Losing my Eyebrows

I've noticed that the longer I use my CPAP, the fewer eyebrows I have. Even if I allow some loss to age, when I get up in the a.m., my eyebrow ridge is red. I wear a full mask (nose and mouth covered), and the mask does not sit on my brows, but just above it. Anyone else had this issue?

I think this is rather uncommon, but let's try to figure out what is happening here.
What make of mask do you wear? Is it new? If this is a new mask, maybe it needs to be prewashed,
as some manufacturing chemicals may remain on the mask/forehead brace, which could cause inflammation.
Redness is often accompanied with a bit of swelling, which may make your hair appear thinner.
If you are seeing loose hair, is it from specific places or all over?--Could you have a deficiency?

If the mask isn't a contributor, thyroid could be. Whether hyper- or hypo-, thyroid imbalance can cause a surprising number of symptoms. Most on-line symptom-checkers, though, may really mis-evaluate you, as they pay a LOT of attention to how regular your menstrual cycle is, pregnancy, etc., all of which makes it say I'm just fine. The survey I ran into never even asked gender, but rated quite a few points based on "Is your menstrual cycle regular? Yes or No.". Just how is a guy supposed to answer that?

I found out that I had an under active thyroid about 10 years ago (before going on CPAP) and the doctor mentioned that my eyebrows at the outer edges were going thin/fall out.
It has stayed about the same even though I take a prescription medication for the under active thyroid. Other people who have an under active thyroid have told me of the same problem, some women who have lost all of their eyebrows have had their eyebrows tattooed on and others use makeup.

My wife has thyroid issues and gets her levels checked semi-annually. Sometimes they need to adjust her dosage. Have you discussed this with your endocrinologist recently?