Hair loss can be caused by genetics (family history), hormonal changes, a reaction to certain medications, radiation therapy, stressful events, or even certain hairstyles and treatments. In all cases, during the typical growth cycle, the hair follicle slowly shrinks over time producing shorter and thinner hairs. At a certain point, the hair follicle eventually stops producing hair altogether.
Hair loss, in men specifically, is thought to be caused by an overproduction of dihydrotestosterone (DHT). DHT is a derivative of testosterone and by blocking the production of this hormone, can slow or reverse hair loss.