Nelson Vergel
Founder, ExcelMale.com
By Nelson Vergel | ExcelMale.com | Updated December 2025
When evaluating testosterone levels, most men focus exclusively on total testosterone—a single number that tells only part of the story. What many don't realize is that a protein called sex hormone binding globulin (SHBG) determines how much of that testosterone is actually available for your body to use. Understanding SHBG is essential for anyone on testosterone replacement therapy (TRT) or considering treatment, because it explains why two men with identical total testosterone levels can have dramatically different symptoms and responses to therapy.This comprehensive guide explains what SHBG is, how it affects your hormone balance, what causes it to be high or low, and how to optimize your TRT protocol based on your SHBG status. Whether you're struggling with symptoms despite "normal" testosterone levels or trying to understand why your current protocol isn't working optimally, SHBG may hold the key.
What Is Sex Hormone Binding Globulin (SHBG)?
Sex hormone binding globulin (SHBG) is a glycoprotein produced primarily by the liver that binds to and transports sex hormones through the bloodstream. Think of SHBG as a carrier protein that holds onto testosterone (and other sex hormones like estradiol and dihydrotestosterone) as they travel through your circulation.The critical point is this: testosterone bound to SHBG is not available for your tissues to use. Only "free" testosterone—the small fraction not bound to any protein—can readily enter cells and activate androgen receptors. This is why measuring SHBG alongside total testosterone gives a much more accurate picture of your actual hormonal status.
How Testosterone Is Distributed in Your Blood
In the general circulation, testosterone exists in three main fractions:1. SHBG-Bound Testosterone (~44-65%): Tightly bound to SHBG with high affinity. This testosterone is essentially "locked up" and unavailable to tissues. It serves as a reservoir but cannot directly activate androgen receptors.
2. Albumin-Bound Testosterone (~33-54%): Loosely bound to albumin, the most abundant protein in blood. Because albumin binding is weak, this testosterone can dissociate and become available to tissues relatively easily. It's considered part of "bioavailable" testosterone.
3. Free Testosterone (~1-3%): Unbound testosterone that can immediately enter cells and exert biological effects. This is the most metabolically active fraction.
Bioavailable testosterone = free testosterone + albumin-bound testosterone. This represents the testosterone actually available for your body to use. When SHBG is high, more testosterone gets locked up; when SHBG is low, more testosterone remains bioavailable.
SHBG Binding Affinity for Different Hormones
Hormone | Relative Binding Affinity | Clinical Implication |
| DHT | Highest (5x testosterone) | Most strongly bound; limited free DHT |
Testosterone | High | Primary determinant of free T levels |
Estradiol | Moderate (1/5 of testosterone) | More free E2 at low SHBG states |
DHEA | Weak | Minimally affected by SHBG changes |
Why SHBG Matters for Men on TRT
The Hidden Cause of Symptoms Despite "Normal" Testosterone
One of the most common frustrations in men's health is having symptoms of low testosterone while lab work shows "normal" total testosterone levels. SHBG often explains this paradox:High SHBG Scenario: A man with total testosterone of 600 ng/dL but SHBG of 70 nmol/L may have free testosterone in the single digits—well below the reference range—and experience all the classic symptoms of hypogonadism: low libido, fatigue, difficulty building muscle, brain fog, and depressed mood. His "normal" total testosterone is misleading because most of it is bound and unavailable.
Low SHBG Scenario: Conversely, a man with total testosterone of 400 ng/dL but SHBG of 15 nmol/L may have free testosterone in the upper-normal range and feel fine—or he may struggle with estrogen-related side effects because low SHBG also means more free estradiol.
This is why the Endocrine Society and other professional organizations recommend measuring free testosterone (or calculating it from total testosterone and SHBG) when evaluating men for hypogonadism, particularly when total testosterone is borderline or when clinical presentation doesn't match lab values.
SHBG Determines Your Optimal TRT Protocol
Your baseline SHBG level significantly influences how you'll respond to TRT and what protocol is likely to work best:Men with Low SHBG (<20 nmol/L): Tend to clear testosterone quickly. Often do better with smaller, more frequent injections (daily or every-other-day) to maintain stable levels and prevent excessive peaks that drive estrogen conversion. Large, infrequent doses can cause dramatic fluctuations and side effects.
Men with Normal SHBG (20-50 nmol/L): Usually respond well to standard twice-weekly injection protocols. Have flexibility in dosing schedules.
Men with High SHBG (>50 nmol/L): May require higher total testosterone doses to achieve adequate free testosterone levels. Some clinicians suggest less frequent injections work fine since high SHBG provides a "buffer," though individual response varies.
Normal SHBG Levels in Men
Reference ranges vary by laboratory, but typical adult male SHBG ranges are:SHBG Reference Ranges by Age
Age Group | SHBG Range (nmol/L) | Clinical Notes |
| Young Adult Male (18-30) | 10-57 nmol/L | Average ~30-35 nmol/L in healthy men |
Middle-Aged Male (40-60) | 18-76 nmol/L | SHBG tends to increase with age |
Older Male (>60) | 20-85+ nmol/L | Explains declining free T with age |
Men on TRT | Often decreases | Exogenous T typically lowers SHBG |
What Causes High SHBG?
Elevated SHBG reduces bioavailable testosterone even when total testosterone appears normal. Understanding the causes can help identify correctable factors:Medical Conditions
• Hyperthyroidism: Excess thyroid hormone strongly increases SHBG production. Men with undiagnosed hyperthyroidism often present with low free testosterone symptoms.• Liver Disease/Cirrhosis: Paradoxically, while the liver produces SHBG, cirrhosis often elevates SHBG levels through complex hormonal changes.
• HIV Infection: Associated with elevated SHBG, contributing to hypogonadal symptoms common in HIV-positive men.
• Anorexia/Severe Caloric Restriction: Extended caloric restriction raises SHBG significantly.
Medications and Substances
• Anticonvulsants: Phenytoin, carbamazepine, and other anticonvulsants can increase SHBG substantially.• Oral Estrogens: Oral estrogen therapy (including in transgender women) dramatically increases SHBG.
• Alcohol Use: Chronic alcohol consumption can elevate SHBG levels.
Age and Genetic Factors
SHBG naturally increases with age—approximately 1-2% per year after age 40. Combined with declining testosterone production, this creates a "double hit" that explains why many older men have low free testosterone even with borderline-normal total testosterone. Genetic polymorphisms in the SHBG gene also contribute to individual variation.What Causes Low SHBG?
Low SHBG is increasingly common and is strongly associated with metabolic dysfunction. While it means more free testosterone is available, low SHBG itself is often a marker of underlying health issues:Metabolic Conditions
• Obesity: Strong inverse relationship between BMI and SHBG. Visceral adiposity (belly fat) is particularly impactful. Weight loss typically raises SHBG.• Insulin Resistance/Type 2 Diabetes: Hyperinsulinemia directly suppresses hepatic SHBG production. Low SHBG is an independent predictor of type 2 diabetes development—often appearing years before diagnosis.
• Metabolic Syndrome: The combination of central obesity, dyslipidemia, hypertension, and glucose intolerance is strongly associated with low SHBG. Research shows low SHBG independently predicts cardiovascular disease and metabolic syndrome.
• Non-Alcoholic Fatty Liver Disease (NAFLD): Hepatic steatosis impairs SHBG production.
Hormonal Factors
• High Androgen Levels: Testosterone and other androgens suppress SHBG production. This is why SHBG typically decreases on TRT—exogenous testosterone feeds back to lower SHBG.• Growth Hormone/IGF-1 Excess: Conditions like acromegaly lower SHBG.
• Hypothyroidism: Low thyroid hormone reduces SHBG production (opposite effect of hyperthyroidism).
Medications
• Anabolic Steroids: Oral 17-alpha-alkylated steroids (like oxandrolone, stanozolol) dramatically lower SHBG by directly affecting liver production.• Glucocorticoids: Chronic corticosteroid use (Cushing's syndrome or therapeutic) reduces SHBG.
• Certain Progestins: Some progestational agents lower SHBG levels.
Optimizing TRT Based on Your SHBG Status
If You Have Low SHBG (<20 nmol/L)
Men with low SHBG face unique challenges on TRT. Because SHBG acts as a "reservoir" for testosterone, low SHBG means:• Total testosterone levels may appear lower than expected for a given dose
• Free testosterone may be high relative to total testosterone
• Testosterone clears more quickly, causing fluctuations
• Free estradiol may be disproportionately elevated, causing estrogen-related side effects
Protocol Recommendations: Most men with low SHBG do better with daily or every-other-day (EOD) injections of smaller testosterone doses. This maintains more stable blood levels and helps prevent the peaks that drive excessive aromatization to estradiol. Large, infrequent injections (e.g., 200mg once weekly) often cause significant fluctuations and side effects in low-SHBG men.
Address Underlying Causes: Because low SHBG is often a marker of metabolic dysfunction, addressing obesity, insulin resistance, and fatty liver can improve SHBG levels and overall health. Weight loss, improved diet, and better glucose control may naturally raise SHBG over time.
If You Have High SHBG (>50 nmol/L)
Men with high SHBG often need higher testosterone doses to achieve adequate free testosterone levels. Their challenges include:• Total testosterone may appear "normal" while free testosterone is low
• May require doses that push total testosterone above typical ranges to achieve symptom relief
• Less prone to estrogen-related side effects (SHBG also binds estradiol)
Protocol Recommendations: Higher weekly doses may be necessary. Some clinicians suggest that high-SHBG men can do well with once or twice weekly injections since the SHBG provides a buffer against fluctuations. However, response varies individually. The key metric is achieving adequate free testosterone levels for symptom resolution.
Evaluate Underlying Causes: Check thyroid function—hyperthyroidism is a common, treatable cause of elevated SHBG. Review medications that may be elevating SHBG. Consider liver function testing.
Strategies to Lower Elevated SHBG
If high SHBG is limiting your response to TRT, several strategies may help:1. Treat Underlying Conditions: Correct hyperthyroidism, address liver disease, discontinue medications that elevate SHBG if possible.
2. Increase Protein Intake: Higher protein diets are associated with lower SHBG levels. Vegetarian/vegan diets with low protein intake tend to increase SHBG.
3. Boron Supplementation: Some studies suggest boron (3-10mg daily) may modestly reduce SHBG, though evidence is mixed.
4. Stinging Nettle Root: May bind to SHBG and displace testosterone, though clinical evidence is limited.
5. Optimize Testosterone Dose: Exogenous testosterone itself lowers SHBG over time. Higher doses have greater SHBG-lowering effect, though this must be balanced against side effects.
6. DHT-Derived Compounds: In clinical settings, DHT-derived medications like proviron (mesterolone) or low-dose danazol can significantly lower SHBG. These require medical supervision.
Strategies to Raise Low SHBG
Raising chronically low SHBG is more challenging than lowering high SHBG. The focus should be on addressing the metabolic dysfunction that typically underlies low SHBG:1. Weight Loss: Losing excess body fat, particularly visceral fat, is the most effective way to raise SHBG. Even modest weight loss improves SHBG levels.
2. Improve Insulin Sensitivity: Exercise, dietary changes (reducing refined carbohydrates), and if needed, medications like metformin can improve insulin sensitivity and raise SHBG.
3. Reduce Alcohol Consumption: Moderating alcohol intake supports liver health and SHBG production.
4. Address Fatty Liver: NAFLD impairs SHBG production. Weight loss, dietary changes, and avoiding hepatotoxic substances can improve liver function.
5. Thyroid Optimization: Ensure thyroid function is normal—hypothyroidism lowers SHBG.
Important Note: Very low SHBG that doesn't respond to lifestyle interventions may simply be genetic. In such cases, focus on optimizing your TRT protocol (more frequent, smaller doses) rather than trying to change SHBG levels.
Testing SHBG: What You Need to Know
SHBG should be part of any comprehensive hormone evaluation. Here's what to know about testing:When to Test: SHBG should be measured at baseline before starting TRT and periodically during treatment (typically annually or when adjusting protocols). Test when total testosterone doesn't explain symptoms, when adjusting dosing, or when experiencing unexplained side effects.
Test Timing: Unlike testosterone, SHBG levels are relatively stable throughout the day and don't require early morning testing. However, testing alongside morning testosterone (before 10 AM) ensures all values are comparable.
Calculating Free Testosterone: With total testosterone, SHBG, and albumin values, free testosterone can be calculated using validated equations (like the Vermeulen equation). This calculated free testosterone correlates well with equilibrium dialysis, the gold-standard method.
Free Androgen Index (FAI): The ratio of total testosterone to SHBG (FAI = Total T / SHBG × 100) provides a quick estimate of androgenic activity. However, FAI becomes unreliable at extreme SHBG levels and should not substitute for calculated or measured free testosterone in clinical decision-making.
Conclusion: SHBG Is Essential to Understanding Your Testosterone Status
Sex hormone binding globulin is far more than a technical footnote in hormone testing—it's a critical determinant of how much testosterone is actually available to your tissues. Men with identical total testosterone levels can have dramatically different symptoms and treatment responses based on their SHBG status.For men on TRT or considering treatment, understanding your SHBG level helps explain why certain protocols work better than others and guides optimization. Low SHBG men typically benefit from frequent, smaller doses; high SHBG men often need higher overall doses to achieve adequate free testosterone.
Beyond TRT, SHBG provides valuable metabolic information. Low SHBG is an independent predictor of type 2 diabetes, metabolic syndrome, and cardiovascular disease—often appearing years before these conditions develop. Monitoring SHBG and addressing underlying causes like obesity and insulin resistance can improve both hormonal status and overall health.
The bottom line: if you're evaluating testosterone status or optimizing TRT, always measure SHBG. It transforms a one-dimensional total testosterone number into a meaningful assessment of your actual hormonal environment.
Related ExcelMale Forum Discussions
Explore these community discussions for additional insights and real-world experiences:• Sex Hormone Binding Globulin (SHBG): Is It Good or Bad? – Comprehensive overview of SHBG's role in men's health
• What Is the Purpose of Sex Hormone Binding Globulin (SHBG)? – Discussion of SHBG's biological functions
• How to Lower Your Sex Hormone Binding Globulin – Strategies for managing elevated SHBG
• How to Lower SHBG and Increase Free Testosterone – Practical approaches to optimizing free testosterone
• SHBG - Study Confirms Wide Variation in Serum Levels – Research supporting routine SHBG testing
• Injection Frequency Effect on SHBG – How dosing protocols impact SHBG levels
• Low SHBG Guys: Will Increasing Injection Frequency Make Any Difference? – Experiences with protocol adjustments for low SHBG
• Low SHBG = Lower Dosed, More Frequent Injections. Why? – Understanding the rationale for low-SHBG protocols
• Help Me Understand SHBG Levels and Injection Protocols – Guidance on matching protocols to SHBG status
• Minimum Dosage or Frequency for Low SHBG – Dosing strategies for men with low SHBG
Key References
1. Goldman AL, Bhasin S, Wu FC, et al. A Reappraisal of Testosterone's Binding in Circulation. Endocrine Reviews. 2017. [PMC Full Text]2. Kupelian V, et al. Association of Testosterone and SHBG With Metabolic Syndrome and Insulin Resistance. Diabetes Care. 2010. [PMC Full Text]
3. Ramachandran S, et al. TRT: Pre-treatment SHBG levels and age may identify clinical subgroups. Andrology. 2020. [PubMed]
4. Trost LW, Mulhall JP. SHBG and Total T Levels in Men with Adult Onset Hypogonadism. Cardiovasc Diabetol Endocrinol Rep. 2020. [Springer Full Text]
5. Daka B, et al. Inverse association between serum insulin and SHBG. Endocrine Connections. 2012.
6. Sex hormone-binding globulin changes with androgen replacement. J Clin Endocrinol Metab. [PubMed]
7. New Insights in the Diagnostic Potential of SHBG—Clinical Approach. Int J Mol Sci. 2024. [PMC Full Text]
8. MedlinePlus. SHBG Blood Test. [MedlinePlus]
Disclaimer: This article is for informational purposes only and does not constitute medical advice. SHBG interpretation should be done in the context of a comprehensive clinical evaluation. Hormone management requires individualized assessment by a qualified healthcare provider. Always consult your physician before making changes to your treatment protocol.
About ExcelMale.com: ExcelMale is a men's health forum with over 24,000 members and 20+ years of archived discussions on testosterone replacement therapy, hormone optimization, and sexual health. Founded by Nelson Vergel, author of Testosterone: A Man's Guide and Beyond Testosterone, ExcelMale provides evidence-based information and peer support for men navigating hormone health decisions.
