What it measures.
Sex Hormone-Binding Globulin (SHBG) is a protein produced by the liver that binds and transports testosterone and estrogen through the bloodstream. The amount of SHBG determines how much 'free' hormone is available for your tissues. High SHBG reduces bioavailable hormones; low SHBG increases them. Understanding SHBG is essential for interpreting testosterone and estrogen levels.
The blood concentration of SHBG protein, measured in nmol/L. This determines the carrying capacity for sex hormones. Higher SHBG means more hormones are bound (inactive); lower SHBG means more hormones are free (active).
Why it matters.
Total testosterone can be normal while you experience low-T symptoms—because high SHBG is binding too much of it. Conversely, low SHBG with normal total T means high free testosterone. SHBG is the key to understanding the full picture of sex hormone status.
Physiology.
SHBG is synthesized in the liver and regulated by insulin, thyroid hormones, and sex hormones themselves. Testosterone lowers SHBG production while estrogen increases it. Insulin and obesity suppress SHBG. Each SHBG molecule binds one hormone molecule with high affinity, creating a hormone reservoir.
Testing & preparation.
How to prepare
- Morning testing preferred (less important than for testosterone)
- Fasting not strictly required
- Consistent timing between tests for trend monitoring
When to test
When testosterone symptoms don't match total T levels, or investigating metabolic syndrome/insulin resistance
How often
Test with free testosterone calculation; recheck 6-12 weeks after intervention
Interpretation.
High sex hormone-binding globulin
Common causes:
- Hyperthyroidism (thyroid hormone increases SHBG)
- Liver disease (cirrhosis initially raises SHBG)
- Estrogen therapy or oral contraceptives
- Anorexia or very low body fat
- Aging (SHBG rises ~1% per year in men)
- HIV infection
- Anticonvulsants (phenytoin, carbamazepine)
Implications:
- Low free testosterone despite normal total T
- Low-T symptoms (fatigue, low libido, ED) with 'normal' labs
- May indicate hyperthyroidism or liver issues
- In women: may mask hyperandrogenism
Low sex hormone-binding globulin
Common causes:
- Insulin resistance and metabolic syndrome
- Obesity
- Type 2 diabetes
- Hypothyroidism
- PCOS (polycystic ovary syndrome)
- High-dose androgens or anabolic steroids
- Nephrotic syndrome
- Cushing's syndrome
Implications:
- High free testosterone despite normal total T
- In women: may cause hirsutism, acne, symptoms of androgen excess
- Marker of metabolic dysfunction
- Cardiovascular risk factor independent of other markers
Optimization.
Diet
- Address insulin resistance: low glycemic, adequate protein
- Adequate fiber (improves insulin sensitivity)
- Coffee may modestly increase SHBG
- Moderate alcohol (excess lowers SHBG)
Lifestyle
- Lose excess weight (raises SHBG if low from obesity)
- Resistance training (improves metabolic health)
- Treat sleep apnea (improves metabolic markers)
- Avoid very low body fat (can raise SHBG too high)
Supplements
- Berberine or metformin (improve insulin sensitivity → raises SHBG)
- Omega-3s (anti-inflammatory, metabolic support)
- Boron (may slightly lower SHBG if elevated)
FAQs.
My total testosterone is normal—why do I have low-T symptoms?
If your SHBG is high, most of that 'normal' testosterone is bound and inactive. Only 2-3% of testosterone circulates freely. High SHBG can leave you with insufficient free testosterone for tissue effects. Always check SHBG and calculate free testosterone when symptoms don't match total T.
Should I try to lower my SHBG?
Only if it's genuinely too high and causing problems (low free T with symptoms). Many interventions that 'lower SHBG' (boron, high-dose androgens) aren't always healthy. Often high SHBG reflects another condition (hyperthyroidism, aging) that should be addressed directly. Very low SHBG is actually concerning for metabolic health.
Why is low SHBG a metabolic risk marker?
Low SHBG strongly correlates with insulin resistance, metabolic syndrome, and type 2 diabetes risk—independent of obesity. Insulin directly suppresses liver SHBG production. Low SHBG is essentially a marker of hyperinsulinemia and metabolic dysfunction, predicting future diabetes even before glucose rises.