Complex sentence: Gynecomastia is thought to be caused by an altered ratio of estrogens to androgens, an increase in estrogen production, a decrease in androgen production, or a combination of these factors with estrogen acting as a growth hormone to increase the size of male breast tissue.
Split into simpler sentences: Gynecomastia is thought to be caused by an altered ratio of estrogens to androgens mediated by an increase in estrogen production, a decrease in androgen production, or a combination of these two factors. Estrogen acts as a growth hormone to increase the size of male breast tissue.