What Is 5-Alpha Reductase And Dihydrotestosterone: Functions Of Dihydrotestosterone, Isoforms, Medicine

5-alpha reductase is the name of an enzyme responsible for converting testosterone into dihydrotestosterone. The latter represents the most powerful androgen hormone in the body and its activity is 4-5 times higher than that of testosterone.As anticipated, it is produced thanks to the action of the enzyme 5-alpha reductase which makes the double bond existing between carbon 4 and carbon 5 simple. This small modification considerably increases the affinity of dihydrotestosterone for androgen receptors , enhancing them by consequently, the activity.

Functions Of Dihydrotestosterone

The enzyme 5-alpha reductase is mainly expressed in the prostate , testicles , hair follicles and adrenal glands . Its action is already very important during uterine life, when dihydrotestosterone determines the development of the male external genitalia. When the type II 5-alpha reductase isoform is deficient due to a recessive genetic defect, there is an abnormal development of the external genitalia in the fetus , which results in sexual identity disorders and infertility in adulthood .

From puberty onwards, dihydrotestosterone supports the development of so-called secondary sexual characteristics, such as lowering of the tone of voice, growth of facial and body hair , receding hairline and sebaceous secretion. Dihydrotestosterone is also important for the development of muscle mass and, on a psychological level, for the appearance of sexual stimulation. An excess of it is implicated in the onset of prostatic hypertrophy, acne , seborrhea , androgenic baldness and hirsutism . A deficiency of dihydrotestosterone is implicated in the onset of gynecomastia .


In the human body there are two isoforms of the 5-alpha reductase enzyme, encoded by two different chromosomes and with different sensitivity to regulatory factors, respectively called 5-alpha reductase type I and 5-alpha reductase type II.

  • The type I isoform is mainly concentrated in the skin, sebaceous glands , central nervous system and liver .
  • The type II isoform is mainly expressed in the prostate and in the hair follicles .


Two drugs have been developed to inhibit the activity of these enzymes. The first, called Finasteride , Proscar or Propecia , is a type II isoform selective inhibitor. The second, called Dutasteride or Avodart , inhibits both isoforms. These medicines are used in the treatment of benign prostatic hypertrophy ; finasteride is also widely used to combat hair loss.It is probable that in the future also dutasteride will be used with the same purpose (considering the greater effectiveness in blocking the activity of the 5-alpha-reductase enzyme, a more marked action is expected, but also more evident side effects; studies are currently underway to establish this).

The side effects of 5-alpha reductase inhibitors, although rare, mainly affect the sexual sphere and can cause erectile dysfunction , decreased libido and ejaculate volume.

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