SHBG, or sex hormone-binding globulin, is a protein made by your liver that acts as a transport system for sex hormones in your blood. It binds to testosterone and estrogen, controlling how much of each hormone is actually available for your body to use. When SHBG is high, less hormone is free to act on tissues. When it’s low, more hormone circulates freely. This balance matters more than most people realize, because your total hormone level on a blood test can look perfectly normal while the amount your body can actually use is too high or too low.
How SHBG Controls Hormone Activity
Sex hormones don’t just float freely through your bloodstream. Most testosterone and estrogen molecules are bound to proteins, and SHBG is the primary carrier with the strongest grip. A smaller fraction binds loosely to albumin, a general-purpose blood protein present in much higher concentrations but with far weaker binding strength. Only a tiny percentage of your total testosterone or estrogen, typically 1 to 3 percent, circulates completely unbound.
That unbound fraction is called “free” testosterone or estrogen, and it’s the portion that can enter cells and trigger biological effects: building muscle, regulating mood, maintaining bone density, influencing sex drive. Hormones locked onto SHBG are essentially inactive until they’re released. So SHBG functions like a dimmer switch. It doesn’t change how much hormone your body produces, but it determines how much of that hormone actually reaches its targets.
Why Doctors Test SHBG
A standard hormone panel measures total testosterone or total estrogen. But if your SHBG is unusually high, a large share of that total is bound and unavailable, meaning your “free” hormone level could be low even though the total looks fine. The reverse is also true: low SHBG can mean more free hormone than the total number would suggest.
This is why SHBG testing exists. Doctors use it alongside total testosterone to calculate free testosterone, most commonly through a ratio called the free androgen index. The formula is straightforward: total testosterone multiplied by 100, then divided by SHBG. More sophisticated calculations exist that account for the complex way SHBG binds hormones (the two halves of the SHBG molecule actually influence each other’s binding strength), but the basic ratio gives a useful clinical picture. In practice, an SHBG test is most often ordered when someone has symptoms of hormone imbalance but their total testosterone or estrogen levels come back in the normal range.
Normal SHBG Ranges
SHBG is measured in nanomoles per liter (nmol/L), and normal ranges differ significantly between men and women. Based on data from a large U.S. population study published in The Journal of Clinical Endocrinology and Metabolism, here are the reference ranges for healthy adults:
- Men ages 20 to 49: 12.3 to 79.6 nmol/L
- Men ages 50 to 80: 23.5 to 114.7 nmol/L
- Women ages 20 to 29: 14.5 to 249.8 nmol/L
- Women ages 30 to 80: 21.9 to 186.0 nmol/L
Women generally have higher SHBG than men, with an overall average around 62 nmol/L compared to about 34 nmol/L in men. These ranges are wide, so a number near the high or low end isn’t automatically a problem. Context matters: your symptoms, other hormone levels, and overall health all factor into interpretation.
How SHBG Changes With Age
SHBG doesn’t stay constant throughout your life. In men, levels rise steadily with age. A man in his twenties might sit comfortably around the lower end of the range, while a man in his sixties could have levels two to three times higher. This age-related increase is one reason older men can have normal total testosterone but still experience symptoms of low testosterone: more of it is bound up and unavailable.
In women, the pattern is more complex. SHBG follows a U-shaped curve across the lifespan, declining from the twenties through the fifties, then rising again after age 60. The drop through midlife is influenced heavily by body weight and insulin levels, both of which tend to increase during those decades. The rise after 60 may reflect declining hormone production. During pregnancy, SHBG increases substantially because of high estrogen levels, which directly stimulate the liver to produce more of the protein.
What Causes High SHBG
Several conditions push SHBG above the normal range. Liver disease is one of the most common causes, since the liver is where SHBG is manufactured and liver damage can disrupt its regulation. An overactive thyroid (hyperthyroidism) also raises SHBG, as thyroid hormones stimulate its production. Certain eating disorders, particularly those involving extreme calorie restriction, are associated with elevated levels as well.
Medications containing estrogen, including birth control pills and hormone replacement therapy, reliably increase SHBG. This is clinically significant: oral contraceptives can push SHBG high enough to substantially reduce free testosterone in women, which sometimes contributes to low libido or other symptoms even while total hormone levels appear adequate.
When SHBG runs high, the net effect is less free hormone available to tissues. In men, this can look like low testosterone: reduced energy, decreased muscle mass, lower sex drive, and difficulty with erections. In women, high SHBG can similarly dampen the effects of both testosterone and estrogen.
What Causes Low SHBG
Obesity is the strongest and most consistent driver of low SHBG. The connection runs through insulin: excess body fat increases insulin levels, and insulin directly suppresses the liver’s production of SHBG. A large Swedish population study confirmed this inverse relationship, finding that people with type 2 diabetes had significantly lower SHBG than those without diabetes, even after adjusting for age. Low SHBG has itself become recognized as an independent marker of metabolic syndrome and a predictor of future type 2 diabetes and cardiovascular disease risk in both men and women.
Polycystic ovary syndrome (PCOS) is another common cause of low SHBG in women. The combination of insulin resistance and elevated androgen production that characterizes PCOS creates a cycle: low SHBG leaves more free testosterone circulating, which worsens symptoms like acne, excess hair growth, and irregular periods. In men, low SHBG means more free testosterone is available, which sounds beneficial but can contribute to issues like oily skin, hair loss, and in some cases, an overestimation of how much active hormone is actually needed if testosterone replacement is being considered.
Diet, Weight, and SHBG
Your dietary habits influence SHBG in measurable ways. A study of older men found that fiber intake was positively correlated with SHBG levels, meaning higher fiber consumption was linked to higher SHBG. Protein intake showed the opposite pattern: more dietary protein was associated with lower SHBG. Interestingly, total calorie intake, fat (whether animal or vegetable), and carbohydrate intake had no significant effect on SHBG concentrations.
Body mass index was one of the two strongest predictors of SHBG in that study, alongside age. This means weight management is one of the most effective levers for shifting SHBG. Losing excess weight tends to raise SHBG in people whose levels are low, while gaining significant weight can suppress it. For someone with PCOS or metabolic syndrome whose low SHBG is contributing to symptoms, even modest weight loss can produce a meaningful change in the balance between bound and free hormones.
SHBG and Metabolic Health
Beyond its role as a hormone shuttle, SHBG has emerged as a useful window into metabolic health. Low levels are consistently linked to insulin resistance, metabolic syndrome, type 2 diabetes risk, and cardiovascular disease. The relationship appears to be bidirectional: insulin suppresses SHBG production, and the resulting hormone imbalance may itself worsen metabolic function.
This means an SHBG result on your blood work carries information beyond just your hormone status. A low reading, especially combined with a higher waist circumference or elevated fasting glucose, can be an early signal that your body is becoming less sensitive to insulin, sometimes years before blood sugar levels cross into the diabetic range. In postmenopausal women, low SHBG has shown particular value as an independent risk factor for diabetes, making it a useful addition to standard metabolic screening.