The estrogen-to-androgen balance behind glandular growth
Gynecomastia is the growth of true glandular breast tissue in males, driven primarily by a shift in the ratio of estrogen to androgen activity in breast tissue rather than the absolute level of either alone. When estrogenic signalling rises relative to androgenic signalling, the ductal tissue proliferates. This distinguishes it from pseudogynecomastia, which is simply fat deposition without glandular growth.
A portion of a man's estrogen comes from the conversion of testosterone by the enzyme aromatase, which is more active in fatty tissue. This is one reason the balance can tilt with higher body fat or with age.
Why the ratio shifts
Common contributors include puberty and ageing (both periods of natural hormonal flux), liver or kidney disease, certain medications, and conditions that lower testosterone or raise estrogen. Many cases in adolescence are transient and resolve on their own. Persistent, rapidly growing, firm, or one-sided breast tissue warrants evaluation because a small minority of cases reflect a more serious underlying cause.
This is general educational information, not medical advice. Any new breast tissue change in a man should be assessed by a qualified clinician.