The Complete Guide to TRT Blood Tests: What to Monitor and Why

Starting testosterone replacement therapy without regular blood monitoring is like driving with your eyes closed. You might feel great for a while, but you won't see the problems coming until it's too late.

TRT can be genuinely life-changing when managed properly — improved energy, mood, body composition, and libido. But testosterone is a powerful hormone that affects virtually every system in your body, and keeping tabs on your blood work is the only reliable way to ensure you're getting the benefits without the risks.

Here's your complete guide to the blood tests that matter on TRT — and why each one deserves your attention.

Why Blood Tests Matter on TRT

Testosterone replacement therapy doesn't just change your testosterone levels. It ripples across your entire endocrine system and affects your blood, liver, kidneys, and cardiovascular health. Without regular monitoring, you risk:

• Polycythaemia — an excessive rise in red blood cells that thickens your blood and increases stroke risk
• Oestrogen imbalance — testosterone aromatises (converts) to oestradiol, and too much can cause gynecomastia, water retention, and mood disturbances
• Suppressed fertility — exogenous testosterone suppresses your pituitary gland, shutting down sperm production
• Liver and kidney strain — especially relevant with oral testosterone preparations
• Cardiovascular risk changes — shifts in cholesterol profile and haematocrit that need tracking

The British Society for Sexual Medicine (BSSM) guidelines are clear: blood tests should be performed before starting TRT, at 3 months, 6 months, 12 months, and then annually as a minimum. Many clinicians prefer quarterly monitoring, particularly in the first year.

The Essential Hormone Markers

Total testosterone

This is the headline number — the total amount of testosterone circulating in your blood, both bound and unbound. On TRT, you're aiming for levels in the upper half of the normal range, typically between 15–30 nmol/L, depending on your clinic's protocol and your individual response.

The timing of your blood draw matters enormously. If you're on injectable testosterone (e.g., testosterone enanthate or cypionate), blood should ideally be drawn at the trough — just before your next injection — to capture your lowest level. If your trough is within range, you know your levels are adequate throughout the cycle.

Free testosterone

Only about 2-3% of your total testosterone is "free" — unbound to any protein and available for your body to use. The rest is bound to sex hormone-binding globulin (SHBG) or albumin.

Free testosterone is arguably more clinically relevant than total testosterone because it reflects what your tissues can actually access. You can have a perfectly normal total testosterone but low free testosterone if your SHBG is elevated — which is common in older men, those with liver conditions, or those taking certain medications.

A healthy free testosterone level is typically 0.2–0.6 nmol/L, though ranges vary between laboratories.

SHBG (sex hormone-binding globulin)

SHBG is the protein that binds most of your circulating testosterone, rendering it inactive. Understanding your SHBG level is essential for interpreting your testosterone results correctly.

• High SHBG (above 50-60 nmol/L) means more of your testosterone is bound and unavailable — you might need a higher dose to achieve adequate free testosterone
• Low SHBG (below 20 nmol/L) means more free testosterone relative to total — but can also indicate insulin resistance or metabolic syndrome

SHBG tends to increase with age, thyroid hormone excess, and liver disease, while it decreases with obesity, insulin resistance, and androgen use.

Practical takeaway: Always request SHBG alongside total testosterone. Without it, you're only seeing half the picture.

DHEA-S

Dehydroepiandrosterone sulphate (DHEA-S) is a precursor hormone produced by the adrenal glands. It serves as a reservoir that your body can convert into both testosterone and oestrogen. Monitoring DHEA-S on TRT provides insight into your broader hormonal landscape and adrenal function.

Low DHEA-S levels may suggest adrenal fatigue or insufficiency, which can undermine the benefits of TRT. The normal range for adult men is approximately 2.0–15.0 µmol/L, declining naturally with age.

Cortisol

Cortisol is your primary stress hormone, and it has an antagonistic relationship with testosterone. Chronically elevated cortisol can suppress testosterone production, impair muscle recovery, increase abdominal fat, and disrupt sleep — essentially undoing many of the benefits you're seeking from TRT.

A morning cortisol level (taken before 9am) between 170–540 nmol/L is considered normal. If your cortisol is persistently elevated despite adequate testosterone levels and you're not feeling the expected improvements, stress management becomes a priority alongside your TRT protocol.

Practical takeaway: If TRT isn't delivering the results you expected, cortisol may be the missing piece. Chronic stress can blunt your response to testosterone therapy.

Full Blood Count: The Safety Net

The full blood count (FBC) is arguably the most important safety test on TRT. Testosterone stimulates erythropoiesis — the production of red blood cells — and in some men, this effect is too strong.

Haematocrit and haemoglobin

Haematocrit measures the percentage of your blood volume occupied by red blood cells. A normal range is 0.40–0.52 L/L for men. If it climbs above 0.54 L/L, your blood becomes significantly thicker, increasing the risk of blood clots, stroke, and cardiovascular events.

The Endocrine Society guidelines recommend reducing the TRT dose or suspending therapy if haematocrit exceeds 0.54 L/L. Some clinicians also recommend therapeutic blood donation (venesection) to bring levels down.

Haemoglobin works alongside haematocrit — a level above 185 g/L warrants attention.

Other FBC markers

Your white blood cell count and platelet count should remain stable on TRT. Significant changes may indicate other health issues that need investigation.

Practical takeaway: Never skip the full blood count. Polycythaemia is the most common safety concern on TRT, and it's entirely manageable if caught early through regular blood testing.

Thyroid: The Hidden Factor

Many men start TRT expecting dramatic improvements in energy and metabolism, only to be disappointed. In a significant number of cases, the culprit is an underperforming thyroid that was never checked.

TSH, Free T4, and Free T3

Thyroid-stimulating hormone (TSH) is the primary screening marker. A level between 0.4–4.0 mIU/L is technically "normal," but many specialists consider optimal TSH to be between 1.0–2.5 mIU/L. An elevated TSH suggests your thyroid is struggling to keep up.

Free T4 and Free T3 measure the actual thyroid hormones circulating in your blood. Free T3 is the biologically active form and is particularly important — you can have a normal TSH and Free T4 but low Free T3, which indicates poor conversion.

Symptoms of subclinical hypothyroidism (fatigue, weight gain, brain fog, cold intolerance) overlap significantly with symptoms of low testosterone. If you're on TRT and still experiencing these symptoms despite adequate testosterone levels, thyroid function is the first place to look.

Practical takeaway: Ask for a full thyroid panel (TSH, Free T4, Free T3) at least once a year while on TRT. Thyroid issues are common, treatable, and can dramatically affect how well you respond to testosterone therapy.

How Often to Test

Following the BSSM guidelines, here is a sensible testing schedule:

Timing	What to Test
Baseline (before starting TRT)	Total T, Free T, SHBG, LH, FSH, Prolactin, Oestradiol, FBC, PSA, Lipids, HbA1c, Liver function, Thyroid
3 months	Total T, Free T, SHBG, Oestradiol, FBC, PSA, Liver function
6 months	Full panel (as baseline)
12 months	Full panel
Annually thereafter	Full panel, with quarterly FBC if haematocrit is trending upward

If your dose changes, restart the monitoring cycle from the 3-month point.

Red Flags to Watch For

Contact your prescribing doctor promptly if your blood tests show:

• Haematocrit above 0.54 L/L — dose reduction or venesection needed
• PSA rising rapidly — may indicate prostate issues requiring further investigation
• Oestradiol significantly elevated — may need aromatase inhibitor adjustment
• Liver enzymes markedly elevated — especially relevant with oral testosterone
• Symptoms worsening despite adequate levels — suggests other underlying issues (thyroid, cortisol, sleep apnoea)

TRT is not a "set and forget" therapy. It's a partnership between you and your clinician, guided by regular blood work. The men who get the best results from testosterone replacement are invariably those who take their monitoring seriously — catching problems early and fine-tuning their protocol based on objective data rather than guesswork.

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