Testosterone blocker impedes the action of dihydrotestosterone. DHT is the product of a biochemical pathway,
a pathway that exists in the human scalp. At the start of that pathway, an androgen hormone in the scalp produces the hormone called testosterone.Testosterone is a hormone that is familiar to most people.
Testosterone can be found in both males and females, although it is a hormone that is associated with the development of the male reproductive system. Testosterone in the scalp can mix with and bind to a particular enzyme, an enzyme called five alpha reductase.
As testosterone combines with that enzyme, the reductase initiates a biochemical reaction. The reductase catalyzes that same reaction. That reaction leads to the production of dihydrotestosterone.
Testosterone blockers interferes in some way with the biochemical pathway that can furnish the scalp with a healthy amount of DHT. In particular men, hair follicles in the scalp are 
genetically predisposed to attract DHT molecules.
Those DHT molecules bind to the hair follicles and thus cut off the blood supply to those follicles. Deprived of blood, the hair on the affected scalp ceases to grow.
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