Allergic reactions to deodorants usually cause contact dermatitis, which includes itching and a red, bumpy rash that may blister, explains About.com. Itchy underarms that don't have a rash usually have another cause, such as stress, extreme cold or heat, medication, parasites, or pregnancy.
Ingrown hairs, scabies, mites, fungal infection or a healing wound may cause underarm itching, explains Healthgrades. Itching may also be a symptom of anaphylaxis, chronic kidney disease and obstructive biliary disease. Scratching may provide temporary relief but may cause tears in the skin that can become infected. In many cases, those suffering from itchy underarms should allow skin to heal.