Yin and yang are the foundation of Chinese medicine, which directly stems from Chinese philosophy. They are relative terms that only have meaning in a situation of comparison. For example a substance can neither be yin nor yang in and of itself, but substance A can be more yin or yang in comparison to substance B. The following chart gives some examples:
All phenomenon can be ascribed to yin and yang, and all phenomenon possess both a yin and yang aspect. Chinese medicine uses these terms to describe the body’s physiology, pathologies, diagnosis, and treatments. Understanding how yin and yang relate to your condition can provide great insight into your treatment.
Therefore, when one is out of balance the other also must be taken into consideration.
Note: when yin is deficient, yang is in relative excess. This not only produces symptoms of yin deficiency but of excess yang (heat). The treatment consists of nourishing yin (bringing it up), and depending on how much the yang is in excess, yang will need to be decreased. One must always weigh both aspects and gauge how much each must be treated. For example, one can get headaches because too much yang is rising up. This can occur from a yang excess (fire rising) where the yin is not truly deficient (but is only relatively deficient); or it can occur from a yin deficiency where the yin is not able to hold the yang down, and it floats up (this is yang’s nature). The treatment is to nourish the yin to help indirectly bring the yang down, and actually bring down the yang (itself).
In general one can start to think about any problem with relative yin and yang imbalances. For example, hyperthyroidism which usually produce hyperactive signs such as fast pulse, red complexion, warm body, nervousness, increased activity, increased appetite, etc. are all signs of (relative) yang excess. Hypothyroid usually produces opposite signs of hypofunction showing a predomination of yin.
Chinese medicine does get more sophisticated when it starts pairing yin and yang with other criteria, for example, one can have a kidney, spleen, lung, liver, or heart yin deficiency, as well as yang deficiencies of various organs and combinations of both.