Here's something most GPs won't tell you: the single number on your testosterone test result probably isn't telling you what you think it is.
Most men, if they bother to get tested at all, look at total testosterone, see they're "in range," and move on. The problem is that "in range" on a standard NHS test covers everything from 8 to 30 nmol/L. That's not a range. That's a canyon. A man at 9 nmol/L and a man at 28 nmol/L are having completely different hormonal experiences — but both get told they're normal.
And total testosterone, on its own, doesn't even tell you how much testosterone your body can actually use.
That's where SHBG comes in.
What SHBG Actually Does
Sex hormone-binding globulin is a carrier protein produced by your liver. Its job is to bind to testosterone and float it around in the bloodstream. When testosterone is bound to SHBG, it's inactive — unavailable to your tissues, your muscles, your brain.
Your circulating testosterone exists in three states:
- Bound to SHBG — locked up, biologically unavailable (roughly 60–70%)
- Bound to albumin — loosely held, partially bioavailable (20–30%)
- Free testosterone — completely unbound, actively working (1–3%)
Free testosterone, plus loosely-albumin-bound testosterone, makes up what's often called your bioavailable testosterone. This is what your body can actually use.
The landmark paper establishing how to calculate free testosterone properly — Vermeulen et al., published in the Journal of Clinical Endocrinology and Metabolism in 1999 — made clear that calculated or directly-measured free testosterone is essential for meaningful diagnosis. The paper is still cited in clinical guidelines today. Total testosterone alone is an incomplete picture.
So: if your SHBG is high, more of your testosterone is bound and unavailable. You could have a total testosterone of 700 ng/dL and feel like rubbish. Conversely, a man at 500 ng/dL with low SHBG might feel excellent. The absolute number matters less than what's actually free to work.
Why SHBG Rises With Age
This is the part that most men over 40 are not aware of: SHBG naturally increases as you get older, independent of what's happening to your total testosterone.
The European Male Ageing Study — a large, multi-centre study following middle-aged and older men across eight European countries — found that while total testosterone declines modestly with age, SHBG rises. The net effect is that free testosterone falls significantly faster than total testosterone. Two men at the same total testosterone level at 30 and 50 respectively will have meaningfully different free testosterone levels.
This is why it's possible to have "normal" testosterone on paper at 45 and feel functionally low. Your total T looks fine. Your SHBG has crept up. Your free T has quietly dropped. Your GP sees the total number, says everything's within range, and offers you antidepressants.
This isn't a criticism of GPs — they're working within a system that hasn't caught up with the evidence. But it's why self-education on this matters.
What Drives SHBG Up
Several factors push SHBG higher, which is generally not what you want:
Insulin resistance. This is probably the most impactful modifiable factor. Chronically elevated insulin — from a diet high in refined carbohydrates, sedentary lifestyle, excess body fat — is consistently associated with elevated SHBG in men with metabolic dysfunction. It's not entirely understood why, but fixing insulin sensitivity almost always improves free testosterone.
High oestradiol. Oestrogen stimulates SHBG production in the liver. Fat tissue produces aromatase, the enzyme that converts testosterone to oestrogen. Overweight men therefore often have a compounding problem: lower total testosterone, higher aromatisation of what's left, and rising SHBG as a result.
Thyroid dysfunction. Hyperthyroidism is a well-established cause of elevated SHBG. Thyroid function is rarely checked by men looking at their hormones, but it should be. If your testosterone is low and your SHBG is high, get thyroid function tested.
Ageing. As above — this one you can't stop, but you can offset.
Excessive alcohol. Alcohol stresses the liver, which produces SHBG. Heavy, consistent drinking impairs liver function in ways that raise SHBG and worsen testosterone metabolism broadly.
What Drives SHBG Down
Resistance training. Heavy, compound lifting — squats, deadlifts, pressing — consistently lowers SHBG over time in men who train regularly. Not single sessions; habitual training. This is one of the most underappreciated hormonal benefits of lifting. It doesn't just increase testosterone acutely; it improves the long-term ratio of free to bound T.
Improving insulin sensitivity. Lowering fasting insulin — through lower-carbohydrate eating, time-restricted eating, regular zone 2 cardio, or weight loss — reliably lowers SHBG in insulin-resistant men.
Zinc. Zinc plays a regulatory role in SHBG production. Most men are mildly deficient. Research published in Nutrition found that dietary zinc restriction in healthy men significantly reduced serum testosterone, and supplementation in marginally zinc-deficient men raised it. 20–30 mg daily (not megadosing) is sensible. Food sources: red meat, oysters, pumpkin seeds.
Boron. The evidence is modest but there's enough of it to mention. A study by Naghii et al. in the Journal of Trace Elements in Medicine and Biology found that 10 mg of boron per day for one week significantly reduced SHBG and increased free testosterone in healthy men. The effect size isn't enormous, but 5–10 mg daily is safe and cheap. Avocados and nuts contain boron naturally.
Managing body composition. Getting leaner lowers aromatase activity, which lowers oestradiol, which lowers SHBG. It's not a linear relationship, but the direction is clear.
Reference Ranges: What You're Looking For
The NHS lab reference ranges are wide, designed to capture 95% of the male population. They tell you whether you're in the bottom 2.5% or top 2.5% of all men — not whether your levels are optimal for how you want to function.
| Marker | Standard Lab Range | Optimal (Men Over 40) | |--------|-------------------|----------------------| | Total Testosterone | 7.7–30.4 nmol/L | 16–26 nmol/L | | Free Testosterone | 225–725 pmol/L | 400–700 pmol/L | | SHBG | 19–76 nmol/L | 25–50 nmol/L | | Oestradiol | <192 pmol/L | 70–150 pmol/L | | LH | 1.7–8.6 IU/L | 4–8 IU/L | | FSH | 1.5–12.0 IU/L | 2–8 IU/L |
The "optimal" column above reflects what you commonly see in clinically well men in their 40s who feel good. It is not a clinical diagnostic range. You are an individual, your symptoms matter, and your trend over time matters as much as a single reading.
How to Get Tested in the UK (and What to Ask For)
Your GP can order testosterone testing, but free testosterone and SHBG are not standard unless you push — and even then it varies by practice. If you want a complete picture, private testing is the path of least resistance.
Medichecks offers a full male hormone panel (total testosterone, free testosterone, SHBG, LH, FSH, prolactin, oestradiol) for around £60–80. Order online, either go to a local nurse appointment or do a finger-prick sample at home. Results within 24–48 hours, with interpretation. This is where I started.
Monitor My Health runs a similar service at a comparable price point. Good turnaround time, reliable processing.
What to request if you're going via your GP:
- Total testosterone (fasted, 9–11 AM when levels are highest)
- SHBG
- Free testosterone (or they can calculate it from total T + SHBG + albumin)
- LH and FSH
- Oestradiol
If they'll only run total testosterone, do it anyway — then get a private test to fill in the gaps.
The Practical Takeaway
If you've tested your testosterone and you're looking at a single total T number, you have half the information. SHBG is the other half. It determines whether your testosterone is available to your body or just circulating, doing nothing.
The steps:
- Get a complete hormone panel — not just total T.
- Look at free testosterone and SHBG together.
- If SHBG is above 50 nmol/L, it's worth understanding why. The usual suspects: insulin resistance, excess body fat, underactive thyroid, heavy drinking.
- The best interventions are also the most boring: consistent resistance training, better body composition, managing insulin sensitivity.
- Zinc and boron are cheap insurance. They won't transform you, but the evidence is reasonable.
SHBG isn't complicated once you understand it. The frustrating thing is that most men — and too many practitioners — are still treating testosterone as a single number rather than a system. It isn't. Start looking at the right numbers.