The most common cause of hair loss is known as Androgenetic Alopecia. This is closely correlated to the person's genetics. In women, this can begin after puberty, however it is more common after menopause.
The second most common, after Androgenetic Alopecia, is Telogen Efflivium. This is general loss and thinning of the hair and usually occurs after a shock to the person's system. Most commonly associated with stress, however is also linked to pregnancy and childbirth, sudden weight loss, a fever, etc. The effects are only temporary and the hair generally grows back after approximately 6 months.
One in 100 people suffer with Alopecia Areata, which is the development of patches of baldness. This is very common in teenagers and young adults. It is associated with problems within the immune system where the body's own defensive cells attack the hair follicles, inducing hair loss. In 60% - 80% of cases, the hair grows back within a year without any treatment. The re-growth of hair is unlikely in cases of Alopecia totalis (total loss of hair on scalp) and Alopecia Universalis (loss of all body hair).
The only proven treatment for hair loss, other than transplants and surgery, is Minoxidil. Using this, up to 25% of women experience a re-growth in hair.
Other causes of hair loss include the following:
Diseases and health issues such as Addison's disease, Hyperthyroidism, iron deficiency, etc.
Traction Alopecia (Excessive pulling or traction on hair)
Trichotillomania (intentional pulling out of healthy hair)
Anagen Effluvium (Chemical damage)