A thick tendon in the rib cage below the left breast may be caused by diseases of the breast itself, inflammation of the mammary gland, and diseases of the chest wall. 1. Diseases of the breast itself, the common breast hyperplasia, dilated ducts of the breast will appear breast pain, and sometimes feel the cord-like structure, tenderness is very obvious, in the process of the menstrual cycle is more obvious, with the end of the menstrual period may gradually reduce the pain. 2. Inflammation of the breast, including mastitis during breastfeeding, may cause pain in the lower part of the breast. 3. Diseases of the chest wall, which may be in organs other than the breast, including costal neuritis and costochondritis, the pain mainly manifests as stabbing pain in paroxysmal episodes. In addition, a thick tendon under the left breast rib cage should also be considered soft tissue injury and vascular disease and other causes, the patient’s left breast has a thick tendon under the rib cage, it is recommended to go to the hospital in time, diagnosed by a professional doctor, timely treatment.