shed and replaced with sparse, abnormal hair growth. The new hair is typically fine, short and brittle, and may lack pigmentation. Baldness may be present by the time the subject is 25 years old.<ref name="Dawber2004pp53-54">{{cite book | last1 = Dawber | first1 = Rodney P. R. | last2 = Van Neste | first2 = Dominique | year = 2004 | title = Hair and scalp disorders: common presenting signs, differential diagnosis and treatment | edition = 2nd | publisher = Informa Health Care | isbn = 978-1-84184-193-9 | pages = 53–54 }}</ref>

== Signs and symptoms == [[File:Andre Agassi (2011).jpg|thumb|upright|A case of mid-frontal baldness: [[Andre Agassi]]]] Symptoms of hair loss include hair loss in patches usually in circular patterns, dandruff, skin lesions, and scarring. Alopecia areata (mild – medium level) usually shows in unusual hair loss areas, e.g., eyebrows, backside of the head or above the ears, areas the male pattern baldness usually does not affect. In male-pattern hair loss, loss and thinning begin at the temples and the crown and hair either thins out or falls out. Female-pattern hair loss occurs at the [[frontal lobe|frontal]] and [[parietal lobe|parietal]].

People have between 100,000 and 150,000 hairs on their head. The number of strands normally lost in a day varies but on average is 100.<ref>{{EMedicine|article|837994|Hair growth}}</ref> In order to maintain a normal volume, hair must be replaced at the same rate at which it is lost. The first signs of hair thinning that people will often notice are more hairs than usual left in the hairbrush after brushing or in the basin after shampooing. Styling can also reveal areas of thinning, such as a wider parting or a thinning crown.{{citation needed|date=August 2015}} [[File:Uudenvuodenpuhe 1959.jpg|thumb|Throughout his political career, [[Urho Kekkonen]], the [[President of Finland]], was well known for his baldness. He was last known to have had hair in about the 1920s.<ref>[https://www.is.fi/kotimaa/art-2000001019668.html Kuvat: Kekkonen ei ollut aina kalju – tältä tuleva presidentti näytti teini-ikäisenä] (in Finnish)</ref> This photo is of Kekkonen in 1959.]]

=== Skin conditions === A substantially blemished face, back and limbs could point to cystic acne. The most severe form of the condition, [[acne vulgaris|cystic acne]], arises from the same hormonal imbalances that cause hair loss and is associated with [[dihydrotestosterone]] production.<ref>{{cite journal |last1=Bergler-Czop |first1=Beata |last2=Brzezińska-Wcisło |first2=Ligia |title=Czynniki hormonalne w etiologii tradziku pospolitego |trans-title=Hormonal factors in etiology of common acne |language=pl |journal=Polski Merkuriusz Lekarski |date=May 2004 |volume=16 |issue=95 |pages=490–492 |pmid=15518435 }}</ref>

=== Psychological ===

The [[psychology]] of hair thinning is a complex issue. Hair is considered an essential part of overall identity: especially for women, for whom it often represents femininity and attractiveness. Men typically associate a full head of hair with youth and vigor. People experiencing hair thinning often find themselves in a situation where their physical appearance is at odds with their own [[self-image]] and commonly worry that they appear older than they are or less attractive to others. Psychological problems due to baldness, if present, are typically most severe at the onset of symptoms.<ref>{{cite journal |last1=Passchier |first1=Jan |last2=Erdman |first2=Jeroen |last3=Hammiche |first3=Fatima |last4=Erdman |first4=Ruud A. M. |title=Androgenetic Alopecia: Stress of Discovery |journal=Psychological Reports |date=September 2016 |volume=98 |issue=1 |pages=226–228 |doi=10.2466/PR0.98.1.226-228 |pmid=16673981 |s2cid=11586141 }}</ref> Two meta analyses on androgenetic alopecia revealed psychosocial distress levels to be moderate, on average, rather than severe and that balding men were no more likely to have depression or self esteem issues compared to non-balding men.<ref>{{Cite journal |last1=Frith |first1=Hannah |last2=Jankowski |first2=Glen S. |date=2024-04-20 |title=Psychosocial impact of androgenetic alopecia on men: A systematic review and meta-analysis |journal=Psychology, Health & Medicine |volume=29 |issue=4 |pages=822–842 |doi=10.1080/13548506.2023.2242049 |issn=1354-8506 |pmid=37605428|doi-access=free }}</ref><ref>{{Cite journal |last1=Huang |first1=Chun-Hsien |last2=Fu |first2=Yun |last3=Chi |first3=Ching-Chi |date=2021-08-01 |title=Health-Related Quality of Life, Depression, and Self-esteem in Patients With Androgenetic Alopecia |journal=JAMA Dermatology |language=en |volume=157 |issue=8 |pages=963–970 |doi=10.1001/jamadermatol.2021.2196 |pmid=34232264 |pmc=8264758 |issn=2168-6068 }}</ref>

Hair loss induced by cancer [[chemotherapy]] has been reported to cause changes in [[self-concept]] and [[body image]]. Body image does not return to the previous state after regrowth of hair for a majority of patients. In such cases, patients have difficulties expressing their feelings ([[alexithymia]]) and may be more prone to avoiding family conflicts. Family therapy can help families to cope with these psychological problems if they arise.<ref>{{cite journal |last1=Poot |first1=F |title=Le retentissement psychologique des pathologies chroniques du cheveu |trans-title=Psychological consequences of chronic hair diseases |language=fr |journal=Revue Médicale de Bruxelles |date=September 2004 |volume=25 |issue=4 |pages=A286–288 |pmid=15516058 |url=https://www.amub.be/revue-medicale-bruxelles/article/le-retentissement-psychologique-des-pathologies-ch-108 |access-date=2020-06-23 |archive-date=2020-06-26 |archive-url=https://web.archive.org/web/20200626093102/https://www.amub.be/revue-medicale-bruxelles/article/le-retentissement-psychologique-des-pathologies-ch-108 }}</ref>

== Causes == Although not completely understood,{{citation needed|date=September 2013}} hair loss can have many causes:

=== Pattern hair loss === {{Main|Pattern hair loss}}

<!-- Content is copied here from lead of Main article, per [[WP:SYNC]]. Please do not change this without first:: a) adding content to the body of the main article; b) evaluating whether that content should also be in the lead of that article; c) updating the lead of that article; d) copying that here --> <!-- Cause and diagnosis --> Male pattern hair loss is believed to be due to a combination of genetics and the [[androgen|male hormone]] [[dihydrotestosterone]].<ref name=Var2015 /> The cause in female pattern hair loss remains unclear.<ref name=Var2015 />

=== Infection === * [[Dissecting cellulitis of the scalp]] * [[Fungal infection]]s (such as [[tinea capitis]]) * [[Folliculitis]] from various causes ** [[Demodex folliculitis]], caused by ''[[Demodex folliculorum]]'', a microscopic mite that feeds on the sebum produced by the [[sebaceous gland]]s, denies hair essential nutrients and can cause thinning. ''Demodex folliculorum'' is not present on every scalp and is more likely to live in an excessively oily scalp environment. * Secondary [[syphilis]]<ref>{{cite web |url=http://www.keratin.com/aq/aq010.shtml |title=Infectious hair disease – syphilis |publisher=Keratin.com |access-date=2011-11-17 |archive-date=2011-09-28 |archive-url=https://web.archive.org/web/20110928205313/http://www.keratin.com/aq/aq010.shtml }}</ref>

=== Drugs === * Temporary or permanent hair loss can be caused by several medications, including those for [[blood pressure]] problems, [[Diabetes mellitus|diabetes]], [[heart disease]] and [[cholesterol]].<ref name="WebMD Drug-Induced Hair Loss">{{cite web |title=Drug-Induced Hair Loss |url=https://www.webmd.com/skin-problems-and-treatments/hair-loss/drug-induced-hair-loss-1 |website=WebMD }}</ref> Any that affect the body's hormone balance can have a pronounced effect: these include the contraceptive pill, [[Hormone replacement therapy (menopause)|hormone replacement therapy]], [[steroid]]s and [[Acne vulgaris|acne]] medications.<ref>{{cite web |title=Drug Induced Hair Loss |url=https://www.americanhairloss.org/drug_induced_hair_loss/ |website=American Hair Loss Association |access-date=2020-06-23 |archive-date=2020-08-01 |archive-url=https://web.archive.org/web/20200801131432/https://www.americanhairloss.org/drug_induced_hair_loss/ }}</ref> * Some treatments used to cure [[fungus|mycotic]] infections can cause massive hair loss.<ref>{{cite journal |last1=Pappas |first1=Peter G. |last2=Kauffman |first2=CA |last3=Perfect |first3=J |last4=Johnson |first4=PC |last5=McKinsey |first5=DS |last6=Bamberger |first6=DM |last7=Hamill |first7=R |last8=Sharkey |first8=PK |last9=Chapman |first9=SW |last10=Sobel |first10=JD |title=Alopecia Associated with Fluconazole Therapy |journal=Annals of Internal Medicine |date=1 September 1995 |volume=123 |issue=5 |pages=354–357 |doi=10.7326/0003-4819-123-5-199509010-00006 |pmid=7625624 |s2cid=53091175 }}</ref> * Medications (side effects from drugs, including [[chemotherapy]], [[anabolic steroids]], and [[birth control pill]]s<ref>{{cite web |title=Alopecia |url=https://www.healthgrades.com/right-care/skin-hair-and-nails/alopecia |website=Healthgrades |date=26 June 2014 }}</ref><ref name="WebMD Drug-Induced Hair Loss"/>)

=== Trauma === * [[Traction alopecia]] is most commonly found in people with [[ponytail]]s or [[cornrows]] who pull on their hair with excessive force. In addition, rigorous brushing and heat styling, rough scalp massage can damage the [[Cuticle (hair)|cuticle]], the hard outer casing of the hair. This causes individual strands to become weak and break off, reducing overall hair volume. * [[Frictional alopecia]] is hair loss caused by rubbing of the hair or follicles, most infamously around the ankles of men from socks, where even if socks are no longer worn, the hair often will not grow back. * [[Trichotillomania]] is the loss of hair caused by compulsive pulling and bending of the hairs. Onset of this disorder tends to begin around the onset of puberty and usually continues through adulthood. Due to the constant extraction of the hair roots, permanent hair loss can occur. * Traumas such as childbirth, major surgery, poisoning, and severe stress may cause a hair loss condition known as [[telogen effluvium]],<ref>{{cite journal |last1=Nnoruka |first1=Nkechi Edith |title=Hair loss: is there a relationship with hair care practices in Nigeria? |journal=International Journal of Dermatology |date=October 2005 |volume=44 |issue=s1 |pages=13–17 |doi=10.1111/j.1365-4632.2005.02801.x |pmid=16187950 |s2cid=26608122 }}</ref> in which a large number of hairs enter the resting phase at the same time, causing shedding and subsequent thinning. The condition also presents as a side effect of [[chemotherapy]] – while targeting dividing cancer cells, this treatment also affects hair's growth phase with the result that almost 90% of hairs fall out soon after chemotherapy starts.<ref>{{cite web |title=Causes of Hair Loss |url=https://www.americanhairloss.org/women_hair_loss/causes_of_hair_loss.html |website=American Hair Loss Association }}</ref>{{failed verification|date=May 2025}} * Radiation to the scalp, as when radiotherapy is applied to the head for the treatment of certain cancers there, can cause baldness of the irradiated areas.

=== Pregnancy === Hair loss often follows [[childbirth]] in the [[postpartum period]] without causing baldness. During pregnancy, the hair is thicker owing to increased circulating [[estrogen]]s. Approximately three months after giving birth (typically between 2 and 5 months), estrogen levels drop and hair loss occurs, often particularly noticeably around the hairline and [[Temple (anatomy)|temple]] area. Hair typically grows back normally and treatment is not indicated.<ref>{{cite journal |last1=Schiff |first1=Bencel L. |last2=Kern |first2=A B |title=Study of Postpartum Alopecia |journal=Archives of Dermatology |date=1 May 1963 |volume=87 |issue=5 |pages=609–611 |doi=10.1001/archderm.1963.01590170067011 |pmid=13991677 }}</ref><ref>{{cite journal |last1=Eastham |first1=John H |title=Postpartum Alopecia |journal=The Annals of Pharmacotherapy |date=February 2001 |volume=35 |issue=2 |pages=255–258 |doi=10.1345/1542-6270(2001)035<0255:pa>2.0.co;2 |pmid=11215848 }}</ref> A similar situation occurs in women taking the fertility-stimulating drug [[clomiphene]].

=== Other causes === * '''Autoimmune disease'''. [[Alopecia areata]] is an [[autoimmune diseases|autoimmune disorder]] also known as "spot baldness" that can result in hair loss ranging from just one location (''Alopecia areata monolocularis'') to every hair on the entire body (''Alopecia areata universalis''). Although thought to be caused by hair follicles becoming dormant, what triggers alopecia areata is not known. In most cases the condition corrects itself, but it can also spread to the entire scalp ([[alopecia totalis]]) or to the entire body ([[alopecia universalis]]). * '''Skin diseases and cancer'''. Localized or diffuse hair loss may also occur in cicatricial alopecia ([[lupus erythematosus]], lichen plano pilaris, folliculitis decalvans, [[central centrifugal cicatricial alopecia]], postmenopausal frontal fibrosing alopecia, etc.). Tumours and skin outgrowths also induce localized baldness (sebaceous nevus, basal cell carcinoma, squamous cell carcinoma). Tumor alopecia is the hair loss in the immediate vicinity of either benign or malignant tumors of the scalp.<ref name="Andrews762">James, William; Berger, Timothy; Elston, Dirk (2005). ''Andrews' Diseases of the Skin: Clinical Dermatology''. (10th ed.). Saunders. {{ISBN|0-7216-2921-0}}.</ref>{{rp|762}} * '''[[Hypothyroidism]]''' (an under-active [[thyroid]]) and the side effects of its related medications can cause hair loss, typically frontal, which is particularly associated with thinning of the outer third of the eyebrows (also seen with syphilis). [[Hyperthyroidism]] (an over-active thyroid) can also cause hair loss, which is parietal rather than frontal.<ref>[http://health.yahoo.com/beauty-treatment/alopecia-areata/healthwise--ug2838spec.html Alopecia Areata] {{webarchive|url=https://web.archive.org/web/20081013064923/http://health.yahoo.com/beauty-treatment/alopecia-areata/healthwise--ug2838spec.html |date=2008-10-13 }}, by Maria G. Essig, MS, ELS, Yahoo! Health</ref>{{MEDRS|date=August 2013}} * '''Sebaceous cysts'''. Temporary loss of hair can occur in areas where [[sebaceous cyst]]s are present for considerable duration (normally one to several weeks). * [[Triangular alopecia|Congenital triangular alopecia]] – It is a triangular, or oval in some cases, shaped patch of hair loss in the temple area of the scalp that occurs mostly in young children. The affected area mainly contains vellus hair follicles or no hair follicles at all, but it does not expand. Its causes are unknown, and although it is a permanent condition, it does not have any other effect on the affected individuals.<ref>{{cite web|url=http://www.keratin.com/af/af005.shtml|title=Congenital triangular alopecia|access-date=2010-06-29|archive-date=2011-07-13|archive-url=https://web.archive.org/web/20110713144349/http://www.keratin.com/af/af005.shtml}}</ref> * '''Hair growth conditions'''. Gradual thinning of hair with age is a natural condition known as ''involutional alopecia''. This is caused by an increasing number of [[hair follicle]]s switching from the growth, or anagen, phase into a resting phase, or telogen phase, so that remaining hairs become shorter and fewer in number. An unhealthy scalp environment can play a significant role in hair thinning by contributing to miniaturization or causing damage.{{citation needed|date=November 2015}} * '''Obesity'''. Obesity-induced stress, such as that induced by a high-fat diet (HFD), targets hair follicle stem cells (HFSCs) to accelerate hair thinning in mice. It is likely that similar molecular mechanism play a role in human hair loss.<ref>{{Cite journal |last1=Morinaga |first1=Hironobu |last2=Mohri |first2=Yasuaki |last3=Grachtchouk |first3=Marina |last4=Asakawa |first4=Kyosuke |last5=Matsumura |first5=Hiroyuki|last6=Oshima |first6=Motohiko |last7=Takayama |first7=Naoya |last8=Kato |first8=Tomoki |last9=Nishimori |first9=Yuriko |last10=Sorimachi |first10=Yuriko |last11=Takubo |first11=Keiyo |date=July 2021 |title=Obesity accelerates hair thinning by stem cell-centric converging mechanisms |journal=Nature|language=en|volume=595|issue=7866|pages=266–271|doi=10.1038/s41586-021-03624-x |pmid=34163066 |pmc=9600322 |bibcode=2021Natur.595..266M|s2cid=235625692|issn=1476-4687}}</ref>

Other causes of hair loss include: * [[Alopecia mucinosa]] * [[Biotinidase deficiency]] * [[Chronic inflammation]] * [[Diabetes]]<ref>{{cite web|url=http://www.medicalbug.com/what-is-alopecia-what-causes-alopecia/|title=What is Alopecia: What Causes Alopecia?|date=6 February 2012|publisher=MedicalBug|access-date=28 March 2012|url-status=live|archive-url=https://web.archive.org/web/20130122081220/http://www.medicalbug.com/what-is-alopecia-what-causes-alopecia/|archive-date=22 January 2013}}</ref> * [[Pseudopelade of Brocq]] * [[Telogen effluvium]] * [[Tufted folliculitis]]

===Genetics=== Genetic forms of localized autosomal recessive hypotrichosis include:

{| class="wikitable" |- ! Type ! [[OMIM]] ! Gene ! Locus |- | LAH1 | {{OMIM|607903||none}} | ''[[DSG4]]'' | 18q12 |- | LAH2 | {{OMIM|604379||none}} | ''[[LIPH (gene)|LIPH]]'' | 3q27 |- | LAH3 | {{OMIM|611452||none}} | ''[[P2RY5]]'' | 13q14.12-q14.2 |}

== Pathophysiology == [[Hair follicle]] growth occurs in cycles. Each cycle consists of a long growing phase ([[anagen phase|anagen]]), a short transitional phase ([[anagen phase|catagen]]) and a short resting phase ([[anagen phase|telogen]]). At the end of the resting phase, the hair falls out (exogen) and a new hair starts growing in the follicle, beginning the cycle again.

Normally, about 40 (0–78 in men) hairs reach the end of their resting phase each day and fall out.<ref name="Yamada et al 2013">{{Cite journal|pmid=23554099|year=2013|last1=Yamada|first1=T|last2=Hara|first2=K|last3=Umematsu|first3=H|last4=Kadowaki|first4=T|title=Male pattern baldness and its association with coronary heart disease: A meta-analysis|volume=3|issue=4|doi=10.1136/bmjopen-2012-002537|doi-access=free|pmc=3641488|journal=[[BMJ Open]]|article-number=e002537}} {{open access}}</ref> When more than 100 hairs fall out per day, clinical hair loss ([[telogen effluvium]]) may occur.{{Citation needed|date=June 2012}} A disruption of the growing phase causes abnormal loss of anagen hairs ([[anagen effluvium]]).

== Diagnosis == Because they are not usually associated with an increased loss rate, male-pattern and female-pattern hair loss do not generally require testing. If hair loss occurs in a young man with no family history, drug use could be the cause. * '''The pull test''' helps to evaluate diffuse scalp hair loss. Gentle traction is exerted on a group of hairs (about 40–60) on three different areas of the scalp. The number of extracted hairs is counted and examined under a microscope. Normally, fewer than three hairs per area should come out with each pull. If more than ten hairs are obtained, the pull test is considered positive.<ref>{{cite web|url=http://www.keratin.com/ab/ab017.shtml|title=The hair pull test|publisher=Keratin.com|access-date=28 March 2012|archive-date=22 January 2013|archive-url=https://web.archive.org/web/20130122134633/http://www.keratin.com/ab/ab017.shtml}}{{self-published inline|date=June 2020}}</ref> * '''The pluck test''' is conducted by pulling hair out "by the roots". The root of the plucked hair is examined under a microscope to determine the phase of growth, and is used to diagnose a defect of telogen, anagen, or systemic disease. Telogen hairs have tiny bulbs without sheaths at their roots. Telogen effluvium shows an increased percentage of hairs upon examination. Anagen hairs have sheaths attached to their roots. Anagen effluvium shows a decrease in telogen-phase hairs and an increased number of broken hairs.{{citation needed|date=June 2020}} * '''Scalp biopsy''' is used when the diagnosis is unsure; a biopsy allows for differing between scarring and nonscarring forms. Hair samples are taken from areas of inflammation, usually around the border of the bald patch.{{citation needed|date=June 2020}} * '''Daily hair counts''' are normally done when the pull test is negative. It is done by counting the number of hairs lost. The hair from the first morning combing or during washing should be counted. The hair is collected in a clear plastic bag for 14 days. The strands are recorded. If the hair count is >100/day, it is considered abnormal except after shampooing, where hair counts will be up to 250 and be normal.{{citation needed|date=February 2012}} * '''[[Trichoscopy]]''' is a noninvasive method of examining hair and scalp. The test may be performed with the use of a handheld [[dermoscope]] or a video dermoscope. It allows differential diagnosis of hair loss in most cases.<ref>{{Cite journal|vauthors=Rudnicka L, Olszewska M, Rakowska A, Kowalska-Oledzka E, Slowinska M |title= Trichoscopy: a new method for diagnosing hair loss |journal= J Drugs Dermatol |volume=7 |issue=7 |pages=651–654 |year=2008 |pmid=18664157 }}</ref>

There are two types of identification tests for female pattern baldness: the Ludwig Scale and the Savin Scale. Both track the progress of diffused thinning, which typically begins on the crown of the head behind the hairline, and becomes gradually more pronounced. For male pattern baldness, the [[Hamilton–Norwood scale]] tracks the progress of a receding hairline and/or a thinning crown, through to a horseshoe-shaped ring of hair around the head and on to total baldness.{{citation needed|date=June 2020}}

In almost all cases of thinning, and especially in cases of severe hair loss, it is recommended to seek advice from a doctor or [[dermatologist]]. Many types of thinning have an underlying genetic or health-related cause, which a qualified professional will be able to diagnose.{{citation needed|date=June 2020}}

== Management == {{See also|Management of hair loss|List of investigational hair loss drugs}}

=== Hiding hair loss === [[File:Syngman Rhee and Douglas MacArthur.jpg|thumb|General [[Douglas MacArthur]] with a [[comb over]]]]

==== Head ==== One method of hiding hair loss is the [[comb over]], which involves restyling the remaining hair to cover the balding area. It is usually a temporary solution, useful only while the area of hair loss is small. As the hair loss increases, a comb over becomes less effective.

Another method is to wear a hat or a hairpiece such as a [[Wig (hair)|wig]] or [[toupee]]. The wig is a layer of artificial or natural hair made to resemble a typical hair style. In most cases the hair is artificial. Wigs vary widely in quality and cost. In the United States, the best wigs{{snd}}those that look like real hair{{snd}}cost up to tens of thousands of dollars. Organizations also collect individuals' donations of their own natural hair to be made into wigs for young [[cancer]] patients who have lost their hair due to [[chemotherapy]] or other cancer treatment in addition to any type of hair loss.

==== Eyebrows ==== Though not as common as the loss of hair on the head, chemotherapy, hormone imbalance, forms of hair loss, and other factors can also cause loss of hair in the eyebrows. Loss of growth in the outer one third of the eyebrow is often associated with [[hypothyroidism]]. Artificial eyebrows are available to replace missing eyebrows or to cover patchy eyebrows. Eyebrow embroidery is another option which involves the use of a blade to add pigment to the eyebrows. This gives a natural 3D look for those who are worried about an artificial look and it lasts for two years. [[Micropigmentation]] (permanent makeup tattooing) is also available for those who want the look to be permanent.

=== Medications === [[File:Petróleo Químico Nally - Ilustração Portugueza (6Set1941).png|thumb|Portuguese advertisement for a hair-loss product from the 1940s]] Treatments for the various forms of hair loss have limited success. Three medications have evidence to support their use in male pattern hair loss: [[minoxidil]], [[finasteride]], and [[dutasteride]].<ref name=Ban2013 /><ref name=Roger2008 /> They typically work better to prevent further hair loss, than to regrow lost hair.<ref name=Ban2013 /> On June 13, 2022, the U.S. Food and Drug Administration (FDA) approved Olumiant (baricitinib) for adults with severe alopecia areatal. It is the first FDA approved drug for systemic treatment, or treatment for any area of the body.<ref>{{cite web|url=https://www.fda.gov/news-events/press-announcements/fda-approves-first-systemic-treatment-alopecia-areata |archive-url=https://web.archive.org/web/20220613195220/https://www.fda.gov/news-events/press-announcements/fda-approves-first-systemic-treatment-alopecia-areata |archive-date=June 13, 2022 |title=FDA Approves First Systemic Treatment for Alopecia Areata &#124; FDA |publisher=Fda.gov |date=2022-06-13 |access-date=2022-06-29}}</ref> * [[Minoxidil]] (Rogaine) is a nonprescription medication approved for male pattern baldness and alopecia areata. In a liquid or foam, it is rubbed into the scalp twice a day. Some people have an allergic reaction to the propylene glycol in the minoxidil solution and a minoxidil foam was developed without propylene glycol. Not all users will regrow hair. Minoxidil may also be taken orally although this route of administration is not approved by the FDA.<ref>{{Cite web |date=2022-08-22 |title=An Old Medicine Remedies Hair Loss for Pennies a Day, Doctors Say - The New York Times |website=[[The New York Times]] |url=https://www.nytimes.com/2022/08/18/health/minoxidil-hair-loss-pills.html |access-date=2022-08-22 |archive-url=https://web.archive.org/web/20220822035952/https://www.nytimes.com/2022/08/18/health/minoxidil-hair-loss-pills.html |archive-date=2022-08-22 }}</ref> The longer the hair has stopped growing, the less likely minoxidil will regrow hair. Minoxidil is not effective for other causes of hair loss. Hair regrowth can take 1 to 6 months to begin. Treatment must be continued indefinitely. If the treatment is stopped, hair loss resumes. Any regrown hair and any hair susceptible to being lost, while Minoxidil was used, will be lost. Most frequent side effects are mild scalp irritation, [[allergic contact dermatitis]], and unwanted hair in other parts of the body.<ref name=Roger2008>{{cite journal |last1=Rogers |first1=Nicole E. |last2=Avram |first2=Marc R. |title=Medical treatments for male and female pattern hair loss |journal=Journal of the American Academy of Dermatology |date=October 2008 |volume=59 |issue=4 |pages=547–566 |doi=10.1016/j.jaad.2008.07.001 |pmid=18793935 }}</ref> * [[Finasteride]] (Propecia) is used in male-pattern hair loss in a pill form, taken 1 milligram per day. It is not indicated for women and is not recommended in pregnant women (as it is known to cause birth defects in fetuses). Treatment is effective starting within 6 weeks of treatment. Finasteride causes an increase in hair retention, the weight of hair, and some increase in regrowth. Side effects in about 2% of males include decreased [[libido|sex drive]], [[erectile dysfunction]], and ejaculatory dysfunction. Treatment should be continued as long as positive results occur. Once treatment is stopped, hair loss resumes.<ref name=Roger2008 /> * [[Corticosteroid]]s injections into the scalp can be used to treat alopecia areata. This type of treatment is repeated on a monthly basis. Oral pills for extensive hair loss may be used for alopecia areata. Results may take up to a month to be seen.{{citation needed|date=June 2020}} * [[Immunosuppressant]]s applied to the scalp have been shown to temporarily reverse alopecia areata, though the side effects of some of these drugs make such therapy questionable.<ref>{{cite journal |last1=Joly |first1=Pascal |title=The use of methotrexate alone or in combination with low doses of oral corticosteroids in the treatment of alopecia totalis or universalis |journal=Journal of the American Academy of Dermatology |date=October 2006 |volume=55 |issue=4 |pages=632–636 |doi=10.1016/j.jaad.2005.09.010 |pmid=17010743 }}</ref> * There is some tentative evidence that [[anthralin]] may be useful for treating alopecia areata.<ref>{{cite journal |last1=Shapiro |first1=Jerry |title=Current Treatment of Alopecia Areata |journal=Journal of Investigative Dermatology Symposium Proceedings |date=December 2013 |volume=16 |issue=1 |pages=S42–S44 |doi=10.1038/jidsymp.2013.14 |pmid=24326551 |doi-access=free }}</ref> * [[Hormonal modulators]] ([[oral contraceptive pill|oral contraceptives]] or antiandrogens such as [[spironolactone]] and [[flutamide]]) can be used for female-pattern hair loss associated with [[hyperandrogenism|hyperandrogenemia]].{{citation needed|date=June 2020}}

=== Surgery === [[Hair transplantation]] is usually carried out under [[local anaesthetic|local anesthetic]]. A surgeon will move healthy hair from the back and sides of the head to areas of thinning. The procedure can take between four and eight hours, and additional sessions can be carried out to make hair even thicker. Transplanted hair falls out within a few weeks, but regrows permanently within months.<ref>'Hair Transplants', WebMD: {{cite web |url=http://www.webmd.com/skin-problems-and-treatments/hair-loss/hair-transplants |title=Hair Transplant Procedures: Average Cost, What to Expect, and More |access-date=2013-09-21 |url-status=live |archive-url=https://web.archive.org/web/20130921061040/http://www.webmd.com/skin-problems-and-treatments/hair-loss/hair-transplants |archive-date=2013-09-21 }}</ref> * Surgical options, such as follicle transplants, scalp flaps, and hair loss reduction, are available. These procedures are generally chosen by those who are self-conscious about their hair loss, but they are expensive and painful, with a risk of infection and scarring. Once surgery has occurred, six to eight months are needed before the quality of new hair can be assessed. ** Scalp reduction is the process of decreasing of the area of bald skin on the head. In time, the skin on the head becomes flexible and stretched enough that some of it can be surgically removed. After the hairless scalp is removed, the space is closed with hair-covered scalp. Scalp reduction is generally done in combination with hair transplantation to provide a natural-looking hairline, especially those with extensive hair loss. ** [[Hairline lowering]] can sometimes be used to lower a high hairline secondary to hair loss, although there may be a visible scar after further hair loss. * Wigs are an alternative to medical and surgical treatment; some patients wear a wig or hairpiece. They can be used permanently or temporarily to cover the hair loss. High-quality, natural-looking wigs and hairpieces are available.

=== Chemotherapy === [[Hypothermia cap]]s may be used to prevent hair loss during some kinds of [[chemotherapy]], specifically, when [[taxanes]] or [[anthracyclines]] are administered.<ref>{{cite journal |last1=Grevelman |first1=E.G. |last2=Breed |first2=W.P.M. |title=Prevention of chemotherapy-induced hair loss by scalp cooling |journal=Annals of Oncology |date=March 2005 |volume=16 |issue=3 |pages=352–358 |doi=10.1093/annonc/mdi088 |pmid=15642703 |doi-access=free}}</ref> It is not recommended to be used when cancer is present in the skin of the scalp or for lymphoma or leukemia.<ref>{{cite journal |last1=Breed |first1=Wim P. M. |title=What is wrong with the 30-year-old practice of scalp cooling for the prevention of chemotherapy-induced hair loss? |journal=Supportive Care in Cancer |date=1 January 2004 |volume=12 |issue=1 |pages=3–5 |doi=10.1007/s00520-003-0551-8 |pmid=14615930 |s2cid=25031894 }}</ref> There are generally only minor side effects from scalp cooling given during chemotherapy.<ref>{{cite journal |last1=Komen |first1=Manon M.C. |last2=Smorenburg |first2=Carolien H. |last3=van den Hurk |first3=Corina J.G. |last4=Nortier |first4=J.W.R. (Hans) |title=Hoofdhuidkoeling tegen alopecia door chemotherapie |trans-title=Scalp cooling for chemotherapy-induced alopecia |language=nl |journal=Nederlands Tijdschrift voor Geneeskunde |date=2011 |volume=155 |issue=45 |pages=A3768 |pmid=22085565 |url=https://www.ntvg.nl/system/files/publications/a3768.pdf }}</ref>

=== Embracing baldness === {{Main|Head shaving}} [[File:Roshi with group of nuns.jpg|alt=A group of Buddhist nuns with shaved heads smiling at the camera|thumb|Buddhist nuns with shaved heads]] Instead of attempting to conceal their hair loss, some people embrace it by either doing nothing about it or sporting a [[Head shaving|shaved head]].<ref>{{Cite news|url=https://www.theguardian.com/fashion/shortcuts/2013/feb/12/10-rules-bald-men-david-cameron|title=The 10 rules for bald men|last=Benedictus|first=Leo|date=February 2, 2013|work=The Guardian|access-date=December 1, 2018|language=en-GB|issn=0261-3077}}</ref><ref>{{cite web|url=https://www.pastemagazine.com/articles/2015/10/the-20-greatest-bald-heads-in-the-history-of-socce.html|title=The 20 Greatest Bald Heads in the History of Soccer|last=Rockwell|first=Taylor|date=October 16, 2015|website=pastemagazine.com|language=en|access-date=December 1, 2018|archive-date=December 15, 2018|archive-url=https://web.archive.org/web/20181215224724/https://www.pastemagazine.com/articles/2015/10/the-20-greatest-bald-heads-in-the-history-of-socce.html}}</ref> The general public became more accepting of men with shaved heads in the early 1950s, when Russian-American actor [[Yul Brynner]] began sporting the look; the resulting phenomenon inspired many of his male fans to shave their heads.<ref>{{cite book|last=Brynner|first=Rock|title=Empire & odyssey: the Brynners in Far East Russia and beyond|year=2006|publisher=Steerforth Press}}</ref> Male celebrities then continued to bring mainstream popularity to shaved heads,<ref>{{cite book|last=Crouse|first=Richard|title=Reel Winners: Movie Award Trivia|publisher=Dundurn|year=2005|url=https://archive.org/details/reelwinnersmovie0000crou|url-access=registration|page=[https://archive.org/details/reelwinnersmovie0000crou/page/171 171]}}</ref><ref>{{cite book|last=Doyle|first=Hubert|title=Ventures with the World of Celebrities, Movies & TV|year=2008|publisher=Joshua Tree |isbn=978-0-9768677-6-0|url=https://books.google.com/books?id=IywAvmhXLPkC&pg=PA112}}</ref><ref>{{cite book|last=Douty|first=Linda|title=How Did I Get to Be 70 When I'm 35 Inside?: Spiritual Surprises of Later Life|year=2011|publisher=SkyLight Paths |isbn=978-1-59473-297-3|url=https://books.google.com/books?id=WWL0FrvB0PIC&pg=PA50}}</ref> including athletes such as [[Michael Jordan]]<ref>{{cite news |last1=Gaines |first1=Cork |title=Michael Jordan once turned down a huge endorsement deal because he didn't like the product's name and another one because he was going bald |url=https://www.businessinsider.com/michael-jordan-beanee-weenee-endorsement-deal-rejected-2019-3 |work=Business Insider |date=17 April 2020 }}</ref> and [[Zinedine Zidane]] and actors such as [[Dwayne Johnson]],<ref>{{cite news |last1=Bryant |first1=Kenzie |title=Dwayne 'The Rock' Johnson Explains Why He's Bald |url=https://www.vanityfair.com/style/2017/04/dwayne-the-rock-johnson-bald |magazine=Vanity Fair |date=3 April 2017 }}</ref> [[Ben Kingsley]],<ref>{{cite web |last1=Steele |first1=Francesca |title=Ferdinand Kingsley interview: 'Yeah, but mum's dad was totally bald too!' |url=https://www.spectator.co.uk/article/ferdinand-kingsley-interview-yeah-but-mum-s-dad-was-totally-bald-too- |website=Spectator |access-date=8 June 2020 |date=19 April 2014}}</ref> and [[Jason Statham]].<ref>{{cite web |last1=Huynh |first1=Mike |title=Jason Statham Is Showing Bald Men How To Look Stylishly Masculine |url=https://www.dmarge.com/2019/07/jason-statham-suit.html |website=DMARGE |access-date=9 June 2020 |date=24 July 2019}}</ref>

To a lesser extent, female baldness can also be neutrally and positively interpreted in various parts of the world.<ref>{{cite web |title=Cultural Perceptions of Hair & Baldness |url=https://www.hairscientists.org/hair-and-scalp-conditions/social-religious-cultural-perceptions-of-hair-baldness |website=The Trichological Society }}{{self-published inline|date=June 2020}}</ref><ref>{{cite journal |last1=Ramos |first1=Paulo Müller |last2=Miot |first2=Hélio Amante |title=Female Pattern Hair Loss: a clinical and pathophysiological review |journal=Anais Brasileiros de Dermatologia |date=2015 |volume=90 |issue=4 |pages=529–543 |doi=10.1590/abd1806-4841.20153370 |pmid=26375223 |pmc=4560543 }}</ref> For example head shaving is not uncommon among Black women, Buddhist nuns, lesbians and some feminists can represent a release from superficial and restrictive appearance norms.<ref>{{Cite journal |last=Zalcberg |first=Sima |date=2007-09-01 |title="Grace Is Deceitful and Beauty Is Vain": How Hassidic Women Cope with the Requirement of Shaving One's Head and Wearing a Black Kerchief |journal=[[Gender Issues (magazine)|Gender Issues]] |language=en |volume=24 |issue=3 |pages=13–34 |doi=10.1007/s12147-007-9043-3 |issn=1936-4717}}</ref><ref>{{Cite journal |last1=Clarke-Jeffers |first1=Paige |last2=Keyte |first2=Rebecca |last3=Connabeer |first3=Kathrina |date=2024-01-26 |title="Hair is your crown and glory" – Black women's experiences of living with alopecia and the role of social support |url=https://hpr.termedia.pl/-Hair-is-your-crown-and-glory-Black-women-s-experiences-of-living-nwith-alopecia,177730,0,2.html |journal=Health Psychology Report |volume=12 |issue=2 |pages=154–165 |doi=10.5114/hpr/177730 |issn=2353-4184 |pmc=11016946 |pmid=38628276}}</ref>

=== Alternative medicine === Dietary supplements are not typically recommended.<ref name=Roger2008 /> There is only one small trial of [[saw palmetto]] which shows tentative benefit in those with mild to moderate androgenetic alopecia.<ref name=Roger2008 /> There is no evidence for [[biotin]].<ref name=Roger2008 /> Evidence for most other alternative medicine remedies is also insufficient.<ref name=Blu2011 /> There was no good evidence for [[ginkgo]], [[aloe vera]], [[ginseng]], [[bergamot essential oil|bergamot]], [[hibiscus]], or [[sophora]] as of 2011.<ref name=Blu2011>{{cite journal|last1=Blumeyer|first1=A|last2=Tosti|first2=A|last3=Messenger|first3=A|last4=Reygagne|first4=P|last5=Del Marmol|first5=V|last6=Spuls|first6=PI|last7=Trakatelli|first7=M|last8=Finner|first8=A|last9=Kiesewetter|first9=F|last10=Trüeb|first10=R|last11=Rzany|first11=B|last12=Blume-Peytavi|first12=U|last13=European Dermatology Forum|first13=(EDF)|title=Evidence-based (S3) guideline for the treatment of androgenetic alopecia in women and in men.|journal=Journal of the German Society of Dermatology|date=October 2011|volume=9|issue=Suppl 6|pages=S1–57|pmid=21980982|doi=10.1111/j.1610-0379.2011.07802.x|s2cid=29821046}}</ref>

Many people use unproven treatments to treat hair loss.<ref name=Ban2013>{{cite journal|last=Banka|first=N|author2=Bunagan, MJ |author3=Shapiro, J |title=Pattern hair loss in men: diagnosis and medical treatment|journal=Dermatologic Clinics|date=January 2013|volume=31|issue=1|pages=129–40|pmid=23159182|doi=10.1016/j.det.2012.08.003}}</ref> [[Egg oil]], in Indian,<ref>{{cite book |last1=Panda |first1=H. |title=Handbook On Ayurvedic Medicines With Formulae, Processes And Their Uses |date=2004 |publisher=NIIR Project Consultancy Services |isbn=978-81-86623-63-3 |page=146 }}</ref> Japanese, [[Unani]] (Roghan Baiza Murgh)<ref>{{cite book |last1=Babu |first1=S. Suresh |title=Home Made Herbal Cosmetics |date=2005 |publisher=Pustak Mahal |isbn=978-81-223-0775-7 |page=103 }}</ref> and Chinese<ref>{{cite book |last1=Zhou |first1=Zhongying |last2=Jin |first2=Hui De |title=Clinical Manual of Chinese Herbal Medicine and Acupuncture |date=1997 |publisher=Churchill Livingstone |isbn=978-0-443-05128-9 |page=222 }}</ref> [[traditional medicine]], was traditionally used as a treatment for hair loss.{{medical citation needed|date=October 2015}}

== Research == Research is looking into connections between hair loss and other health issues. While there has been speculation about a connection between early-onset male pattern hair loss and heart disease, a review of articles from 1954 to 1999 found no conclusive connection between baldness and coronary artery disease. The dermatologists who conducted the review suggested further study was needed.<ref>{{cite journal |last1=Rebora |first1=Alfredo |title=Baldness and Coronary Artery Disease: The Dermatologic Point of View of a Controversial Issue |journal=Archives of Dermatology |date=1 July 2001 |volume=137 |issue=7 |pages=943–947 |pmid=11453815 |url=https://jamanetwork.com/journals/jamadermatology/fullarticle/vol/137/pg/943 }}</ref>

Environmental factors are under review. A 2007 study indicated that smoking may be a factor associated with age-related hair loss among Asian men. The study controlled for age and family history, and found statistically significant positive associations between moderate or severe male pattern hair loss and smoking status.<ref>{{cite journal |last1=Su |first1=Lin-Hui |last2=Chen |first2=Tony Hsiu-Hsi |title=Association of Androgenetic Alopecia With Smoking and Its Prevalence Among Asian Men |journal=Archives of Dermatology |date=1 November 2007 |volume=143 |issue=11 |pages=1401–1406 |doi=10.1001/archderm.143.11.1401 |pmid=18025364 |doi-access=free }}</ref>

Vertex baldness is associated with an increased risk of [[coronary heart disease]] (CHD) and the relationship depends upon the severity of baldness, while frontal baldness is not. Thus, vertex baldness might be a marker of CHD and is more closely associated with atherosclerosis than frontal baldness.<ref name="Yamada et al 2013" />

=== Hair follicle aging ===

A key aspect of hair loss with age is the aging of the hair follicle.<ref name="pmid26912687">{{cite journal|author-link2=Cheng-Ming Chuong |vauthors=Lei M, Chuong CM |title=STEM CELLS. Aging, alopecia, and stem cells |journal=Science |volume=351 |issue=6273 |pages=559–560 |year=2016 |pmid=26912687 |doi=10.1126/science.aaf1635 |bibcode=2016Sci...351..559L |doi-access=free }}</ref> Ordinarily, hair follicle renewal is maintained by the stem cells associated with each follicle. Aging of the hair follicle appears to be primed by a sustained cellular response to the DNA damage that accumulates in renewing stem cells during aging.<ref name="pmid26912707">{{cite journal |vauthors=Matsumura H, Mohri Y, Binh NT, Morinaga H, Fukuda M, Ito M, Kurata S, Hoeijmakers J, Nishimura EK |title=Hair follicle aging is driven by transepidermal elimination of stem cells via COL17A1 proteolysis |journal=Science |volume=351 |issue=6273 |article-number=aad4395 |year=2016 |pmid=26912707 |doi=10.1126/science.aad4395 |s2cid=5078019 }}</ref> This damage response involves the proteolysis of [[Collagen, type XVII, alpha 1|type XVII collagen]] by [[neutrophil elastase]] in response to DNA damage in hair follicle stem cells. Proteolysis of collagen leads to elimination of the damaged cells and, consequently, to terminal hair follicle miniaturization.

=== Hedgehog signaling ===

In June 2022 the University of California, Irvine announced that researchers have discovered that [[hedgehog signaling]] in murine fibroblasts induces new hair growth and hair multiplication while hedgehog activation increases fibroblast heterogeneity and drives new cell states.<ref>{{cite journal |last1=Liu |first1=Yingzi |title=Hedgehog signaling reprograms hair follicle niche fibroblasts to a hyper-activated state |journal=Developmental Cell |date=30 June 2022 |volume= 57|issue= 14|pages= 1758–1775.e7|doi=10.1016/j.devcel.2022.06.005 |pmid=35777353|pmc=9344965 |s2cid=250203913 }}</ref> A new signaling molecule called SCUBE3 potently stimulates hair growth and may offer a therapeutic treatment for androgenetic alopecia.<ref>{{cite journal |last1=Liu |first1=Yingzi |title=Team discovers signaling molecule that potently stimulates hair growth |website=Phys.org |date=1 July 2022 |volume= |issue= |pages= |pmid= |url=https://phys.org/news/2022-06-team-molecule-potently-hair-growth.html }}</ref>

== Etymology == The term ''alopecia'' ({{IPAc-en|ˌ|æ|l|ə|ˈ|p|iː|ʃ|i|ə}}) is from the [[Classical Greek]] [[wikt:ἀλώπηξ|ἀλώπηξ]], ''alōpēx'', meaning "fox". The origin of this usage is because this animal sheds its coat twice a year, or because in ancient Greece foxes often lost hair because of [[mange]].

== See also == * [[Alopecia in animals]] * [[Lichen planopilaris]] * [[List of conditions caused by problems with junctional proteins]] * [[Locks of Love]] – charity that provides hair prosthetics to alopecia patients * [[Psychogenic alopecia]]

== References == {{reflist}}

== External links == * {{Commons category-inline}} * {{Wiktionary-inline}}

{{Medical resources |DiseasesDB = 14765 |ICD10 = {{ICD10|L|65|9|l|60}} |ICD9 = {{ICD9|704.09}} |MedlinePlus = 003246 |MeshID = D000505}} {{Disorders of skin appendages}} {{Human hair}} {{Authority control}}

[[Category:Conditions of the skin appendages]] [[Category:External signs of ageing]] [[Category:Hair diseases]] [[Category:Human hair]] [[Category:Radiation health effects]] [[Category:Wikipedia medicine articles ready to translate]] [[Category:Wikipedia neurology articles ready to translate]]