Look at Traditional Chinese Medicine tongue diagnosis because different areas of the tongue relate to different organs and systems. I have noticed markings on my tongue, red patches, thick white coating, red underlying, the thicker the white coating near the front and edges the worse I feel and the more tired that I feel.
I have had a red tongue with virtually no coating and felt terrific, skin was the best it has ever been and had bags full of energy. The more coating that there is the worse that I feel.
The Yin/Yang balance can be seen in the tongue and many people are yang deficient because most illnesses produce Yin excess and so do many supplements such as high dose Vit C. It's about balance and not going to extremes!