One way to help narrow down the cause hair loss is to determine where it is falling out. Elevated levels of dihydrotestosterone (DHT) will cause hair loss near the temples and back top of the head first then thin out on the rest of the top. This is referred to as male pattern baldness. The reason those areas suffer hair loss and not the sides or back is the higher number of DHT receptors in the temple area and the top of the head.
If the hair is falling out all over evenly then thyroid or autoimmunity (very rare cause) are more likely.
If the hair is coming out in patches then I would suspect an infection.