Androgenetic alopecia
Definition & Epidemiology
- AGA = androgen-driven non-scarring alopecia with progressive miniaturization of terminal hairs to vellus hairs.
- Incidence: Affects ~50% men / ~40% women by the age of 50 (women primarily postmenopausal).
- Polygenic inheritance (AR gene polymorphism + other loci) — paternal and maternal lines contributing.
Pathogenesis
- Key enzyme: Central to the hair follicle is 5α-reductase type II — conversion testosterone → DHT (DHT = dihydrotestosterone).
- DHT effect: Binding to androgen receptor → shortening of anagen phase + miniaturization of the follicle over several hair cycles.
- Genetics: AR polymorphisms + EDA2R + Chr 20p11 etc. — > 60 gene loci identified.
Clinical Features & Classification
Men (Hamilton-Norwood Scale)
- Stage I–VII: beginning bitemporally ("receding hairline") + vertex (crown), up to complete baldness with preservation of the occipital zone ("horseshoe pattern").
Women (Ludwig Scale / Olsen Classification)
- Stage I–III Ludwig: Mid-parting widening ("Christmas-Tree-Pattern") with preservation of the frontal hairline.
- Olsen complements the frontal accent pattern.
Diagnostics
- History: Onset, family history, medications (testosterone, anabolics, anti-estrogens), hormone status (women).
- Trichoscopy / Dermoscopy: Typical for AGA are caliber variation > 20%, predominantly in the vertex region (occiput = reserve zone).
- Trichogram / TrichoScan: Anagen / telogen ratio.
- In women: Iron + ferritin + TSH + androgens (testosterone, DHEA-S, SHBG, free androgen index, possibly 17-OH-progesterone) to rule out endocrine triggers / PCOS.
Differential diagnoses
- Telogen effluvium
- Anagen effluvium
- Alopecia areata
- Trichotillomania
- Folliculitis decalvans
- Central centrifugal cicatricial alopecia (CCCA)
- Frontal fibrosing alopecia
Practise Androgenetic alopecia in the app
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In the DermaFuchs app: 4 flashcards · 3 clinical images
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More diseases of the skin appendages
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- Hidradenitis suppurativa (acne inversa)
- Acne vulgaris
- Alopecia areata
- Folliculitis (infectious)
- Lichen planopilaris
- Milia (milium)
- Perioral dermatitis
Note: This page is intended for medical education and does not replace diagnosis or treatment decisions in individual cases. Treatment and follow-up content is available in the app.