Glossary

5α-reductase (Steroid 5α-reductase)

The enzyme that converts testosterone into the more potent androgen DHT. Two clinically relevant isoforms exist: type 1 (sebaceous glands) and type 2 (hair follicle and prostate).

Also: 5-alpha-reductase, 5AR, SRD5A

Alopecia areata (Alopecia areata (AA))

Autoimmune hair loss producing sharply-bordered, smooth round patches. About half of patchy cases regrow spontaneously within 12 months. Severe disease responds to JAK inhibitors.

Also: AA, alopecia totalis, alopecia universalis

Androgen receptor (Androgen receptor (AR))

The intracellular receptor that binds testosterone and DHT to drive androgen-mediated gene expression. Variants in the AR gene determine how susceptible an individual's follicles are to DHT-driven miniaturisation.

Also: AR

Androgenetic alopecia (Androgenetic alopecia (AGA))

Pattern hair loss driven by DHT acting on genetically susceptible follicles. The most common cause of hair loss in both men and women.

Also: AGA, pattern hair loss, male pattern hair loss, female pattern hair loss

Dermal papilla

The cluster of specialised cells at the base of each hair follicle that controls the growth signal. The site of greatest 5α-reductase activity in the follicle.

Also: dermal papillae

DHT (Dihydrotestosterone)

A potent androgen made from testosterone by the enzyme 5α-reductase. The main driver of pattern hair loss in genetically susceptible follicles.

Also: dihydrotestosterone

Dutasteride

A 5α-reductase inhibitor that blocks both type 1 and type 2 enzymes, lowering scalp DHT by approximately 90%. FDA-approved for BPH; off-label for male AGA in most countries (approved for AGA in South Korea and Japan).

Also: Avodart

Female pattern hair loss (Female pattern hair loss (FPHL))

The female-specific presentation of androgenetic alopecia: diffuse central thinning with the frontal hairline preserved. Treated with minoxidil, spironolactone, low-dose oral minoxidil, and (off-label) finasteride.

Also: FPHL

Ferritin

A blood test reflecting body iron stores. The single most useful blood test for diffuse hair loss. Hair specialists target ferritin above 50 ng/mL, well above the standard 'normal' lower limit of 15 to 30.

Also: serum ferritin

Finasteride

A 5α-reductase type 2 inhibitor that lowers scalp DHT by 64 to 70%. FDA-approved at 1 mg/day for male pattern hair loss. The standard first-line oral treatment for male AGA.

Also: Propecia, Proscar

Frontal fibrosing alopecia (Frontal fibrosing alopecia (FFA))

A scarring alopecia presenting as progressive recession of the frontal hairline, eyebrow loss, and a band of scarred-looking skin behind the new hairline. Predominantly postmenopausal women.

Also: FFA

Hair growth cycle

The repeating sequence each follicle moves through: anagen (growth, 2 to 7 years), catagen (transition, 2 to 3 weeks), telogen (rest, 2 to 4 months), and exogen (shedding).

Also: hair cycle, anagen, catagen, telogen, exogen

JAK inhibitor (Janus kinase inhibitor)

A class of immunomodulator drugs that block JAK-STAT signalling in immune cells. Three are FDA-approved for severe alopecia areata: baricitinib (2022), ritlecitinib (2023), deuruxolitinib (2024).

Also: JAKi

Lichen planopilaris (Lichen planopilaris (LPP))

A scarring alopecia caused by lymphocytic inflammation around the follicle. Presents with patches of progressive loss, perifollicular redness, scaling, and often itching or burning.

Also: LPP

Ludwig scale

A 3-stage visual classification of female pattern hair loss based on central thinning severity, with the frontal hairline typically preserved. The older companion to the Sinclair scale.

Miniaturisation

The progressive shortening of the anagen growth phase in DHT-susceptible follicles, which produces thinner, shorter hair each cycle until the follicle stops producing visible hair.

Also: miniaturization, follicle miniaturisation

Minoxidil

A topical or oral medication that prolongs the anagen growth phase of hair follicles. FDA-approved as a 5% topical for androgenetic alopecia; widely used off-label as a 0.25 to 5 mg/day oral tablet.

Also: Rogaine

Norwood scale (Norwood-Hamilton scale)

The standard 7-stage visual classification of male pattern hair loss. Norwood 1 is no recession; Norwood 7 is the classic horseshoe of advanced loss. Used for staging, treatment decisions, and trial reporting.

Also: Norwood-Hamilton

Pull test

A bedside test for active hair shedding: gently tug a small bunch of about 50 to 60 hairs from the scalp. More than 6 hairs out is positive and suggests active shedding.

Also: hair pull test

Scarring alopecia (Cicatricial alopecia)

Hair loss in which inflammation permanently destroys the follicle. Treatment can halt progression but cannot regrow lost hair. Includes lichen planopilaris, frontal fibrosing alopecia, and central centrifugal cicatricial alopecia.

Also: cicatricial alopecia

Sinclair scale

A 5-point visual classification of female pattern hair loss based on the width of the central part. Sinclair 1 is normal density; Sinclair 5 is severe central thinning. Easy to apply at home with a phone selfie.

Telogen effluvium (Telogen effluvium (TE))

Sudden diffuse hair shedding caused by an event 2 to 4 months earlier (illness, surgery, childbirth, severe stress, new medication, nutritional issue). Almost always reversible.

Also: TE

Terminal hair

Thick, long, pigmented hair anchored deep in the dermis. Scalp hair, beard, eyebrows, and underarm hair are terminal. The opposite of vellus hair.

Also: terminal hairs

Traction alopecia

Hair loss caused by chronic pulling on the hair shaft from tight hairstyles, extensions, weaves, or hair systems. Reversible early; can become scarring and permanent if sustained.

Trichoscopy

Magnified examination of the scalp and hair, usually with a dermatoscope (10x to 70x). The standard tool for distinguishing pattern hair loss, alopecia areata, scarring alopecias, and other conditions.

Also: dermatoscopy of the scalp, scalp dermoscopy

Trichotillomania

A psychiatric condition in which the person pulls out their own hair, usually compulsively. Produces irregular patches of hair of varying lengths in accessible areas. Treated with behavioural therapy and sometimes medication.

Also: hair-pulling disorder, TTM

Vellus hair

Short, fine, soft, lightly pigmented hair that covers most of the body. In pattern hair loss, terminal scalp hairs progressively miniaturise back into vellus hair before disappearing.

Also: vellus