Testosterone, a hormone that is present in high levels in males after puberty, is converted to dihydrotestosterone (DHT) by an enzyme called 5-alpha reductase. DHT has an adverse affect on the hair follicles. Acting on a hormone receptor on the hair follicle it slows down hair production and produces weak, shorter hair, sometimes it stops hair growth from the follicle completely. This process gradually depletes your stock of hair and is normal hair loss.
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Male Pattern Baldness