Sex hormone-binding globulin (SHBG) is a protein made by your liver that binds to testosterone in your bloodstream, controlling how much of the hormone is actually available for your body to use. About 44% of the testosterone circulating in your blood is bound to SHBG, and once bound, that testosterone is essentially locked up and unable to enter your tissues. This makes SHBG one of the most important regulators of testosterone’s real-world effects on your body, sometimes even more relevant than total testosterone itself.
How Testosterone Travels in Your Blood
Testosterone doesn’t float freely through your bloodstream. It hitches a ride on proteins. Roughly 44% binds to SHBG, about 50% binds to albumin (a more common blood protein), around 4% binds to a third carrier protein, and only about 2% remains completely unbound, or “free.”
The distinction matters because SHBG grips testosterone tightly. Albumin holds it loosely, meaning testosterone can detach from albumin relatively easily and slip into cells where it’s needed. Testosterone bound to SHBG, on the other hand, stays locked in the bloodstream and can’t enter target tissues like muscle, bone, or the brain. This is the core of what scientists call the free hormone hypothesis: only testosterone that’s unbound or loosely bound is biologically active. So when your SHBG rises, even if your body is producing the same total amount of testosterone, less of it is available to do its job.
What Happens When SHBG Is Too High
When SHBG levels climb, more testosterone gets captured and less remains free. You can have a total testosterone reading that looks perfectly normal on a lab report while actually experiencing symptoms of low testosterone: fatigue, reduced sex drive, difficulty building muscle, brain fog, and low mood. This is one of the most common reasons people feel “off” despite being told their testosterone is fine.
Several things push SHBG higher. Aging is a major one. In men, SHBG increases in a steady, linear fashion starting around the mid-40s to mid-50s and continuing into old age. Reference ranges reflect this shift clearly: at age 30, normal SHBG for men falls between roughly 13.5 and 57.4 nmol/L, but by age 70, the range jumps to 27.8 to 101 nmol/L. An overactive thyroid also raises SHBG, as does liver disease, certain eating disorders, and estrogen-containing medications like birth control pills or hormone replacement therapy.
What Happens When SHBG Is Too Low
Low SHBG has the opposite effect: more testosterone floats free, which might sound like a good thing but often signals underlying metabolic problems. Reduced SHBG is closely linked to metabolic syndrome, the cluster of conditions that includes high blood sugar, excess abdominal fat, high blood pressure, and unhealthy cholesterol levels. A large analysis of U.S. adults found that for every unit decrease in SHBG below about 77 nmol/L, the odds of having metabolic syndrome increased significantly.
Risk factors for low SHBG include higher body mass index, type 2 diabetes, coronary heart disease, and non-alcoholic fatty liver disease. Low SHBG has also been independently associated with a higher risk of dementia, hypertension, and ischemic stroke. So while low SHBG means more free testosterone in the short term, it’s often a red flag for broader health issues rather than a sign of hormonal advantage.
Why Total Testosterone Alone Can Be Misleading
Standard blood panels often report only total testosterone, which combines all four fractions: SHBG-bound, albumin-bound, the small portion on other carrier proteins, and free. This single number doesn’t tell you how much testosterone your tissues can actually use. Two men with identical total testosterone of, say, 500 ng/dL can have very different free testosterone levels if one has high SHBG and the other has low SHBG.
Doctors can estimate free testosterone using a calculation that factors in your total testosterone, your SHBG level, and an assumed albumin concentration. This formula, developed by endocrinologist Alex Vermeulen, uses the known binding strengths of SHBG and albumin to solve for the unbound fraction. Many labs and online calculators use this method. It’s generally reliable, though it assumes a standard binding strength between SHBG and testosterone, which isn’t always accurate.
Genetics Can Change the Equation
Your genes influence not just how much SHBG your liver produces, but how tightly that SHBG grips testosterone. Certain genetic variations in the SHBG gene alter the protein’s binding strength. This means two people with the same SHBG number on a blood test could have meaningfully different amounts of free testosterone, because one person’s SHBG holds testosterone more tightly than the other’s. Standard free testosterone calculators don’t account for these genetic differences, which can occasionally lead to incorrect clinical assessments.
Diet, Weight, and Lifestyle Factors
Your diet has a measurable effect on SHBG. In a study of men, higher fiber intake was associated with higher SHBG levels, while higher protein intake was associated with lower SHBG. Calorie, fat, and carbohydrate intake showed no significant relationship. This means that for older men already experiencing age-related SHBG increases, a diet very low in protein could push SHBG even higher and further reduce bioavailable testosterone.
Body weight is one of the strongest predictors. Higher BMI consistently correlates with lower SHBG, which partly explains why overweight men often have higher free testosterone but also higher rates of metabolic problems. Losing excess weight tends to raise SHBG, while gaining weight lowers it.
Supplements That May Influence SHBG
Boron is the supplement with the most direct evidence for lowering SHBG. In a small clinical study, men who took about 10 mg of boron daily saw a significant decrease in SHBG within just six hours. After one week of daily supplementation, their free testosterone levels increased significantly and estradiol (a form of estrogen) decreased. The study was small, so these results aren’t definitive, but boron is one of the few over-the-counter options with any published data supporting an effect on SHBG.
Other supplements like magnesium, zinc, and vitamin D are often promoted for testosterone support, but their effects on SHBG specifically are less well-documented. Most of the hormonal benefits attributed to these nutrients appear to work through other pathways rather than directly lowering SHBG.
Getting a Complete Hormonal Picture
If you suspect testosterone-related symptoms but your total testosterone comes back normal, requesting an SHBG test alongside total testosterone gives a much clearer picture. From those two numbers, your doctor or an online calculator can estimate free testosterone. Some labs also measure free testosterone directly, though the calculation method is considered more reliable than many direct assay techniques currently available.
Because SHBG connects to so many other systems, including thyroid function, liver health, insulin sensitivity, and body composition, an abnormal result often points toward something worth investigating beyond hormones alone. A high SHBG finding might prompt a thyroid check, while a low SHBG result could be the first clue toward insulin resistance or early metabolic syndrome, conditions where early intervention makes a real difference.