Role of androgens in hair growth

The influence of androgens on hair depends on the area of the body:

• androgens stimulate tiny, fine, virtually colorless, almost invisible vellus hair in the axilla and pubis of both sexes and greater hair on the face, upper pubic diamond and chest, to turn into much larger deeper follicles forming longer, thicker, heavily pigmented terminal hair (1)
• androgens cause the exact opposite in scalp hair causing terminal hair to transform into miniaturized vellus follicles creating tiny hair (1)

The main male androgen, testosterone is converted to dihydrotestosterone (DHT) by 5 alpha - reductase intracelleualry. There are two isoforms of 5 alpha - reductase:

• type I - expressed widely, the precise physiologic function is unknown
• type II - expressed in androgen-dependent tissues e.g. - prostate and hair follicles (2), appears to play an important role than type I in male pattern baldness (3)

The importance of androgens (especially DHT) in male pattern baldness is supported by the following:

• this condition is not seen in eunuchs (who lack androgens), in individuals who do not have functional androgen receptors and in pseudohermaphrodites who lack 5 alpha reductase
• absence of baldness in those who do not have the gene for type II 5 alpha reductase
• castration of postpubertal men results in arresting of the progression of androgenetic alopecia
• evidence of increased concentrations of 5alpha-reductase, DHT, and the androgen receptors in balding scalps
• drugs which inhibit type 2 5 alpha reductase activity is useful in treatment of hair loss (3)

Reference:

• 1. Randall VA. Androgens and hair growth. Dermatol Ther. 2008;21(5):314-28.
• 2. Olsen EA et al. Evaluation and treatment of male and female pattern hair loss. J Am Acad Dermatol. 2005;52(2):301-11.
• 3. Stough D et al. Psychological effect, pathophysiology, and management of androgenetic alopecia in men. Mayo Clin Proc. 2005;80(10):1316-22