Hair loss on legs can be because of several reasons some of which are not an issue while others can cause serious problems if ignored, so meet your doctor to know the exact reason for the hair fall.
The hair fall can be because of poor blood flow in legs which results in hair fall. What causes the poor blood circulation will be identified by your doctor after tests it can be because of diabetes, heart related issues of kidney failure.
Another reason for the hair fall can be Alopecia areata which is a disease causing hair fall. This can affect any part of your body. Some other infection can also cause hair fall but they are limited to a small area these infections include folliculitis & ringworm.
Some people also lose hair because of wearing tight clothing which results in friction. So if you are in trouble with your hair loss on legs then visit your doctor.