Sex hormone binding globulin (SHBG) is a protein made by your liver that attaches to sex hormones like testosterone and estrogen in your bloodstream. When you see “sex horm binding glob serum” on a lab report, it refers to the amount of this protein measured in a blood sample. SHBG matters because it controls how much of your sex hormones are actually available to your body’s tissues.
How SHBG Controls Hormone Activity
Sex hormones circulate in your blood in two forms: bound to proteins or free-floating. Only the free, unbound hormones can interact with your tissues to do their jobs, things like building bone and muscle, regulating your reproductive system, and influencing mood and energy. When SHBG latches onto testosterone or estrogen, those hormones are essentially locked up and inactive.
This means your total testosterone or estrogen level on a blood test doesn’t tell the whole story. Two people can have the same total testosterone, but if one has much higher SHBG, far less of that testosterone is actually working in their body. That’s why doctors order an SHBG test: it reveals whether the hormones you’re producing are genuinely available or mostly tied up.
SHBG doesn’t bind all hormones equally. It grips DHT (a potent form of testosterone) most tightly, has a medium hold on testosterone and estradiol, and binds only weakly to other hormones like DHEA and androstenedione. This selective binding means SHBG has a particularly strong influence on androgen activity.
Normal SHBG Ranges
SHBG is measured in nanomoles per liter (nmol/L). According to Mayo Clinic Laboratories, the reference ranges for adults are:
- Men (18 and older): 13.3 to 89.5 nmol/L
- Women ages 18 to 46: 18.2 to 135.5 nmol/L
- Postmenopausal women (47 to 91): 16.8 to 125.2 nmol/L
Women generally have higher SHBG levels than men, which is one reason they have less free testosterone circulating in their bodies. Children of all sexes naturally have very high SHBG levels, which keeps sex hormones from affecting body tissues until puberty.
What High SHBG Means
When SHBG is elevated, more of your sex hormones are bound up and unavailable. Even if your body produces a normal amount of testosterone, your tissues may not be getting enough of it. In men, this can look a lot like low testosterone: reduced sex drive, fatigue, difficulty building muscle, and low energy. In women, high SHBG can suppress the small amount of free testosterone needed for libido and overall vitality.
Conditions that commonly raise SHBG include hyperthyroidism (an overactive thyroid), liver disease, and aging. Certain medications, particularly estrogen-containing treatments like oral birth control, can significantly increase SHBG production in the liver. In fact, during treatment for conditions like excess hair growth in women, SHBG levels can climb to twice the upper limit of the normal range when patients are taking estrogen-containing therapy.
What Low SHBG Means
Low SHBG leaves more free testosterone circulating in your body. In women, this excess free testosterone can drive symptoms like acne, oily skin, excess facial or body hair (hirsutism), hair thinning on the scalp, and irregular periods. Low SHBG is a common finding in polycystic ovary syndrome (PCOS). In men, low SHBG is less likely to cause obvious symptoms on its own, but it signals underlying metabolic issues worth paying attention to.
The most significant driver of low SHBG is insulin resistance. When your body needs to produce more insulin than normal to manage blood sugar, SHBG production in the liver drops. Research shows a clear negative relationship: higher body mass index correlates with lower SHBG, and in people with impaired insulin sensitivity, SHBG levels fall as insulin levels rise. Obesity and metabolic syndrome are the most common reasons a person’s SHBG comes back low on a lab test.
Why Doctors Order This Test
An SHBG test is rarely ordered in isolation. It’s typically requested alongside a total testosterone level to get a clearer picture of what’s actually happening with your hormones. The most common reasons include evaluating symptoms of low testosterone in men, investigating excess hair growth or acne in women, helping diagnose PCOS, and sorting out confusing testosterone results.
One of SHBG’s most practical uses is calculating the Free Androgen Index (FAI), a simple ratio that estimates how much testosterone is biologically active. The formula is: total testosterone (in nmol/L) divided by SHBG (in nmol/L), multiplied by 100. This calculation is especially useful when a woman’s total testosterone looks normal but her SHBG is low, meaning she actually has elevated free testosterone driving symptoms like unwanted hair growth. Many women with idiopathic hirsutism have a normal total testosterone but an elevated FAI, which only becomes apparent when SHBG is measured.
Factors That Influence Your SHBG Level
Beyond medical conditions, several everyday factors shift SHBG up or down. Body weight is one of the strongest: carrying excess weight, particularly around the midsection, tends to lower SHBG through its effects on insulin. Losing weight and improving insulin sensitivity can raise SHBG levels over time. Exercise, particularly regular aerobic activity, is associated with modestly higher SHBG.
Age also matters. SHBG tends to rise gradually in men as they get older, which is one reason free testosterone declines with age even when total testosterone stays relatively stable. In women, SHBG fluctuates with hormonal changes across the menstrual cycle, during pregnancy, and through menopause. Thyroid function, liver health, and various medications all play roles as well, which is why your doctor may order additional tests if your SHBG comes back outside the normal range.