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太阴虚寒下利：

“太阴之为病，腹满而吐，食不下，自利益甚，时腹自痛。”

太阴病 is associated with a spleen deficiency that engenders cold-damp (脾虚寒湿). The treatment principle is to warm the center, building spleen qi and getting rid of the damp (温中散寒，健脾燥湿). 理中汤 is indicated here.

少阴虚寒下利：

“五六日自利而渴者，属少阴也， 虚故引水自救，” (line 282b)

If things get worse, then the patient will start experiencing 少阴虚寒下利，where 四逆汤 is indicated for kidney-deficiency that causes cold-damp. This is a more severe version of 虚寒, and hence is described as due to kidney-deficiency and not spleen-deficiency. Remember that the differential trademark here is the thirst, which we don’t see in 太阴-diarrhea.

Combining both:

If you’ve ever heard of 附子理中汤, it’s a combination of both prescription. It is for the case where both spleen and kidney yang are deficient (脾肾阳虚), to warm up the yang energy of both the spleen and the kidney.