We all lose a certain amount of hair each day - anywhere from 50 to 100 to 125 hairs. True hair loss occurs when these hairs we lose don't grow back, or when the amount of hair we lose each day exceeds this normal range.
Hair loss can occur as a result of medications, such as chemotherapy treatments or blood thinners, which can damage the telogen hairs, or stop the natural cell division that then produces weakened hair that is susceptible to breaking. High doses of vitamin A can also lead to hair loss as well. The most common type of hair loss, however, is where more and more hair follicles enter what is called the resting phase (telogen phase) in the hair growth cycle.
Overall, hair grows continuously from the scalp, but it moves away from the scalp in 3 phases. Hair is not all uniformly in one phase. Different parts of the scalp will be in different phases, so at any one time you should have hair follicles in all 3 stages. Generally, most (90%) of the hair will be in the anagen phase, 10 to 14% of the hair will be in the telogen phase, and only 1 to 2% of hair in the catogen phase.
The first phase is the Anagen phase, which is the growth phase. This can last anywhere between 2 and 8 years. A shorter anagen phase will limit how long your hair can grow. Hair cells at the root divide rapidly, which lengthen the hair shaft.
In the next phase, the catagen phase, the outer root of the hair follicle shrinks and attaches to the root of the hair. Hair growth stops here. This phase lasts 1 to 2 weeks.
The telogen phase is the resting phase. This lasts from 5 to 6 weeks in normal hair. Hair doesn't grow in this phase, but it stays firmly rooted in the scalp as long as the follicle stays in a resting phase below it. New growth begins at the end of the resting phase, and this is when natural hair shedding will happen, as the new growth pushes the old hair out.
In male pattern baldness, more hairs enter the telogen phase. This effects of this are an increase in hair shedding. Hair gradually becomes thinner and shorter, and in the end, the hair follicles shut down.