TRT Blood Tests UK: What to Monitor Before, During, and After Testosterone Replacement Therapy

Medically reviewed content. Last updated: February 2026.

Testosterone replacement therapy (TRT) is one of the fastest-growing prescriptions in the UK. NHS data shows testosterone prescribing rose by nearly 90% over a single decade, and the private TRT clinic market has expanded rapidly since 2020. Whether you are exploring treatment for clinically diagnosed hypogonadism or already on a protocol through your GP or a specialist, one thing is non-negotiable: regular blood monitoring.

A TRT blood test is not a formality. It is the primary tool your clinician uses to confirm diagnosis, set your starting dose, flag safety concerns like rising haematocrit or PSA, and fine-tune your protocol over time. Without it, you are flying blind — and the consequences can range from suboptimal results to genuinely dangerous side effects.

This guide covers every marker you need, when to test, what the results mean, and how to monitor TRT safely from home in the UK.

Why Blood Tests Are Essential for TRT

Blood testing serves three distinct purposes across your TRT journey, and each one matters.

1. Establishing Baseline Values

Before a single milligram of testosterone enters your body, your clinician needs a comprehensive snapshot of your hormonal, metabolic, and haematological status. This baseline serves multiple functions:

• Confirming the diagnosis. The BSSM guidelines state that testosterone deficiency (TD) should only be diagnosed when total testosterone is consistently below 12 nmol/L (or below 8 nmol/L for clear deficiency) on at least two morning samples, combined with symptoms. A single low reading is not sufficient.
• Identifying the cause. LH and FSH levels distinguish primary hypogonadism (testicular failure, where LH/FSH are elevated) from secondary hypogonadism (pituitary or hypothalamic dysfunction, where LH/FSH are low or inappropriately normal). This changes the treatment approach entirely.
• Setting safety reference points. Your pre-TRT haematocrit, PSA, liver enzymes, and lipid values become the benchmark against which all future results are compared.

2. Monitoring Safety

TRT is not without risks. The most significant include:

• Polycythaemia — testosterone stimulates erythropoiesis (red blood cell production). A haematocrit above 0.54 significantly increases the risk of venous thromboembolism (VTE) and major adverse cardiovascular events (MACE). Research published in the Journal of Urology found that developing polycythaemia during TRT is an independent risk factor for these events in the first year of therapy.
• Prostate safety — while current evidence does not support the claim that TRT causes prostate cancer, PSA monitoring remains essential for early detection, particularly in men over 40.
• Liver and kidney function — oral testosterone preparations (rarely used in the UK now) carry hepatotoxicity risk, but all forms warrant periodic liver and kidney function checks.
• Cardiovascular markers — TRT can alter lipid profiles, particularly reducing HDL cholesterol. Regular monitoring catches adverse trends early.

3. Optimising Your Dose

The goal of TRT is not to push testosterone as high as possible. It is to achieve stable levels in the mid-to-upper normal range while minimising side effects. Blood tests reveal whether your dose is too low (persistent symptoms, low trough levels), too high (elevated haematocrit, oestradiol conversion, acne, sleep disturbance), or just right.

Pre-TRT Baseline Blood Panel: The Complete List

The following table details every marker that should be included in your baseline panel before starting testosterone replacement therapy. The BSSM practical guide and Endocrine Society guidelines both recommend a comprehensive baseline assessment.

That is 16 markers minimum — and the reason clinics charge what they do for comprehensive TRT panels. The good news is that at-home blood tests now cover this entire panel in a single kit.

TRT Monitoring Schedule: When to Test

The BSSM guidelines recommend a structured monitoring schedule with more frequent testing in the first year, tapering to annual checks once your protocol is stable. The following schedule aligns with BSSM and Endocrine Society recommendations.

Important: If haematocrit exceeds 0.54 at any point, the BSSM recommends dose reduction or a change in preparation, and the patient should be evaluated for sleep apnoea and hypoxia. Do not wait for the next scheduled blood test — re-test within 2-4 weeks of any intervention.

What to Watch For: Red Flags in Your TRT Blood Results

Not every out-of-range result requires panic, but certain patterns demand attention. Here are the key warning signs to discuss with your clinician.

Haematocrit Above 0.54 (Polycythaemia)

This is the single most important safety concern with TRT. Testosterone stimulates the kidneys to produce erythropoietin, which increases red blood cell production. A haematocrit above 0.54 significantly raises the viscosity of your blood, increasing the risk of blood clots, stroke, and cardiovascular events.

The Endocrine Society considers a haematocrit above 0.50 a relative contraindication to starting TRT, while the European Association of Urology uses 0.54 as the absolute threshold requiring intervention. The BSSM aligns with the 0.54 threshold, recommending dose reduction, preparation switch, or therapeutic phlebotomy (blood donation or medical blood removal).

Risk factors for TRT-related polycythaemia: injectable testosterone (particularly Sustanon, which causes higher peaks), smoking, sleep apnoea, living at altitude, obesity, and older age.

Rising Oestradiol (Aromatisation)

Testosterone is converted to oestradiol by the aromatase enzyme, which is concentrated in adipose (fat) tissue. When testosterone levels rise on TRT, oestradiol production increases proportionally — and sometimes disproportionately in men carrying excess body fat.

Symptoms of elevated oestradiol include breast tenderness or gynecomastia (breast tissue growth), water retention and bloating, emotional lability and irritability, reduced libido (paradoxically, since oestradiol competes with testosterone at receptors), and fat redistribution to hips and thighs.

Serum oestradiol levels above approximately 150 pmol/L (40 pg/mL) in men on TRT commonly produce symptoms, though individual sensitivity varies. Management options include dose reduction, switching from injections to gels (which produce more stable levels with less aromatisation), weight loss (reducing aromatase activity), and in some cases, low-dose aromatase inhibitors under specialist supervision.

PSA Rise

Current evidence does not support the historical belief that TRT causes prostate cancer. Multiple large-scale studies, including those referenced in the 2023 BSSM guidelines, found no increased risk of prostate cancer diagnosis, progression, or higher Gleason grades in men on TRT.

However, monitoring remains essential. A PSA rise of more than 1.4 ng/mL within 12 months of starting TRT, or an absolute value exceeding 4.0 ng/mL, warrants referral to a urologist for further investigation. Expect a modest PSA increase of 0.3-0.5 ng/mL in the first 6-12 months of treatment — this is normal and typically plateaus.

Liver Enzyme Elevation

ALT and AST elevations are uncommon with modern TRT preparations (injections and gels) but can occur. More commonly, elevated liver enzymes pre-TRT indicate non-alcoholic fatty liver disease (NAFLD), which is itself associated with low testosterone and metabolic syndrome. Track the trend rather than reacting to a single elevated result.

Lipid Panel Changes

TRT can reduce HDL cholesterol by 5-10%, particularly with injectable preparations. Monitor your cholesterol ratios and triglycerides over time. If HDL drops below 1.0 mmol/L or the total cholesterol/HDL ratio exceeds 6.0, discuss cardiovascular risk management with your doctor.

Mood and Sleep Changes

While not measured in a blood test, mood disturbance, irritability, poor sleep, or increased aggression on TRT often correlate with supraphysiological testosterone levels or elevated oestradiol. If you experience these symptoms, get a blood test rather than waiting for the next scheduled check — the results usually point to an easy dose adjustment.

TRT Forms Available in the UK

The form of testosterone you use directly affects when and how you should time your blood tests. Here is an overview of the main preparations prescribed in the UK.

Transdermal Gels: Testogel and Tostran

Gels are the most commonly prescribed TRT form in the UK. Applied daily to the shoulders, upper arms, or abdomen, they provide relatively stable testosterone levels without the peaks and troughs of injections.

• Testogel — available as 16.2 mg/g gel in pump dispensers or 50 mg sachets
• Tostran — 2% testosterone gel in a metered-dose canister

Blood test timing: Test 2-6 hours after applying the gel. The target is to achieve a mid-range testosterone level (approximately 15-25 nmol/L). A random blood test is appropriate for gel users, as levels remain relatively stable throughout the day.

Short-Acting Injections: Sustanon 250

Sustanon 250 contains four testosterone esters with different half-lives, providing a testosterone release over 2-3 weeks. It is typically injected every 2-3 weeks intramuscularly, though many clinics now recommend more frequent injections (weekly or twice-weekly subcutaneous) to reduce peaks and troughs.

Blood test timing: Test at trough — on the day of your injection, before injecting. The aim is a trough level of 10-15 nmol/L. If testing at the peak (approximately 48-72 hours post-injection), levels of 25-30 nmol/L are expected and acceptable. Ideally, both peak and trough levels should be measured at least once to assess the full picture.

Long-Acting Injections: Nebido (Testosterone Undecanoate)

Nebido is a slow-release injection given every 10-14 weeks (the first interval is typically shorter, at 6 weeks). It provides the most stable levels of any injectable form and is administered intramuscularly into the gluteal muscle.

Blood test timing: Test mid-way between injections (approximately 5-6 weeks after the last injection for a standard 10-12 week interval). The target is a mid-range testosterone level. Trough levels immediately before the next injection should ideally not fall below 10-12 nmol/L.

Patches (Less Common)

Testosterone patches (e.g., Intrinsa, though availability varies) are used less frequently in the UK due to skin irritation and lower patient satisfaction. Blood test timing is similar to gels — test several hours after application.

Optimising Your TRT Dose with Blood Test Results

Getting the dose right is an iterative process. Here is how blood test results guide adjustments.

Understanding Trough vs Peak Testing

The distinction between trough and peak testing is critical for injectable TRT:

• Trough testing (immediately before your next injection) tells you the lowest your testosterone drops between doses. If trough levels are below 10 nmol/L and you have symptoms returning before your next injection, the dose or frequency needs adjusting.
• Peak testing (24-72 hours post-injection for Sustanon) tells you the highest your levels reach. If peak levels are supraphysiological (above 30-35 nmol/L), you may experience side effects — acne, irritability, sleep disruption, elevated haematocrit — even if trough levels are acceptable.

Many modern TRT protocols use more frequent, lower-dose injections (e.g., 62.5 mg Sustanon twice weekly instead of 250 mg every 2-3 weeks) to flatten the curve and reduce the peak-trough differential. This approach often reduces oestradiol conversion and haematocrit elevation.

Target Ranges on TRT

When Dose Adjustments Are Needed

Common scenarios that require a dose change based on blood results:

1. Trough testosterone below 12 nmol/L with persistent symptoms — increase dose by 10-20% or increase injection frequency.
2. Peak testosterone above 35 nmol/L with side effects — reduce dose or split into more frequent smaller doses.
3. Oestradiol above 150 pmol/L with gynecomastia or water retention — reduce dose, switch from injections to gel, address body composition.
4. Haematocrit above 0.52 and rising — switch from injections to gel (gels cause less erythrocytosis), reduce dose, ensure adequate hydration, screen for sleep apnoea.
5. Symptoms resolved but blood values high — reduce to the lowest effective dose. More is not better.

Can You Monitor TRT Without a Clinic?

Yes. While your TRT prescription must come from a doctor — testosterone is a prescription-only medicine (POM) and a controlled substance (Class C) in the UK — the blood monitoring component can be done privately and independently.

This is particularly relevant if you:

• Receive TRT through your GP but want more comprehensive blood panels than the NHS typically provides
• Use a private TRT clinic but want to verify results independently or test more frequently
• Need a baseline panel before your first consultation to arrive with data in hand
• Want to monitor between clinic appointments without booking additional visits

At-home blood tests have transformed TRT monitoring in the UK. A professional phlebotomist visits your home or workplace, draws a venous blood sample (far more accurate than finger-prick capillary samples for hormone testing), and sends it to an accredited laboratory. Results are typically available within 2 working days.

The key advantage is comprehensiveness. Many NHS GPs only check total testosterone and FBC for TRT monitoring. A dedicated male hormone panel adds free testosterone, SHBG, oestradiol, DHT, LH, FSH, and metabolic markers — giving you and your clinician a far more complete picture.

For the most thorough assessment, particularly at your annual review or if you are experiencing unexplained symptoms, a comprehensive panel covering 50-70 biomarkers provides total-body oversight including thyroid function, inflammation markers, vitamin levels, and metabolic health alongside your TRT-specific markers.

For your annual review or a deep-dive into overall health alongside TRT markers, the Peak Insights 70 blood test covers 70 biomarkers including hormones, liver, kidney, thyroid, vitamins, inflammation, and a full lipid panel — everything you need in a single draw.

Frequently Asked Questions About TRT Blood Tests