Gynecomastia is a condition in which the glandular tissue in the breasts becomes enlarged in boys or men, sometimes causing discomfort or psychological distress. It is caused by an imbalance of the hormones estrogen and testosterone, which control sex characteristics in both men and women. Testosterone controls male traits, such as muscle mass and body hair, while estrogen controls female traits, including the growth of breasts. Most people think of estrogen as an exclusively female hormone, but men also produce it, though usually in small quantities. Male estrogen levels that are too high or are out of balance with testosterone levels can cause gynecomastia.

Gynecomastia can affect one or both breasts, sometimes unevenly. Pseudogynecomastia refers to an increase in fat but not glandular tissue in male breasts. Gynecomastia often happens when a preteen or teenage boy is going through the hormonal changes of puberty, but it can also happen to newborn babies and to men as they age. Certain medications and diseases can also cause male breast tissue to swell and get bigger.

Gynecomastia can be diagnosed by a healthcare provider through a physical exam and tests including blood tests, urine tests, a low-dose X-ray of the breast (mammogram), and a small breast tissue sample (a biopsy) that may be removed and checked for cancer cells. Treatment options include medication to adjust a hormone imbalance and surgery to remove the excess breast tissue.