It is important to understand the normal hair growth cycle to understand why hair loss occurs. The hair follicle is an anatomical structure which evolved to produce and extrude (push out) a hair shaft. Hair is made up of proteins called keratins. Human hair grows in a continuous cyclic pattern of growth and rest known as the "hair growth cycle."
Three phases of the hair growth cycle exist: Anagen= growth phase; Catagen= degradation phase; Telogen= resting phase. Periods of growth (anagen) between two and eight years are followed by a brief period, two to four weeks, in which the follicle is almost totally degraded (catagen). The resting phase (telogen) then begins and lasts two to four months. Shedding of the hair occurs only after the next growth cycle (anagen) begins and a new hair shaft begins to emerge. On average 50-100 telogen hairs are shed every day. This is normal hair loss and accounts for the hair loss seen every day in the shower and with hair combing. These hairs will regrow. Not more than 10 percent of the follicles are in the resting phase (telogen) at any time. A variety of factors can affect the hair growth cycle and cause temporary or permanent hair loss (alopecia) including medication, radiation, chemotherapy, exposure to chemicals, hormonal and nutritional factors, thyroid disease, generalized or local skin disease, and stress.
Androgens (testosterone, dihydrotestosterone) are the most important control factors of human hair growth. Androgens must be present for the growth of beard, axillary (underarm), and pubic hair. Growth of scalp hair is NOT androgen-dependent but androgens are necessary for the development of male and female pattern hair loss.
The Hair Growth Cycle
The next figure shows miniaturization of the hair follicle in pattern hair loss. Hair grows for shorter periods of time and becomes smaller, finer, and lighter as it approaches the vellus stage.