Gynecomastia is a condition that causes the breast tissue to develop excessively in boys and men. This happens when there is an imbalance between the amount of estrogen (female hormone) and that of testosterone (male hormone). When a boy goes through puberty or when the body of an older man produces less testosterone, the balance of the two hormones changes.
Sometimes, when this happens, a higher percentage of estrogen causes the development of male breast tissue.
Other causes of gynecomastia: obesity, hyperthyroidism (too much thyroid hormone), malnutrition, chronic liver disease (cirrhosis), familial or spontaneous genetic mutations.