Understanding Your Testosterone Blood Test Results

You asked your GP to check your testosterone, or perhaps you ordered a private blood test. The results have arrived — a handful of numbers and reference ranges that don't mean much without context. If you're staring at your results wondering whether they're "good" or "bad," you're in the right place.

Testosterone blood tests are more nuanced than they first appear. A single number on a page doesn't tell the whole story, and the difference between a lab's "normal" range and what's actually optimal for your health can be surprisingly large.

Let's walk through exactly what your results mean.

Understanding the Basics

When we talk about testosterone in a blood test, we're actually measuring several related things:

• Total testosterone — the entire amount of testosterone in your blood, both bound and free
• Free testosterone — the small fraction (about 2-3%) that is unbound and biologically active
• SHBG — the protein that binds most of your testosterone, making it unavailable to your tissues

Think of total testosterone as your total bank balance. Free testosterone is the cash in your wallet — the amount you can actually spend right now. SHBG is like a savings account that locks your money away.

All three measurements are needed to properly understand your hormonal status. A total testosterone result on its own can be misleading, and unfortunately, many GPs only request total testosterone.

Total Testosterone: What's Normal?

The NHS and most UK laboratories define the normal range for total testosterone as approximately 8.6–29.0 nmol/L. The British Society for Sexual Medicine (BSSM) guidelines use a more clinically useful framework:

• Above 12.0 nmol/L — unlikely to be testosterone deficient
• 8.0–12.0 nmol/L — a "grey zone" where symptoms and clinical judgement guide the decision
• Below 8.0 nmol/L — consistent with testosterone deficiency, especially if symptoms are present

Here's the important nuance: the "normal" range of 8.6–29.0 nmol/L is derived from the statistical distribution of all adult men, including those in their 70s and 80s. A 35-year-old man with a testosterone of 9.0 nmol/L is technically "within range" but is sitting at the bottom end — which may not reflect where his body needs to be for optimal health.

Practical takeaway: Don't just ask whether you're "in range." Ask where you fall within the range and whether your level matches how you feel. A level of 10 nmol/L is very different from 22 nmol/L, even though both are technically "normal."

When to test

Testosterone follows a circadian rhythm, peaking in the early morning and declining throughout the day. For accurate results, blood should be drawn before 10am, fasting where possible. The BSSM recommends confirming a low result with a second sample on a separate day, as levels can fluctuate by 15-20% day to day.

Free Testosterone: Why It Matters More

Free testosterone is the portion that's immediately available to bind to androgen receptors in your muscles, brain, bones, and other tissues. It's the testosterone that actually does the work.

Normal free testosterone for adult men is approximately 0.2–0.6 nmol/L, though this declines with age. Many specialists consider free testosterone to be a more reliable indicator of androgenic status than total testosterone alone.

Here's why free testosterone can disagree with total testosterone: if your SHBG is high, a large proportion of your total testosterone is bound up and unavailable. You might have a total testosterone of 15 nmol/L — perfectly "normal" — but a free testosterone of only 0.15 nmol/L, which is low. Clinically, you'd experience symptoms of testosterone deficiency despite a reassuring total testosterone number.

Practical takeaway: Always request free testosterone alongside total testosterone. If only total T is available, ensure SHBG is tested so that free testosterone can be calculated.

SHBG: The Binding Factor

Sex hormone-binding globulin (SHBG) is a protein produced by your liver that binds to testosterone with high affinity. The normal range is approximately 15–55 nmol/L for adult men.

Understanding your SHBG level is critical for interpreting everything else:

High SHBG (above 50-55 nmol/L)

High SHBG reduces the amount of free testosterone available to your body. Common causes include:

• Ageing — SHBG rises by approximately 1-2% per year after age 40
• Hyperthyroidism — excess thyroid hormone stimulates SHBG production
• Liver disease — particularly cirrhosis
• Certain medications — including some anticonvulsants
• Very low body fat — extremely lean individuals may have elevated SHBG

Low SHBG (below 15-20 nmol/L)

Low SHBG increases free testosterone relative to total, but it's often a marker of underlying metabolic issues:

• Obesity — particularly visceral (abdominal) fat
• Insulin resistance and type 2 diabetes
• Hypothyroidism
• Androgen use — exogenous testosterone suppresses SHBG

Practical takeaway: If your SHBG is unusually high or low, investigate the underlying cause. Treating the root issue (thyroid dysfunction, insulin resistance, weight management) often improves your testosterone picture naturally.

DHEA-S and Cortisol

A comprehensive testosterone panel often includes two additional markers that provide important context.

DHEA-S

DHEA-S (dehydroepiandrosterone sulphate) is produced by your adrenal glands and serves as a precursor to both testosterone and oestrogen. Normal levels range from approximately 2.0–15.0 µmol/L in adult men, declining steadily with age.

Low DHEA-S alongside low testosterone may suggest an adrenal component to your hormone profile. It's a useful marker for overall hormonal resilience and adrenal health.

Cortisol

Cortisol — your primary stress hormone — has a direct antagonistic relationship with testosterone. Chronically elevated cortisol suppresses testosterone production at the hypothalamic-pituitary level and accelerates muscle breakdown.

A morning cortisol (taken before 9am) between 170–540 nmol/L is normal. If cortisol is persistently high and testosterone is low, addressing stress, sleep, and lifestyle factors is essential before considering testosterone therapy.

Practical takeaway: Low testosterone combined with high cortisol and low DHEA-S paints a picture of chronic stress and adrenal strain. This pattern often responds to lifestyle intervention before hormone replacement is needed.

Normal Ranges by Age

Testosterone naturally declines with age, typically by about 1-2% per year after the age of 30. Here's what the general population data looks like:

Age Range	Average Total Testosterone (nmol/L)	Typical Range (nmol/L)
20–29	18–25	12–30
30–39	16–23	10–28
40–49	14–21	9–26
50–59	13–19	8–24
60–69	11–17	7–22
70+	9–15	6–20

Important caveats: These are population averages, and individual variation is substantial. Some 60-year-old men naturally maintain levels of 20+ nmol/L, while some 30-year-olds sit at 12 nmol/L. The question isn't just what's statistically normal for your age — it's whether your level is adequate for your symptoms and wellbeing.

Practical takeaway: Age-related decline is normal, but "normal for your age" doesn't mean "optimal for your health." If you're symptomatic (fatigue, low libido, poor concentration, mood changes, reduced muscle mass) and your levels are in the lower quartile for your age, further investigation is warranted.

Low Testosterone: What Next?

If your results suggest low testosterone (below 12 nmol/L, or low free testosterone with symptoms), here are the appropriate next steps:

• Confirm with a repeat test — a single low reading isn't diagnostic. Retest on a separate morning before 10am
• Rule out reversible causes — obesity, sleep apnoea, excessive alcohol, chronic stress, opioid medications, and certain illnesses can all suppress testosterone temporarily
• Check LH and FSH — these pituitary hormones distinguish between primary hypogonadism (testicular problem — LH/FSH will be high) and secondary hypogonadism (pituitary/hypothalamic problem — LH/FSH will be low or inappropriately normal)
• Optimise lifestyle factors — weight loss, improved sleep, resistance training, and stress reduction can meaningfully raise testosterone in men with borderline levels
• Discuss treatment options — if levels are persistently low and you're symptomatic after addressing lifestyle factors, testosterone replacement therapy may be appropriate

The BSSM guidelines recommend a trial of lifestyle modification for 3-6 months in men with borderline levels before considering TRT, unless levels are severely low or symptoms are significantly affecting quality of life.

The Role of Thyroid

Finally, don't overlook your thyroid. Thyroid hormones interact extensively with testosterone metabolism, and thyroid dysfunction can mimic or worsen symptoms of low testosterone.

Hypothyroidism (underactive thyroid) causes fatigue, weight gain, low mood, and reduced libido — symptoms almost identical to testosterone deficiency. It also raises SHBG, which reduces free testosterone. Correcting an underactive thyroid can improve testosterone symptoms without any testosterone treatment at all.

If your testosterone panel includes thyroid markers (TSH, Free T4, Free T3), review them carefully. A TSH above 2.5 mIU/L with symptoms is worth discussing with your doctor, even if it falls within the standard laboratory range.

Practical takeaway: Before attributing all your symptoms to low testosterone, ensure your thyroid is functioning optimally. Thyroid dysfunction is common, easily diagnosed, and highly treatable — and fixing it can transform how you feel.

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