Most men who look up SHBG do it for the same reason. Their testosterone result came back 'normal', they still feel awful, and somewhere on the report is a line they've never heard of with a number next to it that nobody explained.
That line matters more than most people realise. Sex hormone binding globulin decides how much of the testosterone in your blood is actually available to your body. Two men can have identical total testosterone and feel completely different, and SHBG is usually the reason why.
This article covers what SHBG is, what pushes it up or down, what a high or low result means for your symptoms and for any decision about TRT, and how to get it measured properly.
What SHBG actually is
SHBG is a protein made in the liver. Its job is to bind sex hormones, mainly testosterone, dihydrotestosterone and oestradiol, and carry them around the bloodstream. While a testosterone molecule is attached to SHBG it's inactive. It can't get into a cell, it can't bind to a receptor, and it can't do anything useful for you.
In a typical man, somewhere around 45 percent of the testosterone in his blood is bound tightly to SHBG. Most of the rest is bound loosely to albumin, which lets it go fairly easily. Only about 1 to 3 percent floats completely free. That free fraction, plus the albumin-bound portion that can be released quickly, is what your tissues actually get to use.
So when a lab reports your total testosterone, it's reporting everything: bound and unbound, usable and locked away. The total number is a starting point. It isn't the whole story.
Why SHBG matters for free testosterone
Think of SHBG as the number of seats on a bus. The more seats there are, the more testosterone sits down and stays put. With a lot of SHBG, a large share of your testosterone is bound and your free testosterone falls, even though the total hasn't changed. With very little SHBG, more testosterone is left standing, free and active.
Here's a worked example we use on our free testosterone calculator. A man with total testosterone of 15 nmol/L, SHBG of 35 nmol/L and a normal albumin has a calculated free testosterone of roughly 298 pmol/L, about 2 percent of his total. A total of 15 looks comfortable. A free level of 298 pmol/L sits in the grey zone between 225 and 347 pmol/L. Push his SHBG higher and the same total testosterone produces a free figure that is frankly low.
| Measure | What it is | What it tells you |
|---|---|---|
| Total testosterone | Everything in your blood, bound and unbound | The standard test. Useful, but misleading on its own if SHBG is unusual |
| SHBG | The carrier protein that binds testosterone tightly | How much of your total is locked away |
| Free testosterone | The unbound fraction, usually calculated from total, SHBG and albumin | What your tissues can actually use. Often tracks symptoms more closely than total |
Labs rarely measure free testosterone directly. Direct assays are expensive and the cheaper 'analogue' immunoassays are unreliable enough that the British Society for Sexual Medicine (BSSM) advises against them. Instead, free testosterone is calculated from total testosterone, SHBG and albumin using the Vermeulen equation, which is the method both the BSSM and our free testosterone calculator use.
What raises SHBG
The Endocrine Society's clinical practice guideline lists the conditions known to push SHBG up. The common ones are:
- Age. SHBG rises steadily as men get older, which is one reason free testosterone falls faster than total testosterone with age.
- An overactive thyroid. Hyperthyroidism increases SHBG production.
- Liver disease. Cirrhosis and hepatitis both raise SHBG.
- Some medications. Certain anticonvulsants, and oestrogens, raise SHBG.
- HIV infection.
- Genetics. Variations in the SHBG gene mean some men simply run high for no other reason.
- Very low body weight. Anorexia is associated with raised SHBG.
Insulin also plays a role. Insulin resistance and type 2 diabetes are associated with low SHBG, so a lean, insulin-sensitive man will often have a higher SHBG than a heavier man with the same total testosterone. It's one of the reasons slim, fit men can be surprised by a low free testosterone result.
What lowers SHBG
The same guideline lists what pushes SHBG down:
- Obesity. The most common cause of low SHBG we see.
- Type 2 diabetes and insulin resistance.
- An underactive thyroid. Hypothyroidism lowers SHBG, the mirror image of the overactive picture above.
- Androgens. Testosterone itself, and anabolic steroids, suppress SHBG. Men already using testosterone, prescribed or otherwise, usually have a lower SHBG than they started with.
- Glucocorticoid steroids and some progestins.
- Nephrotic syndrome and acromegaly, both uncommon.
- Genetics, again.
You'll notice a pattern. Many of the things that lower SHBG, such as excess weight and insulin resistance, also lower testosterone production. That's why a heavier man can have a low total testosterone with a free testosterone that looks a little better than expected, and why losing weight tends to raise both his total testosterone and his SHBG at the same time.
High SHBG: what it means for you
High SHBG is the one that catches men out. The total testosterone looks fine, so the GP says you're fine, and the symptoms of low testosterone carry on regardless: flat energy, low libido, poor recovery, a mood that never quite lifts.
What's happened is that a bigger share of a normal-looking total is locked away. The man in the example above, with a total of 15 nmol/L, is a classic case. He will be told he's normal in most GP surgeries. Measure his SHBG, calculate his free testosterone, and the picture changes.
For a TRT decision, high SHBG means three things. First, free testosterone has to be part of the assessment, not an afterthought. Both the BSSM and the Endocrine Society say free testosterone should be calculated whenever total testosterone is borderline or SHBG is likely to be abnormal. Second, the cause of the high SHBG needs looking at. If it's your thyroid or your liver, that needs treating in its own right, and treating it may bring your free testosterone back without any testosterone at all. Third, if you do start treatment, the aim is a healthy free level rather than a total number that looks tidy on paper.
Low SHBG: what it means for you
Low SHBG tends to mean the opposite trap. A borderline total testosterone can hide a free testosterone that is still acceptable, because less of it is bound. That doesn't make the symptoms imaginary, but it does mean the testosterone number may not be the main driver.
More often, low SHBG is a flag for something metabolic. Excess weight, insulin resistance and an underactive thyroid are all on the list, and all of them cause fatigue, low mood and weight gain in their own right. A low SHBG with a low total testosterone usually points to a man whose whole metabolic picture needs attention, which is exactly why a decent panel includes HbA1c, lipids and thyroid function alongside the hormones.
For TRT, low SHBG can also change how a man responds. Because less testosterone is held in reserve, levels can swing more between doses, and his doctor may lean towards smaller, more frequent doses to keep things stable. That's a prescribing judgement based on how you actually respond, which is a good reason to be treated by someone who looks at the full set of numbers.
How SHBG is tested
SHBG is a straightforward blood test, run from the same sample as your testosterone. There is no separate preparation, and the same timing advice as testosterone applies: earlier in the day if you can, no need to fast, and a low result confirmed on a second day before anyone makes a diagnosis. Our guide to diagnosis explains how we read the timing of a sample.
The problem is that SHBG often isn't requested. Many routine NHS testosterone checks measure total testosterone alone, and once that comes back inside the reference range the investigation stops. If you've had a test and there's no SHBG line on it, you don't yet know your free testosterone.
All three of Arc's testosterone blood tests include SHBG and a calculated free testosterone. The at-home finger-prick kit covers total testosterone, SHBG, albumin and free testosterone. The Enhanced blood test adds the rest of a proper diagnostic panel: LH and FSH, prolactin, oestradiol, thyroid function, full blood count, liver and kidney function, HbA1c, lipids and PSA. The Ultimate panel goes further again into cardiovascular and thyroid antibodies. Every result is read by a GMC-registered doctor alongside your symptoms, not just sent to you as raw numbers.
What to do with your result
If you already have a total testosterone and an SHBG, put them into the free testosterone calculator. It uses the same Vermeulen equation a clinician would and gives you free and bioavailable testosterone in seconds.
Then read the number in context. The BSSM uses a free testosterone below 225 pmol/L (0.225 nmol/L) as the level at which treatment is usually needed in a symptomatic man. International recommendations from the 2018 International Consultation for Sexual Medicine add a grey zone of about 225 to 347 pmol/L (65 to 100 pg/mL), where a trial of treatment can be considered at the doctor's discretion. Neither is a hard line, and neither replaces a doctor reading your full panel.
What I'd ask you not to do is dismiss your symptoms because a total testosterone came back 'normal'. If nobody has measured your SHBG, nobody has actually checked whether your testosterone is working.
Thinking about TRT?
Arc TRT is a UK doctor-led testosterone clinic that has helped over 3,000 men investigate their symptoms and, where appropriate, begin properly monitored treatment. Start with a simple at-home blood test.
This article is for general information and does not replace personalised medical advice. Speak to a qualified clinician about your individual circumstances.
Sources: British Society for Sexual Medicine, Guidelines on Male Adult Testosterone Deficiency, with Statements for Practice (Hackett et al., World Journal of Men's Health, 2023); Endocrine Society, Testosterone Therapy in Men with Hypogonadism: An Endocrine Society Clinical Practice Guideline (Bhasin et al., Journal of Clinical Endocrinology and Metabolism, 2018), Table 2; Lab Tests Online UK, Sex Hormone Binding Globulin (SHBG); Vermeulen A. et al., A critical evaluation of simple methods for the estimation of free testosterone in serum, Journal of Clinical Endocrinology and Metabolism, 1999. Morgentaler A et al, Diagnosis and Treatment of Testosterone Deficiency: Updated Recommendations From the Lisbon 2018 International Consultation for Sexual Medicine, Sexual Medicine Reviews, 2019;7(4):636-649.
Keep reading
For the complete picture, see our full guide to TRT in the UK, covering eligibility, blood test thresholds, NHS and private routes, costs and monitoring. If you are still working out whether any of this applies to you, start with the symptoms of low testosterone, or read about normal male testosterone levels and how to get a testosterone blood test in the UK.
