Left underarm pain may cause back pain, but there are some diseases that do not radiate to the back, the possible causes are breast hyperplasia, cardiac lesions, axillary folliculitis, etc. The main cause is estrogen and progesterone imbalance. 1. Breast hyperplasia: the main cause of breast hyperplasia is the imbalance of estrogen and progesterone, clinically it is mostly benign breast tissue hyperplasia, which can be seen as breast nodules and lumps, etc. The common symptom is breast pain, which can be radiated to the armpit area and appear as axillary pain, and the axillary pain can further trigger the radiating pain in the back. 2. Cardiac lesions: usually cardiac lesions can be manifested as pain on the surface of the body, and the main pain site is the left chest, and the pain range of some patients can be changed, such as subxiphoid process, left armpit, toothache, etc. The pain in the left armpit can be reflected to the back, and the pain in the back, but the patients can’t locate it clearly. 3. Axillary folliculitis: sweat stimulation in a hot and humid environment, or the use of hair removal cream and other special cosmetic skin care products stimulation, may lead to axillary skin irritation symptoms, or hair follicles poor excretion, the emergence of folliculitis, which produces pain symptoms, folliculitis produced by the pain symptoms are generally confined to the armpit area, will not cause back pain. The pain in the armpit is usually limited to the armpit, and will not cause back pain. It is recommended to go to the regular hospital in time to identify the cause of the pain and timely treatment.