Hair loss is the result of the efffects of testosterone on predetermined hair follicies. In other words, hair loss is not an illness but rather a physical characteristic which we inherit from our parents and presents itself after puberty, a phase in life which is characterized by an increase in testosterone, the male hormone necessary for the development of the adult male characteristics (beard, muscle mass, etc). Testosterone at the follicular level becomes dihydrotestosterone (DHT). If the hair follicles on the top of the scalp are sensitive to this hormone it will cause them to shrink. This "shrinkage" results in overall thinning with time.

Hair loss is correctly described as the miniaturization of hair through fairly typical patterns but there can be cycles of faster and slower hair loss. It is therefore impossible to know exactly how much hair will be lost and how quickly it will be lost, for any individual.

There are other causes of hair loss which incluide illness, diets, medical therapy, etc.; however in most cases the hair loss resulting from these is usually reversible and follows a different generally more diffuse pattern and only accounts for less than 5% of the cases.