Gynaecomastia — the benign enlargement of glandular breast tissue in males — is a common condition that raises understandable questions about its relationship to intersex or differences of sex development (DSD) conditions. Whilst gynaecomastia is not itself classified as an intersex condition, there is a clinically important overlap: certain DSDs, most notably Klinefelter syndrome, frequently cause gynaecomastia as a secondary feature. Understanding this distinction helps patients and clinicians approach diagnosis, investigation, and management with clarity and confidence. This article explores the hormonal basis of gynaecomastia, its relationship to DSDs, its medical causes, and the treatment options available through the NHS.

What Is Gynaecomastia and How Does It Develop?

Gynaecomastia is benign glandular breast tissue enlargement in males caused by an imbalance between oestrogen and androgen activity. It is most common during puberty and in men over 50, and is fundamentally endocrine in nature.

Gynaecomastia refers to the benign enlargement of glandular breast tissue in males, resulting in a firm or rubbery mass beneath the nipple area. It is distinct from pseudogynaecomastia, which involves fatty tissue accumulation without true glandular growth and is typically associated with obesity. Gynaecomastia is common, particularly during adolescence and older age; population studies suggest it affects a substantial proportion of males at some point during their lifetime, with the highest rates seen during puberty and in men over 50.

The condition develops as a result of an imbalance between oestrogen and androgen activity in breast tissue. Although males naturally produce small amounts of oestrogen, when the ratio of oestrogen to testosterone shifts — whether due to increased oestrogen, reduced testosterone, or altered receptor sensitivity — glandular breast tissue can proliferate. Increased aromatase activity in adipose tissue can also contribute to true gynaecomastia by converting androgens to oestrogens, which is relevant even in the absence of significant obesity. This hormonal interplay is mediated at the level of oestrogen receptors within the breast, making the condition fundamentally endocrine in nature.

During puberty, transient gynaecomastia is considered physiological and typically resolves within six to twenty-four months without intervention. In older men, declining testosterone levels alongside relatively stable oestrogen concentrations can similarly trigger breast tissue growth. Gynaecomastia that persists beyond twelve to eighteen months, presents before puberty, progresses rapidly, or is accompanied by features of hypogonadism warrants clinical evaluation. Understanding this hormonal mechanism is essential for distinguishing gynaecomastia from other breast conditions and for guiding appropriate investigation and management (NICE CKS: Gynaecomastia; NHS: Gynaecomastia).

Understanding Intersex Conditions: NHS and Clinical Definitions

Gynaecomastia is not classified as an intersex or DSD condition; DSDs involve atypical chromosomal, gonadal, or anatomical sex development from conception. However, certain DSDs — particularly Klinefelter syndrome — commonly cause gynaecomastia as a secondary feature.

Intersex conditions — increasingly referred to in clinical settings as differences of sex development (DSDs) — are a group of congenital conditions in which a person's chromosomal, gonadal, or anatomical sex characteristics do not fit typical binary definitions of male or female. The NHS and clinical bodies such as the British Society for Paediatric Endocrinology and Diabetes (BSPED) recognise DSDs as a broad spectrum, encompassing conditions such as congenital adrenal hyperplasia (CAH), androgen insensitivity syndrome (AIS), and Klinefelter syndrome (47,XXY), among others (NHS: Differences in sex development).

It is important to clarify that gynaecomastia is not itself classified as an intersex or DSD condition. Intersex conditions are defined by atypical sex development present from conception or early foetal development, involving chromosomal, gonadal, or anatomical variation. Gynaecomastia, by contrast, is an acquired or physiological response of breast tissue to hormonal signals and does not in itself involve chromosomal abnormality or atypical sex organ development.

However, there is a meaningful clinical overlap worth acknowledging: certain DSD conditions — most notably Klinefelter syndrome (47,XXY), which affects approximately 1 in 600 males — commonly cause gynaecomastia as a secondary feature, alongside small testes, reduced fertility, and elevated gonadotrophins. In such cases, gynaecomastia is a symptom or consequence of the underlying DSD, not its defining characteristic. Clinicians are therefore encouraged to consider whether gynaecomastia — particularly when persistent, severe, or accompanied by features such as small testes, infertility, or tall stature — may warrant investigation for an underlying DSD. When a DSD is suspected, referral to adult endocrinology and, where appropriate, clinical genetics is recommended. This distinction is clinically significant and should be communicated sensitively to patients.

Medical Causes of Gynaecomastia and Their Hormonal Basis

Gynaecomastia causes are categorised as physiological, pathological, or drug-induced, all acting by altering the oestrogen-to-androgen ratio. In approximately 25% of cases no cause is identified, termed idiopathic gynaecomastia.

Gynaecomastia has a wide range of underlying causes, broadly categorised as physiological, pathological, or drug-induced. Identifying the cause is essential for appropriate management and to exclude serious underlying conditions.

Physiological causes include:

• Neonatal gynaecomastia — caused by transplacental transfer of maternal oestrogens

• Pubertal gynaecomastia — due to a transient rise in oestradiol relative to testosterone during adolescence

• Age-related gynaecomastia — associated with declining testosterone in older men

Pathological causes include conditions that alter the oestrogen-to-androgen ratio:

• Klinefelter syndrome (47,XXY) — a chromosomal DSD associated with hypogonadism, elevated gonadotrophins, and increased peripheral aromatisation of androgens to oestrogens

• Hypogonadism — primary or secondary, leading to reduced testosterone production

• Hyperthyroidism — increases sex hormone-binding globulin (SHBG), reducing free testosterone

• Liver disease — impairs oestrogen metabolism and may increase SHBG

• Adrenal or testicular tumours — may secrete oestrogens or oestrogen precursors directly

• Chronic kidney disease — associated with hormonal dysregulation

Drug-induced gynaecomastia is a particularly common and underappreciated cause. Medications implicated include spironolactone, cimetidine, anabolic steroids, anti-androgens (such as bicalutamide, used in prostate cancer), 5-alpha-reductase inhibitors (finasteride, dutasteride), ketoconazole, digoxin, certain calcium channel blockers (such as verapamil and diltiazem), amiodarone, antiretrovirals (including efavirenz), and some antipsychotics. Cannabis use has also been associated with gynaecomastia, though the evidence base is limited. Patients and clinicians should be aware that this list is not exhaustive; the BNF and individual Summary of Product Characteristics (SmPC) documents should be consulted for specific agents (BNF; emc SmPCs).

In approximately 25% of cases, no identifiable cause is found — termed idiopathic gynaecomastia. In line with NICE CKS guidance on gynaecomastia, a structured clinical assessment is recommended, including a thorough medication history, testicular examination, and targeted blood tests. These should include LH, FSH, testosterone, oestradiol, thyroid function, and liver function tests, as well as urea and electrolytes (U&Es). Beta-hCG should be measured when a testicular or other germ cell tumour is suspected; prolactin testing is best reserved for cases where clinical features suggest hyperprolactinaemia. Testicular ultrasound is indicated if a testicular mass is identified on examination or if beta-hCG or oestradiol levels are raised. If male breast cancer is suspected, urgent imaging and referral are required (see below).

Seeking Support and Treatment Through the NHS

Persistent breast tissue enlargement should be assessed by a GP to exclude male breast cancer and identify any underlying cause. Treatment options include watchful waiting, addressing the underlying cause, off-label medical therapy with tamoxifen, or surgical intervention subject to ICB funding.

Anyone who notices persistent breast tissue enlargement, breast tenderness, or nipple discharge should seek assessment from their GP in the first instance. Whilst gynaecomastia is most often benign, it is important to exclude male breast cancer, which, although rare, accounts for approximately 1% of all breast cancer diagnoses in the UK.

Features that should prompt urgent referral on a suspected cancer (two-week-wait) pathway, in line with NICE guideline NG12 (Suspected Cancer: Recognition and Referral), include:

• A hard, irregular, or unilateral lump

• Spontaneous unilateral nipple discharge, particularly if blood-stained

• New nipple inversion or retraction

• Skin tethering, ulceration, or changes overlying the breast

• Associated axillary lymphadenopathy

For confirmed gynaecomastia, management depends on the underlying cause and the degree of distress caused:

• Watchful waiting is appropriate for physiological pubertal gynaecomastia, which typically resolves spontaneously within six to twenty-four months

• Addressing the underlying cause — for example, withdrawing a causative medication or treating hyperthyroidism — may lead to regression of breast tissue, particularly if the condition is of recent onset

• Medical therapy — tamoxifen (a selective oestrogen receptor modulator, SERM) and raloxifene are not licensed specifically for gynaecomastia in the UK and are used off-label in specialist settings for symptomatic or persistent cases. These medicines are typically initiated by a specialist following careful assessment. Patients should be made aware of potential risks, including venous thromboembolism (VTE) and vasomotor symptoms such as hot flushes. Medical therapy is most effective in recent-onset, tender gynaecomastia (generally within the first twelve months); longstanding fibrotic breast tissue responds poorly. Any suspected side effects from medicines should be reported to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk or through the Yellow Card app

• Surgical intervention — subcutaneous mastectomy or liposuction may be considered for longstanding or severe gynaecomastia causing significant psychological distress; however, NHS funding for such procedures is subject to local Integrated Care Board (ICB) funding policies, and an Individual Funding Request (IFR) process may apply

For individuals in whom an underlying DSD such as Klinefelter syndrome is identified, referral to a specialist endocrinology or clinical genetics team is recommended. Psychological support should also be offered, as gynaecomastia — regardless of its cause — can significantly affect body image, self-esteem, and quality of life. Patients are encouraged to discuss concerns openly with their GP, who can facilitate onward referral to appropriate specialist services within the NHS (NICE CKS: Gynaecomastia; NICE NG12; NHS: Gynaecomastia).

Frequently Asked Questions