There are many conditions that can lead to hair loss. As well as requiring different forms of treatment, different conditions will also have different causes. Your GP or dermatologist should be able to determine the cause of any hair loss you may be experiencing and will be able to advise you accordingly.
In androgenetic alopecia, the cause of hair loss is dihydrotestosterone, also known as DHT. People who have a lot of the enzyme 5-alpha reductase make more DHT and this in turn causes hair follicles to produce thinner hairs. Ultimately, follicles can cease to produce hairs altogether.
Alopecia areata works rather differently. It is an autoimmune disorder where they body itself attacks the hair follicles with white blood cells. Hair production will slow considerably and there can be no hair grown for months or even years. As the hair follicle is still alive, it is possible that the hair will eventually grow back.
As was mentioned at the start of the article, the important thing to do is to first consult your GP in order that any hair loss condition can be properly diagnosed.