SHBG stands for sex hormone-binding globulin, a protein made by your liver that acts as a transport vehicle for sex hormones in your blood. When it shows up on a blood test, it helps your doctor figure out how much testosterone or estrogen is actually available for your body to use. About 98% of testosterone in your bloodstream is bound to proteins and essentially inactive, so knowing your SHBG level reveals whether your “usable” hormone levels are truly normal or not.
What SHBG Does in Your Body
Your liver continuously produces SHBG and releases it into your bloodstream, where it latches onto sex hormones and carries them through circulation. In men, roughly 44% of total testosterone is bound to SHBG, another 50% is loosely bound to a general-purpose protein called albumin, and only about 2% floats freely. That small free fraction is the testosterone that can enter cells and do its job: building muscle, maintaining bone density, driving libido, and regulating mood.
The hormones bound to SHBG are essentially locked up. Unlike albumin, which holds hormones loosely and releases them fairly easily, SHBG grips them tightly. This means SHBG acts like a control valve. When SHBG is high, more hormone gets bound and less is available to tissues. When SHBG is low, more hormone circulates freely, which can cause symptoms of excess even if total hormone levels look normal on paper.
Why Your Doctor Ordered This Test
A standard testosterone or estrogen test measures total levels, both bound and free. That number can be misleading. Someone with high SHBG might have a normal total testosterone reading but very little of it is actually usable, leaving them with symptoms of low testosterone like fatigue, low sex drive, or difficulty building muscle. The reverse is also true: low SHBG can mean plenty of active hormone is circulating even when the total number looks unremarkable.
Doctors typically order SHBG when total hormone levels don’t match the symptoms they’re seeing. In women, it’s often part of evaluating conditions like polycystic ovary syndrome (PCOS), unexplained acne, or excess hair growth. In men, it helps clarify borderline testosterone results. SHBG is also used to calculate two important values: the Free Androgen Index (FAI), which is simply total testosterone divided by SHBG expressed as a percentage, and calculated free testosterone. Both give a more accurate picture of androgen activity than total testosterone alone.
What High SHBG Means
High SHBG leaves less free hormone available to your tissues. Conditions linked to elevated SHBG include liver disease, an overactive thyroid (hyperthyroidism), and certain eating disorders. Estrogen-containing medications, including oral contraceptives and hormone replacement therapy, are among the most common causes of elevated SHBG in women. In fact, studies that measure SHBG often exclude people on hormonal birth control because it raises levels so significantly that it skews results.
In men, high SHBG can produce symptoms that mimic low testosterone: reduced energy, decreased muscle mass, low libido, and mood changes. Total testosterone may read as perfectly normal, which is why the SHBG test is so useful. It explains the gap between what the lab says and how you feel.
What Low SHBG Means
Low SHBG lets more free hormone circulate, which can create symptoms of androgen excess in women (acne, oily skin, unwanted hair growth, irregular periods) or complicate metabolic health in both sexes. The relationship between SHBG and insulin is one of the most studied connections in endocrinology. There is a clear inverse relationship: as insulin levels rise, SHBG drops. This means people with insulin resistance, obesity, or type 2 diabetes frequently have low SHBG.
SHBG also correlates negatively with BMI. As body weight increases, SHBG concentrations tend to fall. Low SHBG has been linked to a higher risk of developing metabolic syndrome, type 2 diabetes, and cardiovascular complications. Some researchers consider low SHBG a meaningful marker of metabolic risk on its own, independent of traditional factors like blood pressure and cholesterol. SHBG levels also correlate positively with HDL (“good”) cholesterol and negatively with triglycerides, reinforcing its connection to overall metabolic health.
Other causes of low SHBG include hypothyroidism (underactive thyroid), high-dose androgen use, and conditions that increase androgen production, such as PCOS.
How SHBG Changes With Age
SHBG doesn’t stay constant throughout your life, and the pattern differs between men and women. In men, SHBG rises in a steady linear fashion with age, starting noticeably from the mid-40s onward. This gradual increase means that even though total testosterone may decline only modestly, the amount of free, usable testosterone can drop more significantly because more of it gets bound up.
In women, the pattern is U-shaped. SHBG decreases between roughly ages 45 and 65, which aligns with falling estrogen levels during the menopausal transition. After about age 65, SHBG begins climbing again. This means hormone availability shifts in complex ways across different life stages, and a single SHBG reading is most useful when interpreted alongside your age and hormone levels.
Understanding Your Results
SHBG is measured in nanomoles per liter (nmol/L). Reference ranges vary between labs, and normal values differ by sex and age, so always compare your result to the specific range printed on your lab report. Women generally have higher SHBG levels than men because estrogen stimulates SHBG production while testosterone suppresses it.
Your SHBG result is rarely interpreted alone. It’s most valuable when combined with total testosterone to calculate free testosterone or the Free Androgen Index. The FAI formula is straightforward: divide total testosterone by SHBG (both in the same units) and multiply by 100. A high FAI in women can point toward androgen excess, while a low result in men may confirm functional testosterone deficiency. An increase in SHBG can decrease bioavailable testosterone without any noticeable change in total testosterone, and a decrease in SHBG can do the opposite. This is why SHBG adds a layer of information that total hormone levels simply can’t provide on their own.
Factors That Can Shift Your Levels
Beyond disease states, several modifiable factors influence SHBG. Weight loss has been shown to increase SHBG in people with obesity, consistent with the strong connection between insulin resistance and low SHBG. Medications that improve insulin sensitivity, such as thiazolidinediones, have also been shown to raise SHBG, though metformin does not appear to have the same effect.
If you’re taking oral estrogen (birth control pills or hormone replacement), your SHBG may be substantially elevated. Switching to a non-oral delivery method, like a patch or gel, typically has less impact on SHBG because the hormone bypasses the liver’s first-pass metabolism. This is a practical consideration worth discussing if your SHBG result seems unusually high and you’re on oral hormonal medication.