An SHBG blood test measures the level of sex hormone-binding globulin in your blood, a protein made by the liver that carries sex hormones like testosterone and estrogen throughout your body. The test matters because SHBG controls how much of those hormones are actually available for your tissues to use. It’s most often ordered when symptoms suggest abnormal testosterone levels, especially when a standard testosterone test doesn’t fully explain what’s going on.
What SHBG Does in Your Body
Think of SHBG as a transport vehicle. Your liver produces this protein, and once it enters the bloodstream, it latches onto sex hormones, primarily testosterone and estrogen, and carries them through circulation. While a hormone is bound to SHBG, your cells can’t use it. Only the “free” or unbound portion of testosterone or estrogen is biologically active, meaning it can enter tissues and do its job.
This is why total testosterone alone doesn’t always tell the full story. Two people can have the same total testosterone level but very different amounts of usable testosterone, depending on how much SHBG is in their blood. High SHBG means more hormone is locked up and unavailable. Low SHBG means more hormone is free and active. The SHBG test fills in that gap, giving your doctor a clearer picture of your actual hormonal status.
Why Your Doctor Ordered This Test
The most common reason for an SHBG test is that you have symptoms of a hormone imbalance, and a total testosterone test didn’t match up with what you’re experiencing. In men, this typically means symptoms of low testosterone: reduced sex drive, difficulty getting erections, fatigue, loss of muscle mass, or fertility problems. In women, the concern is usually the opposite, signs of excess testosterone: unusual hair growth on the face or body, deepening voice, acne, weight gain, irregular or absent periods, or fertility issues.
Doctors also order the test when evaluating specific conditions. Polycystic ovary syndrome (PCOS) is one of the most common, since SHBG levels tend to drop in women with PCOS, leaving more free testosterone in circulation. The test is also used to monitor people on hormone replacement therapy, to investigate early puberty in children, and to help sort out complex hormonal pictures where standard tests aren’t providing clear answers.
Normal SHBG Ranges
SHBG levels vary significantly by sex and age. A large study published in The Journal of Clinical Endocrinology and Metabolism established reference ranges using U.S. adults, measured in nanomoles per liter (nmol/L):
- 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
A few patterns stand out. Women generally have higher SHBG than men, which is why women have less free testosterone despite producing some. SHBG also rises with age in men, meaning older men have less bioavailable testosterone even if their total testosterone holds relatively steady. The ranges are wide, so your result needs to be interpreted alongside your symptoms and other hormone levels, not in isolation.
What High SHBG Means
When SHBG is elevated, more of your sex hormones are bound up and unavailable. In men, this can produce symptoms that look exactly like low testosterone (fatigue, low libido, erectile difficulty) even though total testosterone levels appear normal on paper. In women, high SHBG reduces the amount of active estrogen, which can contribute to symptoms during menopause or affect bone health over time.
Several conditions and factors push SHBG higher. Liver disease is one of the most well-documented causes. An overactive thyroid (hyperthyroidism) raises SHBG consistently. Aging is a natural contributor, particularly in men. Anorexia and significant calorie restriction increase it. Growth hormone deficiency plays a role as well. Certain medications, especially those that affect how the liver processes drugs (cytochrome P450 enzyme inducers, including some anti-seizure medications), can elevate SHBG. Oral estrogen-containing contraceptives and pregnancy also push levels up in women, sometimes substantially.
What Low SHBG Means
Low SHBG has the opposite effect: more free hormone circulates in your blood. In women, this often shows up as symptoms of androgen excess, the acne, unwanted hair growth, and menstrual irregularities associated with PCOS. In men, low SHBG can sometimes mask a testosterone problem because total testosterone looks adequate while the hormonal picture is actually off balance.
The most common driver of low SHBG is obesity. Excess body fat, particularly abdominal fat, suppresses SHBG production in the liver. Type 2 diabetes and insulin resistance are closely linked to lower SHBG as well, and researchers consider low SHBG an independent marker of metabolic risk. An underactive thyroid (hypothyroidism) can reduce levels too. PCOS is strongly associated with low SHBG in women, creating a cycle where less binding protein means more free testosterone, which worsens symptoms.
How SHBG Relates to Free Testosterone
Your doctor may use your SHBG result to calculate something called the Free Androgen Index (FAI), which estimates how much testosterone is biologically active. The formula is straightforward: total testosterone divided by SHBG (both in the same units), expressed as a percentage. A high FAI suggests more free testosterone than expected; a low FAI suggests less.
This calculation is particularly useful in women being evaluated for PCOS or other androgen-excess conditions. In men, some labs use SHBG alongside total testosterone to calculate free testosterone directly using more complex formulas. Either way, the SHBG value is what makes these calculations possible, which is why the test adds so much diagnostic value beyond a basic testosterone check.
What to Expect From the Test
The SHBG test is a simple blood draw, typically from a vein in your arm. Unlike some hormone tests, SHBG levels are relatively stable throughout the day and are not as sensitive to meal timing as other markers. That said, your doctor may schedule the blood draw in the morning if testosterone is being measured at the same time, since testosterone peaks in the early morning hours and labs often prefer standardized collection times.
Results usually come back within a few days. Your doctor will interpret the SHBG number alongside your total testosterone, and possibly your estrogen levels, thyroid function, and liver enzymes, to build a complete picture. A single SHBG result on its own rarely leads to a diagnosis. Its power is in context: explaining why your hormone levels don’t match your symptoms, pointing toward an underlying metabolic or thyroid issue, or guiding adjustments to hormone therapy.