There are many reasons your beard may become thin. You may even lose your beard completely if you are affected by certain conditions. Many of these conditions cause only temporary loss of facial hair; others may cause permanent loss, but these are confined to small parts of your beard.
The Natural Cycle of Facial Hair
All hair goes through a growth cycle---even beard hair. When a strand of hair enters the end of its telogen phase, which is also known as the rest phase, it falls out and the follicle begins growing a new hair in its place. As a result of your hair's life cycle, it is natural to lose between 50 and 100 hairs per day from your beard, scalp and other parts of your body.
Telogen effluvium is a condition in which your hairs enter their telogen phase early. This causes your hair to be shed at a faster rate and can affect the thickness and fullness of your beard as well as the hair on your head. You will notice a gradual thinning across your entire beard and scalp due to this condition. It can have a number of causes, including infections, high fever, major surgeries or influenza. Chronic illnesses that can cause beard loss include thyroid disease and low protein or serum iron. The American Academy of Dermatology notes that this type of hair loss is temporary in most cases and your beard will gradually return after the contributing factor has been removed.
Damaged Skin or Hair
Physical damage to the skin on your face can cause the permanent death of hair follicles. If your skin becomes scarred, hair will not typically grow on the scar tissue. Chemical treatments, which are a common cause of hair damage on the scalp, are not as common among the causes of beard hair loss, but excessive coloring of your beard hair can result in damaged hair.
Chemotherapy and Radiotherapy
The hair on your head, body and face may be affected by chemotherapy. If you are undergoing chemotherapy, you may begin to notice this hair loss anywhere from a few days to a few weeks after beginning treatment. Your facial hair, including your beard and eyebrows, can be affected. According to the CancerNet website, this type of hair loss is usually temporary. Fine hair should begin growing again soon after you have finished your treatment, and your full beard should return after three to six months.
Radiotherapy can also cause hair loss. In this case, the hair loss is typically specific to the area being directly treated. If you are receiving radiotherapy on any part of your face that grows facial hair, expect a bald patch to occur. This effect is usually temporary, but can be permanent depending on the length and quantity of radiotherapy you receive.