Hirsutism pathophysiology

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: ; Ogheneochuko Ajari, MB.BS, MS [2] Rasam Hajiannasab M.D.[3]

Overview[edit | edit source]

The growth of hair on sex-specific areas of the body occurs due to androgens. Androgens induce vellus follicles in sex-specific areas to develop into terminal hairs, which are larger and more heavily pigmented.

Pathophysiology[edit | edit source]

The growth of hair on sex-specific areas of the body occurs due to androgens. Androgens stimulate the growth of vellus follicles in sex-specific areas to develop into terminal hairs, which are larger and have increased pigmentation (darker).^[1][2][3][4]

Types of hair[edit | edit source]

• There are approximately 5 million hair follicles at birth in humans with about 80,000 to 150,000 of them on the scalp.
• The quality of hair is determined by hormones and other intrinsic characteristics of the hair follicle.
• There are various types of hair which include:
  • Lanugo hair: The soft hair which covers the fetal skin and it disappears within the first three months after the fetus delivery.
  • Vellus hair: This is fine, soft hair that is not pigmented. It covers most of the body before the pubertal period.
  • Terminal hair: This is long, coarse and pigmented. Pubertal androgens for example dihydrotestosterone (DHT) convert vellus hair to terminal hair.

Phases of Hair Growth Cycle[edit | edit source]

• There are 3 phases of hair growth cycle:
  • The hair growth phase (termed Anagen phase): This phase varies, depending on the body area affected. It is approximately 4 months for facial hair.
  • The involutional phase (Catagen phase): This phase is approximately 2-3 weeks.
  • The resting phase (Telogen phase): The hair shaft separates from the dermal papillae at the base of the follicle, which terminates growth.

Pathogenesis[edit | edit source]

• The growth of sexual hair is dependent on the presence of androgens.
• Androgens induce vellus follicles in sex-specific areas to develop into terminal hairs, which are larger and more heavily pigmented.
• Hirsutism is caused by increased androgen production and/or an increased sensitivity of the hair follicles to androgens.
• Hyperandrogenism, resulting from any factors, prolongs the anagen(growth) phase of androgen-sensitive hairs, resulting in their conversion from fine, light, vellus hairs to coarse, dark, terminal hairs.
• The response of hair follicle to androgens and other factors such as local 5 alpha-reductase activity determines the level of conversion of hair from the vellus type to terminal hair.^[1]

• Growth recommences with the formation of new hair shaft by the reactivation of the dermal papillae, thereby replacing the old hair.
• The hair growth cycle takes months to years to be completed, causing a delay in hair growth response to changes from effects of androgens.
• DHT is a hormone that acts on the hair follicle to produce terminal hair.
• Differences in the activity of DHT explains why women with the same plasma level testosterone, have different degrees of hirsutism.
• It is speculated that insulin, at high enough concentration, stimulates the ovarian theca cells to produce androgens.
• There may also be an effect of high levels of insulin to activate the insulin-like growth factor-I (IGF-1) receptor in those same cells resulting in increased androgen production.

Associated Conditions[edit | edit source]

Common conditions associated with hirsutism include:

• Diabetes (in PCOS )
• Acne
• Fertility problems

References[edit | edit source]

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