SHBG
Also known as: Sex Hormone-Binding Globulin
What Does SHBG Measure?
Sex Hormone-Binding Globulin (SHBG) is a protein produced primarily by the liver that binds tightly to sex hormones in the bloodstream, particularly testosterone, dihydrotestosterone (DHT), and estradiol. A blood test for SHBG measures the concentration of this carrier protein circulating in your blood, typically expressed in nanomoles per liter (nmol/L). When hormones are bound to SHBG, they are essentially 'inactive' and unavailable for use by your body's cells — only the 'free' or unbound fraction of these hormones can exert biological effects.
Why Does SHBG Matter?
SHBG is a critical regulator of hormonal activity because it determines how much testosterone and estrogen are biologically available to your tissues. Even if total testosterone levels appear normal, high SHBG can mean that very little free, active testosterone is available, leading to symptoms of hormonal deficiency such as low libido, fatigue, and mood changes. Conversely, low SHBG means more free hormones are circulating, which can contribute to conditions like polycystic ovary syndrome (PCOS), acne, or excess androgen effects. Measuring SHBG alongside total testosterone allows clinicians to calculate 'free testosterone,' providing a much more accurate picture of a person's true hormonal status. SHBG levels are influenced by many factors including age, insulin resistance, thyroid function, and liver health.
Normal Ranges
Males
10–57 nmol/L (adult males; levels decline with age and may be lower in obese individuals)
Females
18–144 nmol/L (adult females; levels vary with pregnancy, oral contraceptive use, and menopause)
Children
Varies by age and sex; generally higher in prepubertal children (can exceed 100 nmol/L) and decline during puberty
Causes of High Levels
- Hyperthyroidism (overactive thyroid): thyroid hormones directly stimulate SHBG production in the liver
- Liver disease (e.g., cirrhosis or hepatitis): damaged liver cells can paradoxically overproduce SHBG in certain conditions
- Estrogen excess or estrogen therapy: estrogens are potent stimulators of hepatic SHBG synthesis
- Anorexia nervosa or severe caloric restriction: low body fat and low insulin levels drive SHBG upward
- Aging (especially in men): SHBG naturally rises with age, reducing bioavailable testosterone
- Use of oral contraceptives or hormone replacement therapy: synthetic estrogens significantly increase SHBG production
Causes of Low Levels
- Insulin resistance and type 2 diabetes: elevated insulin suppresses hepatic SHBG production, one of the most common causes of low SHBG
- Obesity: excess body fat increases insulin levels and promotes androgen production, both of which lower SHBG
How to Improve Your SHBG
Diet
- Reduce refined carbohydrates and added sugars to lower insulin levels, which is one of the most effective ways to raise SHBG naturally
- Increase dietary fiber intake (aim for 25–38g/day from vegetables, legumes, and whole grains) to improve insulin sensitivity and support healthy SHBG
- Eat adequate protein (0.8–1.2g per kg body weight) to support liver function without overconsumption, as very high protein with low fat may suppress SHBG
- Include cruciferous vegetables (broccoli, Brussels sprouts, kale) which contain indole-3-carbinol to support estrogen metabolism and hormonal balance
- Limit alcohol consumption, as chronic alcohol use impairs liver function and disrupts SHBG regulation
Supplements
- Magnesium (300–400mg/day): may help improve insulin sensitivity and support healthy SHBG levels, particularly in deficient individuals
- Zinc (15–30mg/day): plays a role in androgen metabolism and liver health; deficiency is associated with hormonal imbalances affecting SHBG
- Boron (3–10mg/day): research suggests boron supplementation may increase free testosterone by reducing SHBG levels in those with high SHBG
- Vitamin D (1,000–4,000 IU/day): adequate vitamin D status is associated with healthier SHBG levels and improved hormonal balance
Related Biomarkers
Frequently Asked Questions
What is the difference between SHBG and free testosterone?
SHBG is the protein that carries testosterone through your bloodstream, but while bound to SHBG, testosterone cannot enter cells or exert its effects. Free testosterone is the small fraction (typically 1–4% of total testosterone) that is not bound to any carrier protein and is immediately biologically active. A person can have a normal total testosterone level but still experience low testosterone symptoms if their SHBG is very high, because most of their testosterone is bound and inactive. Testing SHBG alongside total testosterone allows doctors to estimate or calculate free testosterone for a more accurate assessment.
Can SHBG levels affect fertility?
Yes, SHBG can significantly impact fertility in both men and women. In women, very low SHBG (often associated with PCOS) means excess free androgens circulate in the body, which can disrupt ovulation and menstrual cycles. In men, very high SHBG reduces the bioavailability of testosterone, which can impair sperm production and sexual function. Conversely, very low SHBG in men allows excessive free testosterone to be converted to estrogen, which can also negatively affect sperm quality. Maintaining optimal SHBG levels is therefore an important component of reproductive health.
Why is SHBG higher in women than men?
Women naturally have higher SHBG levels than men primarily because estrogen is a potent stimulator of SHBG production in the liver. Since women have much higher estrogen concentrations than men, their livers produce significantly more SHBG. This higher SHBG helps regulate the smaller amounts of testosterone in women's bodies, keeping free testosterone at appropriately low levels. The higher SHBG in women also means that total testosterone measurements alone are not very meaningful without also measuring SHBG to understand hormonal availability.