SHBG: why a normal testosterone can still leave you short
Sex hormone-binding globulin (SHBG) is a protein made by the liver that binds tightly to testosterone in the blood. Testosterone bound to SHBG is essentially locked away, so the SHBG level determines how much of your total testosterone is free to act. It explains a common puzzle: a man with a "normal" total testosterone who has every symptom of a low one. SHBG is a blood test a urologist or primary-care doctor adds when total testosterone and symptoms do not match.
What your number means
| Low | under 10 nmol/L | More testosterone is free, so total testosterone underestimates what tissues receive. Typically seen with obesity, insulin resistance, type 2 diabetes, and androgen or steroid use. Low SHBG is itself a marker of metabolic risk. |
|---|---|---|
| Normal | 10–57 nmol/L | Total testosterone is a reasonable reflection of what is available. Free testosterone can still be calculated from the total, SHBG and albumin if symptoms warrant. |
| High | above 57 nmol/L | Less testosterone is free, so total testosterone overestimates what tissues receive. Seen with ageing, hyperthyroidism, liver disease, low body weight and some medications. This is the classic reason for normal total testosterone with low-T symptoms. |
What it measures
SHBG is a glycoprotein synthesised in the liver that circulates in the blood and binds sex hormones with high affinity, testosterone and DHT more tightly than estradiol. In a typical man, roughly 40–60% of testosterone is bound to SHBG, most of the rest is loosely bound to albumin, and only 1–3% is fully free. Only the free and albumin-bound fractions, together called bioavailable testosterone, can enter cells and act.
Because SHBG-bound testosterone is effectively inert, the same total testosterone means different things at different SHBG levels. A total of 400 ng/dL with an SHBG of 20 nmol/L leaves considerably more usable hormone than the same total with an SHBG of 70 nmol/L. This is why SHBG is measured: not for its own sake, but to interpret testosterone correctly.
SHBG is drawn from the same morning sample as total testosterone. Fasting is not required. Laboratories then calculate free testosterone from the total testosterone, SHBG and albumin using a validated formula, which is more reliable than the direct free testosterone immunoassays many labs offer.
Why it matters for sperm production
SHBG does not act on the testicles directly, and inside the testicle sperm production relies on locally made testosterone that is not controlled by circulating SHBG. Its importance for fertility is indirect but real: it determines whether a man's testosterone is truly adequate, which shapes libido, erectile function and the frequency of intercourse, and it flags the metabolic conditions that do damage sperm.
High SHBG with a normal total testosterone can hide a genuinely low free testosterone. Men in this situation may have reduced sex drive and energy, and a doctor relying on the total alone will miss it. Conversely, a low SHBG can make a borderline total testosterone look worse than the usable fraction really is.
Low SHBG is strongly associated with obesity, insulin resistance and type 2 diabetes, all of which independently reduce sperm concentration and motility and raise DNA fragmentation. In a fertility workup, a low SHBG is a prompt to look at weight, blood sugar and lipids, not just at hormones.
What raises or lowers it
SHBG rises steadily with age, roughly 1–2% per year in adulthood, which is one reason free testosterone falls faster than total testosterone in older men. It also rises with hyperthyroidism, liver cirrhosis and hepatitis, HIV infection, low body weight or restrictive eating, estrogen exposure, and some medications including certain anticonvulsants. Heavy endurance training can push it up.
SHBG falls with insulin: obesity, insulin resistance, metabolic syndrome and type 2 diabetes all lower it, as does non-alcoholic fatty liver disease. Androgens and anabolic steroids, growth hormone, glucocorticoids, hypothyroidism and nephrotic syndrome also reduce it. Genetic variation in the SHBG gene accounts for a meaningful part of the differences between men.
The lifestyle levers are the metabolic ones. Weight loss and improved insulin sensitivity raise a low SHBG toward normal; there is no reliable way to lower a high SHBG through diet or supplements, and treating the underlying cause, such as an overactive thyroid, is the route when one exists.
What your doctor may do about it
SHBG is rarely treated on its own. A urologist or endocrinologist uses it to calculate free testosterone and decide whether the hormone picture explains symptoms. If free testosterone is low despite a normal total, the man is managed as having low testosterone, with the same search for a cause and the same fertility-preserving options, such as clomiphene or hCG, that apply to any man trying to conceive.
A low SHBG usually triggers a metabolic assessment: fasting glucose or HbA1c, lipids, liver enzymes and blood pressure. Treating insulin resistance and losing weight improves SHBG, testosterone and semen parameters together, which makes it one of the more rewarding findings to act on.
A high SHBG prompts a look at thyroid function, liver tests, medications and, in older men, a frank conversation about age-related decline. If an underlying condition is found, treating it typically brings SHBG down and free testosterone up.
When to retest
SHBG is a relatively stable measure day to day, so a single result alongside a confirmed total testosterone is usually enough. If total testosterone is being repeated to confirm a low value, SHBG is drawn again with it so free testosterone can be recalculated.
After weight loss, improved blood sugar control or thyroid treatment, a recheck at 3 to 6 months shows whether SHBG and free testosterone have shifted. A semen analysis at the same interval captures any change in sperm production.
Most men do not need SHBG measured at all. Hera's semen analysis is the usual first step; SHBG is added by a doctor when total testosterone is borderline or when symptoms and the total do not agree.
Common questions
Why is my testosterone normal but I have symptoms of low T?
One common explanation is a high SHBG. Testosterone bound to SHBG is not available to your tissues, so a normal total can hide a low free testosterone. Ask your doctor to measure SHBG and calculate free testosterone. Other causes, such as thyroid problems, sleep apnea, depression or medication effects, produce similar symptoms and are checked at the same time.
Does SHBG affect sperm count?
Not directly. Sperm production depends on testosterone made inside the testicle, which circulating SHBG does not control. SHBG matters for fertility because it determines how much testosterone reaches the rest of the body, which affects libido and sexual function, and because a low SHBG flags obesity and insulin resistance, which do reduce sperm quality.
What causes low SHBG in men?
Insulin is the main driver. Obesity, insulin resistance, metabolic syndrome, type 2 diabetes and fatty liver all lower SHBG. Anabolic steroids, testosterone therapy, hypothyroidism and some medications also reduce it. Low SHBG is considered an early marker of metabolic risk even when blood sugar is still normal.
How is free testosterone calculated from SHBG?
Laboratories use a validated formula, most often the Vermeulen equation, that takes total testosterone, SHBG and albumin and estimates the free fraction based on how tightly each protein binds testosterone. This calculated value is considered more reliable than direct free testosterone immunoassays. Your doctor may also see it reported as "bioavailable testosterone", which adds the albumin-bound fraction.