I know of at least 2 conditions that can cause the loss of hair, however, there are probably many more. All are related to hormones but they are NOT suitable for removing excess hair due to the severity of side effects. I have one of these conditions myself. It is called HYPOTHYROIDism. Among all the possible symptoms and there are a load of them) is loss of the eyebrows. My dermatologist noticed that the outer third of my eyebrows were gone and ordered a blood test for this. I now take “levoxyl” (a synthetic thyroid hormone). Hair does not regrow but the gradual loss of eyebrows has stopped.
The second is seen in women and is called “androgenetic hair loss”. It is caused by the woman having too much androgen in her blood. This is a genetic condition to many women starting at about the pre-menopausal phase of their life. It is common for them to see an increase in facial hair at the same time. My wife has this problem. Her hair became very fine and she lost a lot of her hair. She has been talking about getting a wig. A dermatologist at Massachusetts General Hospital who specializes in hair disorders said at this time there is nothing that can be done.