Thickness. The standard for examining the thickness of the tongue is whether it is "bottom visible" or"bottom invisible". That means the tongue fur with dimly visible body of the tongue is thin fur, while the tongue fur with invisible body of the tongue is thick fur.
The thickness of the tongue fur reflects the degree and severity of the pathogenic factors as well as the development of disease. Generally speaking, thin tongue fur is seen at the primary stage of exogenous disease, suggesting that the pathogenic factors are superficial and the disease is mild; it is also seen in diseases due to internal impairment with deficiency of healthy qi, especially with hypofunction of the spleen and stomach. Thick tongue fur is the sign of exuberance of pathogenic factors, frequently due to internal invasion of exogenous pathogenic factors, or due to internal stagnation of phlegm, dampness and food retention as well as fumigation of gastric qi with turbid substance and pathogenic factors.
During the course of a disease, the change of the tongue fur from thinness to thickness indicates gradual exuberance of pathogenic factors, development of pathogenic factors from the exterior to the interior and progress of pathological conditions from mildness to severity; the change of the tongue fur from thickness to thinness suggests predomination of healthy qi over pathogenic factors, elimination of pathogenic factors internally and externally as well as development of the pathological conditions from severity to mildness.