What Causes The Growth Of Facial Hair To Stop At The Age Of 55?

I am a 55-year old male who is a non-smoker, occasional drinker and is in general good health (I do though have a cardiac condition known as hypertropic cardiomiopathy with arithmia and a 35% ejection fraction factor). In recent months I have experienced a rare condition:the facial hair of my mustache on the right side of my has stopped growing, while the left side has continued its usual growth! Before this situation occurred my mustache facial hair stubs grew evenly on both sides. Now I must shave the left side of what used to be my mustache area every day, or it will look uneven. Is this condition perhaps a sign of an internal illness?