Spironolactone-Induced Breast Enlargement (Gynecomastia) Mechanism
Spironolactone causes male breast enlargement primarily through antiandrogen effects that shift the balance toward estrogen signaling in breast tissue. [1] This process leads to ductal and glandular breast growth in response to relatively increased estrogen effects and relatively decreased androgen effects. [2]
Androgen Receptor Antagonism
Spironolactone can bind to androgen receptors and block androgen action. [1] Reduced androgen signaling removes the inhibitory effect of androgens on breast tissue growth. [2]
Reduced Testosterone Availability
Spironolactone can decrease testosterone production by inhibiting steps in the testosterone synthesis pathway. [2] Reduced testosterone availability further lowers androgenic protection of breast tissue. [2]
Increased Estrogen Effect
Spironolactone increases peripheral conversion (aromatization) of testosterone to estradiol. [3] Spironolactone can also increase the amount of free estrogen available by displacing estrogen from sex hormone-binding globulin (SHBG). [3] Together, aromatization and increased free estrogen shift the functional estrogen-to-androgen balance toward breast growth. [2]
Additional Steroid-Receptor Interactions
Spironolactone can interact with progesterone-related steroid signaling in ways that may contribute to gynecomastia development. [1]
Clinical Course After Discontinuation
Drug-induced gynecomastia from spironolactone is generally reversible after stopping the medication, although persistence can occur. [3]
Key Mechanistic Summary
Spironolactone causes breast enlargement by blocking androgen action and decreasing androgen availability while increasing estradiol effects in target tissues. [2] The net result is an estrogen-dominant hormonal environment in male breast tissue that drives glandular growth. [2]