Does TRT cause gynaecomastia? Testosterone replacement therapy (TRT) is a recognised treatment for hypogonadism in men, but it carries a known risk of causing gynaecomastia — the development of enlarged or tender breast tissue. This occurs because exogenous testosterone can be converted into oestradiol via a process called aromatisation, tipping the androgen-to-oestrogen balance. Not every man on TRT will be affected, and individual risk depends on factors including body composition, formulation, and dose. This article explains the mechanism, warning signs, monitoring strategies, and treatment options available within the NHS.

How Testosterone Replacement Therapy Can Lead to Gynaecomastia

TRT can cause gynaecomastia by converting exogenous testosterone into oestradiol via aromatisation, particularly when testosterone levels reach supraphysiological peaks or body fat is high.

Testosterone replacement therapy (TRT) is a widely used treatment for hypogonadism — a condition in which the body produces insufficient testosterone. While TRT can significantly improve quality of life, energy levels, libido, and mood, it carries a recognised risk of causing gynaecomastia: the development of enlarged or tender breast tissue in men. Gynaecomastia is listed as a known adverse reaction in the Summary of Product Characteristics (SmPC) for all UK-licensed testosterone products, including Nebido (testosterone undecanoate), Sustanon, Testogel, and Tostran.

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Gynaecomastia occurs when there is an imbalance between androgens (such as testosterone) and oestrogens in the male body. When exogenous testosterone is introduced through TRT, a proportion undergoes a natural biochemical process called aromatisation, whereby testosterone is converted into oestradiol — the primary form of oestrogen — by an enzyme called aromatase. This enzyme is found in adipose (fat) tissue, the liver, and other organs. The risk of excess aromatisation is greatest when testosterone levels reach supraphysiological peaks or when total exposure is high; it is less pronounced when TRT is carefully titrated to achieve mid-normal physiological serum testosterone levels.

It is important to note that not every man on TRT will develop gynaecomastia. Individual susceptibility varies considerably depending on factors such as:

• Body composition — men with higher body fat have more aromatase activity

• Age — older men may have altered hormone metabolism

• Genetic sensitivity — breast tissue responsiveness to oestrogen differs between individuals

• TRT formulation and dose — injectable preparations can cause larger fluctuations in hormone levels compared to transdermal gels or patches

Importantly, testosterone is contraindicated in men with known or suspected breast cancer, as stated in the SmPCs for all UK-licensed testosterone products. Understanding the mechanism of gynaecomastia is essential for both patients and clinicians, as it informs how TRT is monitored and adjusted to minimise unwanted side effects whilst maintaining therapeutic benefit.

Why Oestrogen Levels Rise During TRT

Oestrogen rises during TRT because aromatase enzymes convert circulating testosterone into oestradiol; injectable preparations such as Sustanon are more likely to cause spikes than transdermal gels.

The rise in oestrogen during TRT is a direct pharmacological consequence of elevated circulating testosterone. As testosterone levels increase, the aromatase enzyme — present in fat cells, the brain, bone, and other tissues — converts a fraction of this testosterone into oestradiol. This is a normal metabolic pathway, not a malfunction. However, the risk of disproportionate oestradiol production is greatest when testosterone levels reach supraphysiological peaks, as can occur with certain TRT formulations.

Certain TRT delivery methods are more likely to cause oestrogen spikes than others. In the UK, commonly used intramuscular (IM) preparations include Sustanon (a blend of four testosterone esters) and testosterone undecanoate (Nebido), a long-acting ester given every 10–14 weeks per its SmPC. Sustanon can produce notable peaks in testosterone shortly after injection, followed by troughs before the next dose, which may drive higher aromatisation. Nebido has a different pharmacokinetic profile — producing a slower rise and more gradual decline — but peak-related aromatisation can still occur. In contrast, transdermal preparations such as Testogel and Tostran tend to produce more stable daily serum testosterone levels, which may result in more consistent — and potentially lower — oestradiol levels.

Additionally, lifestyle factors play a meaningful role:

• Obesity increases aromatase activity due to greater adipose tissue mass

• Alcohol consumption can impair hepatic oestrogen metabolism

• Certain medications may interact with hormone metabolism

It is worth emphasising that oestrogen is not entirely harmful in men — it plays important roles in bone density, cardiovascular health, and libido. The clinical concern arises when oestradiol rises disproportionately, tipping the androgen-to-oestrogen ratio in a direction that stimulates breast tissue proliferation. Regular monitoring during TRT is therefore important to detect this imbalance early, particularly if symptoms such as breast tenderness develop.

Recognising the Signs and Symptoms of Gynaecomastia

Gynaecomastia typically presents as a firm, rubbery disc of tissue beneath the nipple, often with breast tenderness; hard, irregular, or fixed lumps require urgent clinical assessment to exclude malignancy.

Gynaecomastia can present in a range of ways, from subtle and asymptomatic to noticeably uncomfortable. Recognising the early signs allows for timely intervention and helps distinguish true gynaecomastia from other conditions such as pseudogynaecomastia (fatty tissue accumulation without glandular proliferation) or, in rare cases, breast malignancy.

The hallmark features of gynaecomastia include:

• A firm, rubbery disc of tissue felt beneath the nipple or areola — this is glandular tissue, not simply fat

• Breast tenderness or sensitivity, which may be the first symptom noticed

• Swelling or enlargement of one or both breasts — it can be unilateral or bilateral

• Nipple discharge — though uncommon, this warrants prompt medical review

Symptoms typically develop gradually over weeks to months after starting or adjusting TRT. Some men notice tenderness before any visible enlargement occurs. The condition is graded clinically from mild (minor subareolar tissue) to severe (marked breast enlargement).

It is important to differentiate gynaecomastia from breast cancer in men, which, although rare, can present similarly. In line with NICE guideline NG12 (Suspected cancer: recognition and referral), clinicians should consider an urgent suspected cancer (two-week wait) referral to a breast clinic for men with an unexplained breast lump, and for those aged 50 or over with unilateral nipple changes such as discharge, retraction, or other concerning features. Clinical judgement should be applied in all cases.

Features that should prompt urgent assessment include:

• A hard, irregular, or fixed lump

• Bloody or spontaneous nipple discharge

• Skin changes such as dimpling, redness, or ulceration

• Nipple retraction

• Associated lymph node swelling

Patients on TRT who notice any breast changes — even mild tenderness — are encouraged to report these to their prescribing clinician rather than waiting for a routine review. Early identification allows for dose adjustment or additional management before the condition progresses.

Reducing Your Risk: Monitoring and Dose Adjustments on TRT

Risk can be reduced through regular blood monitoring, dose reduction, switching to transdermal preparations, and lifestyle changes such as weight management; aromatase inhibitors are off-label and specialist-initiated only.

Proactive monitoring is the cornerstone of safe TRT management. In line with UK clinical practice — including guidance from the Specialist Pharmacy Service (SPS) and specialist society recommendations — men on TRT should undergo regular blood tests. Standard monitoring typically includes:

• Serum testosterone — timing of the sample should be appropriate to the formulation (e.g., mid-interval trough for Sustanon; pre-injection trough for Nebido; any time for transdermal preparations)

• Full blood count and haematocrit — at baseline, at 3–6 months, and annually thereafter

• Prostate-specific antigen (PSA) — as per age and individual risk, in line with local policy

• Liver function tests and lipids — based on clinical context

Oestradiol (E2) is not routinely measured in all men on TRT in standard UK practice; it should be checked when clinically indicated — for example, if symptoms of gynaecomastia or other signs of oestrogen excess develop. Luteinising hormone (LH) is not routinely monitored once TRT has commenced, as it will be suppressed by exogenous testosterone; it is primarily useful as a diagnostic marker before treatment begins.

If oestradiol levels are found to be elevated in the context of symptoms, several clinical strategies may be employed:

• Dose reduction — lowering the TRT dose reduces the substrate available for aromatisation

• Changing the delivery method — switching from an IM preparation to a transdermal gel or patch may smooth out hormone fluctuations

• Adjusting injection intervals — for Sustanon, smaller and more frequent doses may reduce peak testosterone levels and thereby limit aromatisation; any adjustment to Nebido (testosterone undecanoate) should follow the product SmPC and be made under specialist guidance, as its long-acting pharmacokinetics differ significantly from shorter-acting esters

• Aromatase inhibitors (AIs) — medications such as anastrozole are off-label in the context of TRT and should only be initiated by a specialist with careful clinical oversight; they carry a risk of suppressing oestradiol to levels that are harmful to bone density and cardiovascular health

Lifestyle modifications also contribute meaningfully to risk reduction:

• Weight management — reducing body fat lowers aromatase activity

• Limiting alcohol — supports healthier hepatic hormone metabolism

• Regular exercise — particularly resistance training, which supports lean body composition

It is essential that any adjustments to TRT are made under medical supervision. Self-adjusting doses or self-medicating with aromatase inhibitors purchased online carries significant risks. Suspected side effects from TRT should be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Treatment Options Available on the NHS

NHS management includes TRT dose adjustment, off-label tamoxifen for active gynaecomastia, and surgical correction for persistent cases, subject to local ICB funding criteria.

When gynaecomastia develops as a result of TRT, the initial management approach is usually to address the underlying hormonal imbalance — as described above — before considering further intervention. In many cases, adjusting the TRT regimen and optimising hormone levels is sufficient to halt progression and, in early-stage cases, may allow partial resolution of breast tissue changes.

However, once glandular breast tissue has become established — particularly if present for more than 12 months — it is unlikely to resolve spontaneously, even if hormone levels are corrected. In such cases, the following options may be considered:

• Pharmacological treatment: Selective oestrogen receptor modulators (SERMs) such as tamoxifen have been used off-label to treat gynaecomastia. Tamoxifen blocks oestrogen receptors in breast tissue and can reduce tenderness and size, particularly in early or active gynaecomastia; it is less effective once tissue is well established. It is not universally commissioned for this indication and its use is off-label; it may be initiated by a specialist such as an endocrinologist or breast surgeon in selected cases, with local policies varying across Integrated Care Boards (ICBs).

• Surgical treatment: For persistent or significant gynaecomastia causing physical discomfort or psychological distress, surgical correction (subcutaneous mastectomy or liposuction-assisted excision) may be considered. NHS funding for this procedure is subject to local ICB funding criteria and is generally reserved for cases where there is documented functional or psychological impact. Patients may be referred to a breast surgery or plastic surgery team for assessment.

It is worth noting that cosmetic concerns alone may not meet NHS funding thresholds, and some patients opt for treatment in the private sector. A frank discussion with a GP or specialist about the available options, likely outcomes, and local funding pathways is advisable. The NICE Clinical Knowledge Summary (CKS) on gynaecomastia provides further guidance on evaluation and management in primary care.

When to Speak to Your GP or Endocrinologist

Men on TRT should seek prompt medical advice for any new breast lump, nipple discharge, or skin changes; NICE NG12 recommends urgent two-week wait referral for unexplained breast lumps.

Men on TRT should maintain regular contact with their prescribing clinician throughout their treatment, but certain symptoms and circumstances warrant more prompt medical attention. Knowing when to seek advice — rather than waiting for a scheduled review — is an important aspect of safe self-management.

Contact your GP or specialist promptly if you notice:

• Any new breast tenderness, swelling, or lump, even if mild

• Nipple discharge of any kind, particularly if bloody or spontaneous

• Rapid or asymmetrical breast enlargement

• A hard, irregular, or fixed lump that does not feel like soft glandular tissue

• Skin changes over the breast, such as dimpling, redness, or ulceration

• Nipple retraction

• Psychological distress related to changes in your body image

In line with NICE NG12, your GP should consider an urgent two-week wait referral to a breast clinic if you have an unexplained breast lump, or if you are aged 50 or over with nipple changes or other concerning features. Do not delay seeking assessment for any of the red-flag symptoms listed above.

Routine monitoring appointments should not be skipped, as blood test results may reveal hormonal imbalances before physical symptoms become apparent. If you are receiving TRT via a private clinic or online prescription service, it remains your responsibility to ensure that appropriate blood monitoring is in place and that results are reviewed by a qualified clinician. Suspected side effects should be reported to the MHRA via the Yellow Card scheme (yellowcard.mhra.gov.uk).

For men who have concerns about whether TRT is the right treatment for them, or who are experiencing multiple side effects, a referral to an NHS endocrinologist can provide specialist assessment and guidance. Endocrinologists can review the full hormonal picture, consider alternative treatment strategies, and coordinate care with other specialists such as breast surgeons if required.

Current evidence does not demonstrate a clear increase in male breast cancer risk with TRT; however, testosterone is contraindicated in men with known or suspected breast cancer, as stated in the SmPCs for all UK-licensed testosterone products. Any unexplained breast change should always be assessed clinically to exclude rarer causes. Open communication with your healthcare team remains the most effective safeguard.

Frequently Asked Questions