What are some reasons to thin eyebrows in men?

Think medical: Thin eyebrows in men can be a result of "normal" male pattern hair loss. But there are several medical conditions that should be explored, such as thyroid disease, autoimmune conditions, fungal infections, alopecia, to name a few.

Many reasons: Could be genetic! could be self plucking! could be thyroid related! could be hair follicle infection! could be chemo related! needs to evaluated in context of other health issues!

Many reasons!: The loss of hair can be from many different sources, self plucking, genetics, thyroid disease, chemo therapy... It must be considered within the context of the full picture. Should see your medical doctor for an evaluation.

Many : Thinning eyebrows could be as a result of generalized hair loss due to thyroid and other metabolic issues, chemotherapy does it and alopecia could do it too. One other cause - a habit of pulling or traumatizing them could cause hair loss.