I have a coated tongue. It was very bad for a while and an acupuncturist told me I was yin DEFICIENT! I started to take all these supplements to help with Yin (meaning lack of female hormones mainly estrogen based) and honestly it didn't help. In fact, I think it got worse. My mouth would be very dry especially second half of cycle.
If it is true that it is a sign of yin excess then it would make sense that my body wasn't producing enough progesterone/cortisol because of estrogen dominance...hence my little adrenal melt down.
Let me know b/c I am very curious about this. I had always assumed that the coated tongue was candida which can be made worse by progesterone supplements so I stayed away from those and took estrogen based supplements.