What is the pathophysiology of hirsutism?

Updated: May 01, 2020
  • Author: George T Griffing, MD; Chief Editor: Michel E Rivlin, MD  more...
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Hormones and the intrinsic characteristics of the hair follicle determine the quality of hair growth. Vellus hairs are fine, lightly pigmented hairs that cover most of the body before puberty. Pubertal androgens promote the conversion of these vellus hairs to coarser, pigmented terminal hairs. The level and duration of exposure to androgens, the local 5-alpha-reductase activity, and the intrinsic sensitivity of the hair follicle to androgen action determine the extent of conversion from vellus to terminal hair. However, some terminal hair growth is androgen-independent (eg, scalp, eyebrows, lashes). [4]

The development of terminal hair or reversion back to a vellus pattern may not be immediately evident because of the characteristics of the hair cycle. This cycle has 2 phases that include active hair growth (anagen phase) and a resting period (telogen phase), which follows the anagen phase. During the resting period, the hair shaft separates from the dermal papillae at the follicle base, and no further growth takes place. Eventually, growth restarts and the new hair shaft formed by the reactivated papillae pushes the old hair out. The cycle may take months to years to complete, and this causes a delay in hair growth response to changes in the androgen milieu.

Dihydrotestosterone is the androgen that acts on the hair follicle to produce terminal hair. This hormone is derived from both the bloodstream and local conversion of a precursor, testosterone. The local production of dihydrotestosterone is determined by 5-alpha-reductase activity in the skin. Differences in the activity of this enzyme may explain why women with the same plasma levels of testosterone can have different degrees of hirsutism.

A study by Willis et al found that self-reported hirsutism in women, as measured using the modified Ferriman-Gallwey score, was associated with menstrual irregularity, bleed lengths of 7 days or more, and heavy menstrual bleeding. [5]

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