Free Testosterone Calculator with Bioavailable Levels

Calculate free testosterone and bioavailable testosterone levels using total testosterone, SHBG, and albumin measurements.

This calculator uses validated mathematical models to determine free testosterone and bioavailable testosterone levels from total testosterone, sex hormone-binding globulin (SHBG), and albumin measurements. Essential for accurate hormone assessment and clinical decision-making.

Example Scenarios

Click on any example to load it into the calculator.

Normal Adult Male

normal-male

Healthy adult male with normal testosterone levels.

Total Testosterone: 650 ng/dL

SHBG: 35 nmol/L

Albumin: 4.2 g/dL

Age: 35 years

Free Testosterone: 15.2 pg/mL

Bioavailable Testosterone: 180.5 ng/dL

Low Testosterone Case

low-testosterone

Adult male with low testosterone levels.

Total Testosterone: 280 ng/dL

SHBG: 45 nmol/L

Albumin: 3.8 g/dL

Age: 45 years

Free Testosterone: 4.8 pg/mL

Bioavailable Testosterone: 65.2 ng/dL

High SHBG Case

high-shbg

Case with elevated SHBG levels affecting bioavailability.

Total Testosterone: 500 ng/dL

SHBG: 80 nmol/L

Albumin: 4.0 g/dL

Age: 40 years

Free Testosterone: 6.2 pg/mL

Bioavailable Testosterone: 85.1 ng/dL

Elderly Male

elderly-male

Older male with age-related testosterone decline.

Total Testosterone: 350 ng/dL

SHBG: 60 nmol/L

Albumin: 3.5 g/dL

Age: 70 years

Free Testosterone: 5.8 pg/mL

Bioavailable Testosterone: 78.3 ng/dL

Other Titles
Understanding Free Testosterone Calculator with Bioavailable Levels: A Comprehensive Guide
Learn about testosterone metabolism, binding proteins, and how to accurately assess hormone levels for optimal health and clinical decision-making.

What is Free Testosterone and Bioavailable Testosterone?

  • Understanding Testosterone Fractions
  • The Role of Binding Proteins
  • Clinical Significance of Free vs Total Testosterone
Testosterone exists in the bloodstream in three main forms: free testosterone (unbound), testosterone bound to sex hormone-binding globulin (SHBG), and testosterone bound to albumin. Only free testosterone and albumin-bound testosterone are biologically active, collectively known as bioavailable testosterone. Understanding these fractions is crucial for accurate hormone assessment and clinical decision-making.
The Three Fractions of Testosterone
Total testosterone represents the sum of all three fractions. Free testosterone (1-3% of total) is the biologically active form that can enter cells and bind to androgen receptors. SHBG-bound testosterone (60-80% of total) is tightly bound and biologically inactive. Albumin-bound testosterone (20-40% of total) is weakly bound and can dissociate to become bioavailable. The balance between these fractions determines overall androgen activity.
Why Free and Bioavailable Testosterone Matter
Total testosterone levels alone can be misleading because binding protein concentrations vary significantly between individuals and conditions. A person with normal total testosterone but high SHBG may have low free testosterone and experience symptoms of androgen deficiency. Conversely, someone with low total testosterone but low SHBG may have adequate free testosterone levels. This is why measuring free and bioavailable testosterone provides more accurate assessment of androgen status.
Clinical Applications and Significance
Free and bioavailable testosterone measurements are essential for diagnosing androgen deficiency, monitoring testosterone replacement therapy, and assessing androgen excess conditions. They are particularly important in cases where total testosterone levels are borderline or when SHBG levels are abnormal due to conditions like obesity, diabetes, thyroid disorders, or liver disease.

Key Concepts:

  • Free Testosterone: Unbound, biologically active form (1-3% of total)
  • Bioavailable Testosterone: Free + albumin-bound testosterone (20-40% of total)
  • SHBG-Bound: Tightly bound, biologically inactive (60-80% of total)
  • Clinical Relevance: More accurate than total testosterone for androgen assessment

Step-by-Step Guide to Using the Free Testosterone Calculator

  • Accurate Input Requirements
  • Understanding the Calculation Process
  • Interpreting Results and Reference Ranges
The free testosterone calculator uses validated mathematical models based on mass action law and equilibrium constants to determine hormone fractions. This approach provides accurate estimates without requiring expensive direct measurement techniques. Understanding the input requirements and calculation process ensures reliable results for clinical decision-making.
1. Required Laboratory Measurements
You need three essential measurements: total testosterone, SHBG, and albumin. Total testosterone should be measured in the morning (8-10 AM) when levels are highest. SHBG is typically measured in nmol/L, while albumin can be in g/dL or g/L. Ensure all measurements are from the same blood draw for accurate calculations. Age is also required to determine appropriate reference ranges.
2. Understanding the Calculation Algorithm
The calculator uses the Vermeulen equation, which applies mass action law to hormone binding. It calculates the equilibrium between free testosterone and binding proteins using known association constants. The algorithm iteratively solves for free testosterone concentration, then calculates bioavailable testosterone as the sum of free and albumin-bound fractions. This mathematical approach has been validated against direct measurement methods.
3. Interpreting Results and Reference Ranges
Results include free testosterone (pg/mL or pmol/L), bioavailable testosterone (ng/dL or nmol/L), and percentages of total testosterone. Reference ranges vary by age and laboratory, but generally: free testosterone should be 1-3% of total, bioavailable testosterone 20-40% of total. Clinical interpretation considers symptoms, age, and individual circumstances alongside numerical results.
4. Clinical Decision-Making and Follow-up
Use results in conjunction with clinical symptoms and other laboratory values. Low free/bioavailable testosterone with symptoms may indicate androgen deficiency requiring treatment. High levels may suggest androgen excess. Always consider the clinical context and consult with healthcare providers for interpretation and treatment decisions.

Calculation Guidelines:

  • Morning Testing: Measure testosterone between 8-10 AM for highest levels
  • Same Blood Draw: Use measurements from the same sample for accuracy
  • Unit Consistency: Ensure all measurements use consistent units
  • Clinical Context: Interpret results with symptoms and medical history

Real-World Applications of Free Testosterone Assessment

  • Clinical Diagnosis and Treatment
  • Research and Population Studies
  • Sports Medicine and Performance
Free testosterone assessment has numerous applications across clinical practice, research, and specialized fields. From diagnosing androgen deficiency to optimizing athletic performance, understanding testosterone fractions provides valuable insights for various medical and health-related decisions.
Clinical Endocrinology and Andrology
Endocrinologists and andrologists use free testosterone measurements to diagnose hypogonadism, monitor testosterone replacement therapy, and assess androgen excess conditions like polycystic ovary syndrome. The ability to distinguish between total and free testosterone helps identify cases where binding protein abnormalities mask true androgen status.
Aging and Geriatric Medicine
Age-related testosterone decline affects millions of men worldwide. Free testosterone assessment helps distinguish between normal aging and pathological hypogonadism. This is crucial for determining who might benefit from testosterone replacement therapy and monitoring treatment effectiveness in elderly populations.
Sports Medicine and Performance Optimization
Athletes and sports medicine professionals use testosterone assessment to monitor training responses, detect overtraining syndrome, and assess recovery status. Free testosterone levels provide more accurate information about anabolic status than total testosterone alone, especially in athletes with varying SHBG levels.
Research and Population Health Studies
Epidemiological studies use free testosterone measurements to investigate relationships between androgen status and various health outcomes including cardiovascular disease, diabetes, osteoporosis, and cognitive function. These studies help establish reference ranges and understand hormone-health relationships across different populations.

Clinical Applications:

  • Hypogonadism Diagnosis: Identify androgen deficiency in symptomatic patients
  • Therapy Monitoring: Track effectiveness of testosterone replacement
  • Aging Assessment: Distinguish normal decline from pathological hypogonadism
  • Performance Optimization: Monitor athletic training and recovery status

Common Misconceptions and Correct Methods

  • Myths About Testosterone Testing
  • Proper Interpretation Guidelines
  • Avoiding Common Pitfalls
Several misconceptions exist about testosterone testing and interpretation. Understanding these myths and following evidence-based guidelines ensures accurate assessment and appropriate clinical decision-making. This section addresses common errors and provides correct approaches to testosterone evaluation.
Myth: Total Testosterone Alone is Sufficient
Many believe that total testosterone measurement alone provides complete information about androgen status. However, binding protein variations can significantly affect free testosterone levels while total testosterone remains normal. Conditions like obesity, diabetes, and thyroid disorders alter SHBG levels, making free testosterone assessment essential for accurate evaluation.
Myth: Direct Measurement is Always Better
Some assume that direct measurement of free testosterone is superior to calculated values. However, direct measurement methods have limitations including poor precision and high cost. Calculated free testosterone using validated equations like Vermeulen's method provides accurate, reproducible results and is the standard approach in clinical practice.
Myth: One Measurement is Definitive
Testosterone levels vary throughout the day and between days. Single measurements may not represent true androgen status. Multiple measurements, preferably in the morning, provide more reliable assessment. Clinical symptoms and other laboratory values should always be considered alongside testosterone measurements.
Myth: Reference Ranges Apply to Everyone
Reference ranges vary by age, laboratory, and population. What's normal for a 25-year-old may be abnormal for a 70-year-old. Individual factors including health status, medications, and lifestyle affect testosterone levels. Clinical interpretation must consider the whole patient, not just numerical results.

Correct Approaches:

  • Multiple Measurements: Test on multiple occasions for reliability
  • Clinical Context: Consider symptoms and medical history
  • Age-Appropriate Ranges: Use age-specific reference values
  • Comprehensive Assessment: Include other hormone and health markers

Mathematical Derivation and Examples

  • The Vermeulen Equation
  • Equilibrium Constants and Binding
  • Practical Calculation Examples
The calculation of free testosterone uses the Vermeulen equation, which applies mass action law to hormone binding equilibrium. Understanding the mathematical principles helps appreciate the accuracy and limitations of calculated free testosterone values. This section provides the theoretical foundation and practical examples.
The Vermeulen Equation Foundation
The Vermeulen equation is based on mass action law: [Free T] × [SHBG] = K1 × [T-SHBG] and [Free T] × [Albumin] = K2 × [T-Albumin], where K1 and K2 are association constants. The equation solves for free testosterone concentration using total testosterone, SHBG, and albumin measurements. This approach has been validated against direct measurement methods.
Equilibrium Constants and Binding Affinity
SHBG has high affinity for testosterone (K1 ≈ 1.6 × 10^9 L/mol), while albumin has low affinity (K2 ≈ 4.0 × 10^4 L/mol). This means SHBG-bound testosterone is tightly bound and biologically inactive, while albumin-bound testosterone can dissociate to become bioavailable. The balance between these binding affinities determines free testosterone levels.
Practical Calculation Examples
Consider a 40-year-old man with total testosterone 500 ng/dL, SHBG 35 nmol/L, and albumin 4.2 g/dL. The calculator determines free testosterone ≈ 15.2 pg/mL (3.0% of total) and bioavailable testosterone ≈ 180.5 ng/dL (36.1% of total). This demonstrates how binding proteins affect hormone bioavailability and why free testosterone assessment is clinically important.
Limitations and Considerations
The calculation assumes normal binding protein function and uses population-based association constants. Individual variations in binding protein structure or function may affect accuracy. The equation works best when SHBG and albumin levels are within normal ranges. Extreme values may require direct measurement methods for confirmation.

Mathematical Concepts:

  • Mass Action Law: [Free T] × [SHBG] = K1 × [T-SHBG]
  • Association Constants: SHBG (K1 ≈ 1.6 × 10^9), Albumin (K2 ≈ 4.0 × 10^4)
  • Bioavailable Calculation: Free T + Albumin-bound T
  • Percentage Calculation: (Fraction/Total) × 100