* A hormonal imbalance, such as from
taking birth control pills
* An underactive thyroid gland
* A tumor (prolactinoma) or other disorder of the pituitary gland
* Chronic breast stimulation, such as from
frequent breast self-exams or sexual activity
A blunt trauma — for instance, the impact of the steering wheel in a car accident or a hard blow to your chest during a sporting activity — can cause nipple discharge in both your breasts.