Gynecomastia: What Could Be Causing It

How the Estrogen-Androgen Balance Controls Breast Tissue

Gynecomastia develops when estrogen signaling at the breast outpaces androgen signaling. Every male produces some estrogen. The enzyme aromatase, concentrated in adipose tissue, converts circulating androgens (testosterone and androstenedione) into estradiol and estrone 1. When this conversion increases or when androgen levels drop, the net estrogenic effect on breast ductal epithelium rises, triggering proliferation of glandular and stromal tissue.

Free testosterone is the biologically active fraction. Sex hormone-binding globulin (SHBG) binds testosterone with higher affinity than estradiol, so any condition that raises SHBG (hyperthyroidism, liver disease, aging) effectively lowers free testosterone while leaving more free estradiol available 2. The result is a functional estrogen excess even when total estradiol levels look normal on a lab report.

This is not a single-pathway problem. Gynecomastia can arise from increased estrogen production, decreased androgen production, increased SHBG, blocked androgen receptors, or exogenous estrogen exposure. Each mechanism points to a different clinical cause, which is why the differential diagnosis spans endocrine, hepatic, renal, pharmacologic, and neoplastic categories.

The Endocrine Society's 2019 clinical practice discussion noted that "gynecomastia should be considered a sign, not a disease, and the underlying etiology should be sought" 3. That framing matters. Treating the breast tissue without investigating the hormonal driver can miss serious pathology.

Physiologic Gynecomastia: Three Age Windows

The most common form of gynecomastia requires no treatment at all. It occurs in three predictable windows tied to normal hormonal physiology.

Neonatal gynecomastia affects 60% to 90% of newborn males due to transplacental transfer of maternal estrogens. The breast tissue typically regresses within weeks as maternal hormones clear 4.

Pubertal gynecomastia appears in 50% to 70% of adolescent boys, usually between ages 12 and 14 5. During early puberty, testicular estradiol production rises before testosterone output fully matures, creating a transient estrogen-dominant window. The tissue is often unilateral at onset, tender, and located directly beneath the areola. In approximately 75% to 90% of cases, it resolves spontaneously within 12 to 24 months without any intervention.

Senescent gynecomastia develops in older men as testosterone production declines and adipose tissue increases. The Baltimore Longitudinal Study of Aging documented a 1.6% per year decline in total testosterone after age 30, with a steeper decline in free testosterone 6. Concurrently, rising adiposity drives higher aromatase activity, compounding the shift toward estrogen dominance. Prevalence estimates in men over 50 range from 24% to 65%, depending on how gynecomastia is defined (palpation versus imaging) 7.

Drug-Induced Gynecomastia: The Most Reversible Cause

Medications account for roughly 10% to 25% of all gynecomastia cases in adults, making this the single most actionable category in the differential 8. The mechanism varies by drug class.

Antiandrogens and GnRH agonists. Bicalutamide, flutamide, enzalutamide, and leuprolide directly block or suppress androgen signaling. In the Early Prostate Cancer trial, bicalutamide 150 mg produced gynecomastia in 73% of patients over 9 months 9. That rate is not a side effect footnote. It is the expected pharmacologic consequence of blocking androgen receptors at breast tissue.

Spironolactone binds the androgen receptor as an antagonist and also inhibits testosterone biosynthesis. Gynecomastia occurs in 4% to 10% of men taking standard doses for heart failure or hypertension, and rates climb with higher doses 10.

5-alpha reductase inhibitors. Finasteride and dutasteride block conversion of testosterone to dihydrotestosterone (DHT). While gynecomastia is reported in only 0.4% to 2.2% of users in key trials, the risk is real and often surprises patients taking these drugs for hair loss 11.

Exogenous estrogens or aromatizable androgens. Testosterone replacement therapy at supraphysiologic doses or with poor monitoring can paradoxically cause gynecomastia via excess aromatization. The same applies to illicit anabolic steroid use, which is an increasingly common cause in younger men 12.

Other well-documented offenders include cimetidine (H2 blocker), ketoconazole (antifungal), digoxin, methadone, and certain HIV antiretrovirals (efavirenz). A thorough medication reconciliation is the first diagnostic step in any new gynecomastia presentation.

Pathologic Causes: When Gynecomastia Signals Disease

When medications and physiologic age windows are excluded, the differential narrows to conditions that disrupt the hypothalamic-pituitary-gonadal (HPG) axis, increase peripheral aromatization, or produce ectopic hormones.

Hypogonadism

Primary hypogonadism (testicular failure) reduces testosterone production directly. Klinefelter syndrome (47,XXY), the most common genetic cause, affects approximately 1 in 660 males and causes gynecomastia in 50% to 80% of cases 13. Acquired causes include orchitis (mumps, HIV), testicular trauma, chemotherapy, and radiation. Lab findings show low testosterone with elevated LH and FSH.

Secondary hypogonadism (pituitary or hypothalamic dysfunction) presents with low testosterone and inappropriately low or normal LH/FSH. Causes include pituitary adenomas (especially prolactinomas), Kallmann syndrome, chronic opioid use, and obesity-related hypothalamic suppression. The European Male Ageing Study (EMAS), which enrolled 3,369 men aged 40 to 79 across eight European centers, found that only men with total testosterone below 8 nmol/L (approximately 230 ng/dL) and free testosterone below 220 pmol/L showed a consistent cluster of hypogonadal symptoms, including gynecomastia 14.

Liver Disease

Cirrhosis raises SHBG production by the liver, increases aromatase activity in the damaged hepatic parenchyma, and impairs clearance of adrenal androstenedione (a substrate for peripheral estrogen synthesis). The prevalence of gynecomastia in men with alcoholic cirrhosis reaches 40% to 50% 15. Alcohol itself has direct toxic effects on Leydig cells, compounding the hormonal imbalance.

Hyperthyroidism

Thyrotoxicosis increases hepatic SHBG production, binding more testosterone than estradiol and shifting the free hormone ratio toward estrogen. Gynecomastia is reported in 10% to 40% of men with untreated Graves disease 16. The breast tissue typically regresses once euthyroidism is restored.

Chronic Kidney Disease

Uremia suppresses the HPG axis at multiple levels. Testosterone levels are low in 40% to 60% of men on hemodialysis, and gynecomastia prevalence in this population is approximately 30% 17. Renal failure also impairs clearance of prolactin, which can further suppress gonadal function.

Estrogen-Secreting and hCG-Secreting Tumors

Rare but clinically urgent. Leydig cell tumors and Sertoli cell tumors can secrete estradiol directly. Adrenocortical carcinomas may produce estrogen precursors. Extragonadal germ cell tumors (particularly mediastinal choriocarcinomas) and hepatocellular carcinoma can secrete human chorionic gonadotropin (hCG), which stimulates testicular aromatase and estrogen production. A detectable serum beta-hCG in a male with gynecomastia warrants urgent cross-sectional imaging 18.

Dr. Glenn Braunstein, who published the seminal 2007 review on gynecomastia in the New England Journal of Medicine, wrote: "The finding of an elevated serum beta-hCG level should prompt a search for an extragonadal germ-cell tumor, particularly in the mediastinum and retroperitoneum" 18.

The Diagnostic Workup: A Step-by-Step Approach

A focused evaluation identifies the cause in most cases. Start with history and physical exam, then layer labs based on findings.

Step 1: History. Document onset and duration, associated symptoms (testicular pain, mass, nipple discharge), medication list (prescription, over-the-counter, supplements, illicit drugs), alcohol intake, and family history of endocrine disorders.

Step 2: Physical exam. Palpate the breast to distinguish true gynecomastia (concentric, firm, rubbery disc beneath the areola) from pseudogynecomastia (diffuse adipose tissue without a discrete disc). Examine the testes for size, masses, or atrophy. Check for signs of liver disease (spider angiomata, palmar erythema, ascites), thyroid enlargement, or visual field deficits (pituitary mass).

Step 3: First-line labs. Order serum total testosterone (morning draw), estradiol, LH, FSH, beta-hCG, liver function panel, and thyroid function (TSH, free T4) 3. If testosterone is borderline, add SHBG and calculate free testosterone.

Step 4: Second-line studies based on results.

| Lab Pattern | Likely Cause | Next Step | |---|---|---| | Low T, high LH/FSH | Primary hypogonadism | Karyotype if young; testicular ultrasound | | Low T, low/normal LH/FSH | Secondary hypogonadism | Pituitary MRI, prolactin level | | Elevated estradiol | Estrogen-producing tumor or increased aromatization | Testicular + adrenal imaging | | Elevated beta-hCG | hCG-secreting tumor | CT chest/abdomen/pelvis | | Elevated SHBG, abnormal LFTs | Liver disease | Hepatology referral | | Low TSH, high free T4 | Hyperthyroidism | Endocrinology referral |

Step 5: Imaging. Breast ultrasound is indicated when the exam is equivocal, the mass is eccentric or hard, or the patient reports bloody nipple discharge. Male breast cancer is rare (approximately 2,800 new U.S. cases annually per the American Cancer Society 19), but features such as a fixed, eccentric, hard mass with skin changes or axillary lymphadenopathy require biopsy.

Treatment Options: From Watchful Waiting to Surgery

Treatment depends on the cause, symptom severity, duration, and patient goals.

Address the underlying cause first. Discontinue offending medications when possible. Treat hypogonadism, hyperthyroidism, or liver disease. In many cases, gynecomastia regresses partially or fully once the hormonal driver is corrected.

Observation. Appropriate for pubertal gynecomastia (high spontaneous resolution rate), mild senescent gynecomastia, and recent-onset cases (<12 months) where the underlying cause has been addressed. Tissue that has been present for more than 12 months often undergoes fibrosis, reducing the likelihood of medical resolution 20.

Tamoxifen. A selective estrogen receptor modulator (SERM) that blocks estradiol at the breast. A retrospective review of 220 patients found tamoxifen 20 mg daily produced partial or complete regression in 78% of patients with gynecomastia of less than two years duration 21. The Endocrine Society suggests tamoxifen as a reasonable off-label option for symptomatic gynecomastia when the underlying cause cannot be corrected 3. Duration is typically 3 to 6 months.

Raloxifene. Another SERM, studied less extensively. A small trial showed 86% improvement in pubertal gynecomastia at 3 to 9 months of treatment 22.

Aromatase inhibitors. Anastrozole has shown mixed results. A randomized placebo-controlled trial of anastrozole 1 mg daily in pubertal boys with gynecomastia found no significant difference in breast volume reduction compared to placebo at 6 months 23. Based on this evidence, aromatase inhibitors are not first-line.

Surgery. Subcutaneous mastectomy (with or without liposuction) is definitive for fibrotic, long-standing, or cosmetically distressing gynecomastia. Satisfaction rates exceed 90% in experienced surgical series 24. Risks include hematoma, seroma, nipple asymmetry, and contour irregularity.

When Gynecomastia Is an Emergency

Most gynecomastia is benign and non-urgent. But certain presentations require expedited evaluation.

Rapid onset in a previously unaffected adult male, especially with testicular asymmetry or a palpable testicular mass, raises concern for a Leydig cell tumor or germ cell tumor. A hard, fixed, eccentric breast mass with skin dimpling or bloody nipple discharge warrants urgent mammography and biopsy to exclude male breast cancer, which has a lifetime risk of approximately 1 in 833 men 19.

A positive serum beta-hCG in a male is never normal. Even low-level elevations demand imaging. Mediastinal germ cell tumors can grow rapidly, and delay in diagnosis worsens prognosis.

Gynecomastia accompanied by galactorrhea (milky nipple discharge), visual field changes, or severe headaches suggests a prolactin-secreting pituitary macroadenoma and requires brain MRI within days, not weeks.

Gynecomastia and Testosterone Replacement Therapy

Men starting testosterone replacement therapy (TRT) sometimes develop gynecomastia during the first weeks to months of treatment. This occurs because exogenous testosterone is a substrate for aromatase, and the initial estradiol rise may outpace the androgenic effect, particularly in men with higher baseline adiposity 25.

Monitoring estradiol at 6 to 12 weeks after initiating TRT is standard practice in guidelines from the American Urological Association 26. If estradiol climbs above 40 to 50 pg/mL and gynecomastia develops, dose reduction, change in injection frequency, or switching from intramuscular to transdermal delivery may lower aromatization. Some clinicians add low-dose anastrozole (0.5 mg twice weekly) to control estradiol in TRT patients, though this use remains off-label and long-term bone density effects are a concern 27.

Body composition changes also help. A 10% reduction in body fat reduces aromatase substrate availability, often improving the estrogen-to-androgen ratio without pharmacologic intervention. For men on TRT who develop breast tenderness without visible enlargement, serum estradiol should be checked before adding any adjunctive medication.