A coating on the tongue can give TCM doctors information about your health. The thickness of a coating is an indicator of the severity of the condition being treated. A thin coating, one in which you can see the tongue through the coating, indicates that any pathogen present is mild or on the exterior. A thick coating that obscures the tongue tells your practitioner that the condition is deeper and more serious.

The condition of the coating also speaks to the condition of fluids in the body. A moist or wet coating indicates poor fluid metabolism, and a dry coating indicates depleted fluids.  A coating that is peeled off, either completely or partially, indicates some kind of heat or damage to the Stomach, possibly a depletion of Stomach Yin, or damage to Stomach Qi.

1. White Tongue Coating
A thin white tongue coating is normal.

A thin white tongue coating can also indicate external Cold patterns when the appropriate clinical symptoms are present.

With a moist and pale tongue, a thicker white coating can indicate Damp-Cold.

A dry white coating can indicate Cold turning to Heat and starting to dry body fluids.

A slightly yellow coating indicates Wind-Heat or Wind-Cold turning to Wind-Heat.

A yellow tongue coat indicates an interior heat pattern. The deeper the color of yellow, the more severe heat. A burnt yellow indicates further accumulation of heat in the body.

If the coating is yellow and moist and the tongue body is swollen and tender, this indicates interior Damp-Heat or Yang Deficiency. In the case of Yang Deficiency, the tongue is showing false heat signs, as the Yin has forced the Yang to the surface of the body. 

3. Gray Tongue Coating
This tongue coating usually indicates an internal pattern of either Heat or Damp-Cold.

A dry gray coating can indicate internal Excess Heat scorching Body Fluids or Yin Deficient Fire.

A wet or moist gray coating usually indicates Cold-Damp Stagnation or Damp-Phlegm retention. 

4. Black Tongue Coating
Similar to the Gray coating above, but more severe.

The black coating usually develops from gray or a burnt yellow coating. 

A dry black coating (usually with cracks) may indicate excessive Heat burning body fluids.

A moist black coating and a pale tongue body may indicate Yang Deficiency, Internal Excess Cold, with or without the presence of Dampness. 

Both Gray and Black tongue coating can indicate extreme Cold (wet tongue) or extreme Heat (dry tongue).

5. Thin Coating
Normal

In disease, it indicates the disease is either external or an internal disease that is not severe. 

If the tongue coating changes from thick to thin, this indicates pathogens are moving to the exterior of the body and the disease is waning.

 6. Thick Coating

A thick coating usually indicates more of an internal disease that is more severe.

It may also indicate that exterior pathogenic factors have penetrated more deeply into the body. 

A thick tongue coating may also indicate retention of food. 

If the tongue coating changes from thin to thick, this indicates pathogens are penetrating deeper into the interior of the body. 

With a mirrored tongue, there is no coating on the tongue. In less severe cases, there may be a partial coating on the tongue. 

If the body of the tongue is also red, it usually indicates that Stomach Qi and Yin is severely damaged. 

If the tongue body is also light in color, this may indicate that Qi and Blood of the Spleen and Stomach is damaged and Deficient. 

If the tongue body is also red or dark, Stomach and Kidney Yin is damaged (body fluids dried up) due to heat.

8) Thick greasy coating
This indicates that your body has lots of dampness and fluid. You may suffer from the following symptoms such as heavy arms and legs, bloating, lack of energy, stiff joints, weight gain, sluggish bowel movement. These people usually have high cholesterol or fatty liver.

This indicates that you have lots of food and heat stagnation in your digestive system. You may have some of the following symptoms such as body heat, easy to perspire, body odour and bad breath, yellow urine, constipation, pungent or dark stool, pass lots of wind, bloating, red face, vivid dreams or nightmares or insomnia. These people may also suffer from high blood pressure, high cholesterol, diabetes or liver/gallbladder disease.