There are various causes of hair loss in men and women. For example acute stress either via work or personal life could cause a rapid and sudden shedding of large amounts of hair.So the causes of hair loss are not always so simple and 'telogen effluvium' (fast hair loss), is not as bad as it would first seem because most, if not all, of the hair will grow back after a short period of time as the body readjusts itself and hormone levels re-adjust.
The more common variety of male hair loss is androgenetic alopecia, known as male pattern baldness, which is caused by a combination of male hormones and a hereditary tendency to lose hair. These men have a scalp that is extra sensitive to the male sex hormones circulating in the blood stream.
The trigger for losing your hair is currently believed to be an enzyme called 5-alpha reductase that converts testosterone into dihydrotestosterone (DHT), which inhibits growth in certain genetically prone hair follicles on the scalp.
It causes the follicles to shrink to the point where they become so small they cannot replace lost hair.The follicles are alive but not longer able to perform their task.
There are other problems that can cause hair loss such as: Hormonal problems, an imbalance caused by an over or under active thyroid gland for instance may make your hair fall out. Also some medicines, especially blood thinners, antidepressants, birth control pills, medicines used to treat gout and cancer, and excessive intake of vitamin A. Also infections could be a cause, such as fungal infections of the scalp.
Finally an underlying disease such as diabetes may cause hair loss and would be an early sign of this problem, in this case it would be important to find out the cause so it can be treated.