Gynecomastia is a condition of overdevelopment or enlargement of the breast tissue in men or boys. It is caused by an imbalance of the hormones estrogen and testosterone, where the amount of testosterone decreases compared to estrogen. This can cause an increase in the amount of breast gland tissue, leading to female-appearing breasts. Gynecomastia can affect one or both breasts, sometimes unevenly.

There are many causes of gynecomastia, including changes in hormone levels during puberty, certain medications, illegal drugs, and some medical conditions. For example, liver diseases, kidney disease, lung cancer, testicular cancer, tumors of the adrenal glands or pituitary gland, and thyroid disorders can cause gynecomastia. Obesity is also a common cause of gynecomastia, as being overweight can increase levels of estrogen, which can cause breast tissue to grow.

Most cases of gynecomastia happen when a preteen or teenage boy is going through puberty, but it can also happen to older men. Researchers have estimated that the prevalence among men ages 50 to 80 is between 24% and 65%, but most men with the condition experience no symptoms.

Gynecomastia can be diagnosed by a healthcare providers exam. Treatment options include medication to adjust a hormone imbalance or surgery to remove the excess breast tissue. However, many cases of gynecomastia resolve on their own without treatment.