Gynecomastia

Gynecomastia is enlargement of male breast tissue from glandular proliferation, driven by estrogen acting relative to androgen rather than by absolute estrogen levels alone. Low thyroid function and elevated prolactin add to the estrogenic load that produces it.

Mechanism

Testosterone converts partly to estradiol outside the testes, through aromatization of adrenal and testicular androgens, and this extraglandular estradiol is what produces feminization "commonly manifested by gynecomastia" when estrogen is relatively or absolutely in excess. Dihydrotestosterone (DHT) blocks these effects directly at the estrogen receptor rather than through the androgen receptor, competing with estradiol for the same binding site; the study that established this proposed DHT as the main protective antiandrogen against gynecomastia in men.

Serum estradiol correlates poorly with this whole-body estrogenic load. Gynecomastia, like central obesity and muscle atrophy, is commonly seen in men with unremarkable serum estradiol but high whole-body estrogenic tone, since almost any stressed or inflamed tissue can produce estrogen locally, and prolactin tracks that whole-body estrogenic tone more reliably than serum estradiol does.

Drug-induced gynecomastia

5-alpha-reductase inhibitors (finasteride, dutasteride) lower DHT while leaving estrogen unchanged, shifting the balance toward estrogen; case reports document gynecomastia from topical as well as oral finasteride. Antiandrogen therapy for prostate cancer (bicalutamide and newer androgen receptor pathway inhibitors) produces gynecomastia at a markedly higher rate than androgen-deprivation therapy alone; a 2025 meta-analysis of 18 randomized trials (5,036 patients) found androgen-receptor-pathway-inhibitor monotherapy raised gynecomastia risk 5.19-fold (95% CI 3.58-7.51) versus androgen-deprivation therapy alone, and prophylactic tamoxifen significantly reduced the incidence of gynecomastia and breast pain from bicalutamide monotherapy.

Treatment

Intramuscular dihydrotestosterone heptanoate (DHT-hp), 200 to 400 mg every 2 to 4 weeks for 16 weeks, reduced breast size 67% to 78% in adolescent boys with persistent pubertal gynecomastia, with no regrowth seen up to 15 months after treatment; serum DHT rose to a mean of 278 ng/dL by week 16 while testosterone and estradiol fell.

In Ray Peat's words

Peat listed gynecomastia among the effects seen with growth hormone (GH) treatment, alongside carpal tunnel syndrome, myalgia, and tumor growth, consistent with GH's close functional overlap with prolactin.

Asked on air about beer drinkers developing breasts, Peat pointed to estrogen produced by the yeast itself rather than the hops:

Yeast can produce estradiol, so they think that the yeast for men is a major source of the breast development in beer drinkers.

— Ray Peat

Practice

Check drugs (especially 5-alpha-reductase inhibitors and antiandrogens), thyroid function, and estrogen load. Get medical evaluation for any breast mass, since true glandular gynecomastia needs to be distinguished from simple fat accumulation. See Estrogen, Prolactin, Testosterone, Thyroid, Estrogen dominance.

See also

• Estrogen
• Prolactin
• Testosterone
• Thyroid
• Estrogen dominance