Does testosterone treatment cause blood clots? Testosterone replacement therapy (TRT) has been linked to an increased risk of venous thromboembolism (VTE), including deep vein thrombosis and pulmonary embolism. The mechanism involves testosterone stimulating red blood cell production, which can increase blood viscosity and potentially raise clot risk. However, the relationship remains complex, with mixed evidence from UK and international studies. The MHRA and EMA advise vigilance but note that a definitive causal link has not been established. For men with confirmed hypogonadism, benefits typically outweigh risks when treatment is properly monitored and individual risk factors are carefully assessed.

Does Testosterone Treatment Cause Blood Clots?

Testosterone replacement therapy (TRT) has been associated with an increased risk of venous thromboembolism (VTE), which includes deep vein thrombosis (DVT) and pulmonary embolism (PE). However, the relationship between testosterone treatment and blood clots remains complex and somewhat controversial within the medical community.

The mechanism behind this potential risk primarily involves testosterone's effect on red blood cell production (erythropoiesis). Testosterone stimulates the bone marrow to produce more red blood cells, leading to increased haemoglobin concentration. This physiological response can result in erythrocytosis or polycythaemia (elevated red blood cell count), which increases blood viscosity and may raise the risk of thrombotic events.

Current evidence from observational studies and meta-analyses presents mixed findings. UK research using the Clinical Practice Research Datalink suggests a modest increase in VTE risk, particularly during the first six months of treatment, whilst other studies have found no significant association. The Medicines and Healthcare products Regulatory Agency (MHRA) and the European Medicines Agency (EMA) have reviewed available data and concluded that whilst a causal relationship has not been definitively established, healthcare professionals should remain vigilant about thrombotic risk.

It is important to note that testosterone therapy is only licensed for confirmed hypogonadism, where benefits typically outweigh potential risks when properly monitored. Injectable testosterone formulations may carry a higher risk of erythrocytosis than transdermal preparations. The absolute risk of blood clots remains relatively low in most patients, but individual risk factors must be carefully assessed before initiating treatment. If VTE is suspected or confirmed during treatment, testosterone should be suspended pending further evaluation.

Risk Factors That Increase Blood Clot Risk During Treatment

Certain patient characteristics and medical conditions significantly elevate the risk of developing blood clots during testosterone therapy. Understanding these risk factors is essential for both clinicians prescribing treatment and patients considering or currently receiving TRT.

Pre-existing thrombotic and haematological conditions represent major risk factors. According to NICE guidance (NG158), patients with a personal or strong family history of VTE, inherited thrombophilias (such as Factor V Leiden or prothrombin gene mutations), or previous thrombotic events face substantially higher risk. Active cancer, major surgery or trauma, and prolonged immobility are also significant risk factors.

Lifestyle and demographic factors play an important role in risk stratification:

• Age over 60 years – older patients have baseline increased thrombotic risk

• Obesity (BMI >30 kg/m²) – adipose tissue produces pro-inflammatory cytokines affecting coagulation

• Smoking – contributes to vascular damage, though is a weaker risk factor than those above

• Prolonged immobility – such as long-haul travel or bed rest following surgery

• Varicose veins with phlebitis – may increase local clot formation risk

Concurrent medications and medical interventions may compound risk. Testosterone can potentiate the effects of coumarin anticoagulants (e.g., warfarin), requiring closer INR monitoring and possible dose adjustment. Oestrogen-containing medications also increase thrombotic risk. Recent major surgery, particularly orthopaedic procedures, substantially increases thrombotic risk for several weeks post-event.

Supraphysiological testosterone doses, whether through excessive prescribed doses or non-medical use (such as in bodybuilding), dramatically increase haematocrit levels and consequently thrombotic risk. According to UK guidance, patients with baseline elevated haematocrit should have this investigated before commencing testosterone therapy. Those with chronic lung disease, obstructive sleep apnoea, or living at high altitude may have pre-existing polycythaemia that testosterone would exacerbate.

Recognising Symptoms of Blood Clots While on Testosterone

Early recognition of blood clot symptoms is crucial for preventing serious complications. Patients receiving testosterone therapy should be educated about warning signs and advised to seek immediate medical attention if they develop concerning symptoms.

Deep vein thrombosis (DVT) typically affects the legs and presents with:

• Swelling in one leg (rarely both), often starting in the calf

• Pain or tenderness, which may feel like cramping or soreness

• Warmth in the affected area

• Redness or discolouration of the skin, which may appear darker or have a bluish tinge

• Visible distension of superficial veins

These symptoms usually develop gradually over hours to days. It is important to note that DVT can occasionally occur without obvious symptoms, particularly in smaller veins.

Pulmonary embolism (PE) occurs when a clot travels to the lungs and represents a medical emergency. Symptoms include:

• Sudden shortness of breath that worsens with exertion

• Sharp chest pain that may worsen with deep breathing or coughing

• Rapid heart rate or palpitations

• Coughing up blood (haemoptysis)

• Feeling lightheaded or fainting

• Anxiety or sense of impending doom

When to seek immediate help: Patients should call 999 or attend A&E immediately if they experience symptoms suggestive of PE, particularly sudden breathlessness or chest pain. For suspected DVT without PE symptoms, patients should contact NHS 111 or their GP urgently the same day, or attend an Urgent Treatment Centre. Do not wait to see if symptoms resolve, as early treatment significantly improves outcomes.

Patients should inform healthcare professionals that they are receiving testosterone therapy, as this information aids diagnostic decision-making. Healthcare professionals use clinical probability scores, D-dimer testing and imaging to confirm diagnosis, and may start anticoagulation treatment if clinical suspicion is high while awaiting test results.

Safety Monitoring and Reducing Your Risk

Appropriate monitoring and risk reduction strategies are essential components of safe testosterone therapy. UK guidance from the British Society for Sexual Medicine and Society for Endocrinology recommends structured surveillance protocols to identify and manage potential complications early.

Baseline and ongoing blood monitoring forms the cornerstone of safety surveillance. Before starting testosterone therapy, patients should undergo:

• Full blood count – to establish baseline haematocrit and haemoglobin

• Prostate-specific antigen (PSA) – in men over 40 or those with prostate symptoms

• Lipid profile and HbA1c – to assess cardiovascular risk factors

• Liver and renal function tests

During treatment, haematocrit monitoring is particularly important, typically performed at 3–6 months after initiation, then annually if stable. According to UK product information, if haematocrit exceeds 54%, testosterone dose should be reduced or treatment temporarily suspended until levels normalise. Some patients may require therapeutic venesection (blood removal) if polycythaemia persists. Injectable testosterone formulations carry a higher risk of erythrocytosis than transdermal preparations and may require more frequent monitoring.

Practical risk reduction measures patients can implement include:

• Maintaining optimal hydration – adequate fluid intake helps prevent blood thickening

• Regular physical activity – promotes healthy circulation and reduces venous stasis

• Smoking cessation – reduces vascular damage and thrombotic risk

• Weight management – achieving healthy BMI through diet and exercise

• Leg exercises during prolonged sitting – particularly important during long journeys

Medication adherence and communication are vital. Patients should attend all scheduled monitoring appointments and inform their GP about any new medications or health changes. Those taking warfarin require closer INR monitoring when starting or adjusting testosterone therapy. Before elective surgery, discuss testosterone therapy with the surgical team, as temporary discontinuation may be advised.

Dose optimisation is preferable to supraphysiological dosing. Treatment should aim for testosterone levels in the mid-normal range rather than upper limits, balancing symptom relief against potential risks. Regular review of treatment necessity should occur, particularly as patients age or develop new health conditions.

If you experience side effects from testosterone treatment, report them through the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

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